Transactions vol ii

omega.com ®

OMEGA®

http://www.omega.com • e-mail: [email protected]

http://www.omega.com/

http://www.omega.com/das/dsc.html


http://www.omega.com/literature/emaildir/dasmail.html


http://www.omega.com/pptst/CT485B.html

http://www.omega.com/toc_asp/sectionSC.asp?section=J&book=temperature


1018

1015

1012

109

106

103

102

101

10-1

10-2

10-3

10-6

10-9

10-12

10-15

10-18

FACTOR PREFIX SYMBOL FACTOR PREFIX SYMBOL

exa

peta

tera

giga

mega

kilo

hecto

deka

E

P

T

G

M

k

h

da

deci

centi

milli

micro

nano

pico

femto

atto

d

c

m

µ

n

p

f

a

DECIMAL FACTOR PREFIXES

Black

Brown

Red

Orange

Yellow

Green

Blue

Violet

Gray

White

Gold

Silver

No Color

resistor value = (1st band)x(2nd band)x(103rd band)

resistor tolerance = 4th band (if specified)

±20%

±2%

±5%

±10%

±20%

COLOR VALUE OF (1st & 2nd bands) (3rd band) (4th band)

0

1

2

3

4

5

6

7

8

9

0

1

2

3

4

5

6

7

8

9

-1

-2

RESISTOR COLOR CODES

1.0

1.8

3.3

5.6

1.1

2.0

3.6

6.2

1.2

2.2

3.9

6.8

1.3

2.4

4.3

7.5

1.5

2.7

4.7

8.2

1.6

3.0

5.1

9.1

STANDARD RESISTOR VALUES

Voltage Source

Current Source

Resistor

Inductor

Capacitor

Joined Wires

Unjoined Wires

Open Circuit

Short Circuit

es appears in the circuit for i

is supplied to the circuit for all v

vR = iR

vL = L or i = vLdt

vc = idt or i = c

Connection is made at the dot

Wires cross without contact

i is zero for all v

v is zero for all i

es

is

vR

vL

+

+R

i

+L

i

vc

+c

i

v

i

v

i

didt

dvcdt

1c

SYMBOLS FOR SIMPLE ELECTRICAL CIRCUITS

A: ampere (amp) ac: alternating currentA/D: analog-to-digital ADC: analog-to-digital converterANSI: American National Standards Institute ASCII: American Standard Code for InformationInterchangeASIC: application-specific integrated circuit b: bitB: bytebps: bits per secondC: capacitanceCAN: Controller Area Network CJC: cold junction compensation CMRR: common-mode rejection ratio COM: Component Object Model CPU: central processing unitCSMA/CD: carrier sense multiple access with collisiondetection D/A: digital-to-analogDAC: digital-to-analog converterDAQ: data acquisitionDAS: data acquisition system dB: decibeldc: direct currentDCE: data communications equipmentDCOM: Distributed Component Object Model DCS: distributed control systemDDE: Dynamic Data ExchangeDIO: digital input/outputDLL: Dynamic Link Library DMA: direct memory accessDSP: digital signal processing DTE: data terminal equipmentEEPROM: electrically erasable programmable ROMEMF: electromotive force EMI: electromagnetic interference EPROM: erasable programmable ROM FIFO: first-in first-outGPIB: General Purpose Interface Bus GUI: graphical user interface

HMI: Human-machine interfacehz: hertzI/O: input/outputi: currentIEEE: Institute of Electrical and Electronic EngineersJTFA: joint time-frequency analysis L: inductanceLED: light emitting diodeLSB: least significant bitMAC: media access controlMFLOPS: million floating-point operations per second MIPS: million instructions per secondMMI: man-machine interface MUX: multiplexerODVA: Open DeviceNet Vendors AssociationOLE: Object Linking and Embedding OSI: Open Systems Interconnect PC: personal computerPCI: Peripheral Component Interconnect PCMCIA: Personal Computer Memory Card InternationalAssociation PGA: programmable gain amplifierPID: proportional-integral-derivative PLC: programmable logic controller R: resistanceRFI: radio frequency interference RMS: root-mean-square ROM: read-only memoryRTD: resistance temperature detector SCADA: supervisory control and data acquisitionSE: single-endedS/H: sample-and-hold SNR: signal-to-noise ratio SPC: statistical process control SQL: Structured Query Language SS: simultaneous samplingSTP: shielded twisted pair TCP/IP: Transmission Control Protocol/Internet Protocol TTL: transistor-to-transistor logic UTP: unshielded twisted pair V: volt

ACRONYMS AT A GLANCE

©1998 Putman Publishing Company and OMEGA Press LLC.

omega.com®

®

Data Acquisition A Technical Reference Series Brought to You by OMEGA

22

VOLUME

I N M E A S U R E M E N T A N D C O N T R O L

06 Volume 2 TRANSACTIONS

Analog I/O Functionality1 • Resolution & Aliasing

• Analog-to-Digital

• Digital-to-Analog

Figure 1-5: Analog Input Flow Diagram

Analog Input

Signals

1

2

3

4

Signal Conditioning

Digital Output

Mul

tipl

exer

Amplifier

Sample/Hold

A/D Converter

Digital I/O Functionality2

10

I N M E A S U R E M E N T A N D C O N T R O L

TABLE OF CONTENTS

VOLUME 2—DATA ACQUISITIONSection Topics Covered Page

• Digital Inputs

• Digital Outputs

• Pulse I/O

Figure 2-1: Signal Processing Requirements for Digital and Analog Signals

Signal conditioning Signal conditioning

Multiplexing

A/D-D/A conversion

Sensors & Actuators

Computer

Digital signals

Analog signals

Digital signals

17

Analog Signal Transmission3

• Analog Signal Types

• Noise & Grounding

• Wire & Cable Options

5

0

Digital Analog

VoltsVolts

Time Time

10

0

-10

Figure 3-2: Digital and Analog Signal Representations

19

Digital Signal Transmission4

• The OSI Network Model

• Physical Layer Options

• Network Topologies

• Fieldbus & Device Networks

Figure 4-9: Dual-Ring Redundant FDDI Network

Intact Broken

24

TRANSACTIONS Volume 2 07

Data Acquistion Hardware5

• Selecting a System

• Plug-in-Cards

• Standalone Components

• Communication Devices

Section Topics Covered Page

Presentation & Analysis6

36

Recording, Printing & Storage 7

• Development Considerations

• Component Architectures

• Off-the-Shelf Offerings

• Definitions and Classifications

• Trend Recorders

• Data Loggers

• Videographic Recorders

Inside Front Cover 02

Table of Contents 06

Editorial 08

About OMEGA 09

48

Figure 7-2: Typical Circular Chart Recorder Trace

Time

2

3

Hole to Accept Chart HubPrinted Line for Manual Entry Of Date

Hole to Accept Chart Positioning and Drive Pin

20 30 40 45 50 55 60 65

19 58

REFERENCE SECTIONS

68 Information Resources

72 Glossary

79 Index

82 List of Figures


A Commitment to StandardsSince OMEGA first opened its doors back in 1962, the practice of data acquisition has changed

at least as much as OMEGA itself. At that time, OMEGA offered a single line of fine-gauge thermocouples; now our scientific

and industrial measurement and control products number more than 68,000. In 1962, scientificand industrial data acquisition systems—if they could be called systems at all—consisted primarilyof data manually logged by technicians on clipboards and reams of analog recorder paper.And while many recorders still do record on paper, the now ubiquitous microprocessor hasrevolutionized pretty much everything else.

The PC and other microprocessor-based devices have replaced analog recording and displaytechnologies in all but the simplest data acquisition applications. And while computers havehad an undeniably positive impact on the practice of data acquisition, they have also addedunprecedented levels of complexity.

Analog-to-digital conversion and other signal conditioning functions come in a myriad ofform factors. Networked devices must be able to communicate flawlessly with

host computers at high speeds. And application programs written by aburgeoning number of independent software developers must readily

interchange ever-increasing amounts of data—and convert this firehose of raw bits into meaningful information.

The secret to success? Standards.Standards like the ISA and PCI backplane buses, RS-232

and GPIB networks, and DDE and OLE for application inte-gration. As your partner in instrumentation and control,it is our responsibility to help ease your integration andcommunication tasks using standards wherever possi-ble. You can also count on OMEGA’s commitment tostandards to preserve your freedom of choice andflexibility now and in the future. We’ll never lock youinto proprietary protocols.

Of course, the world of standards isn’t perfect andstandardization, by its very nature, lags the leading edge

of technology. But rest assured that OMEGA will be withyou every step of the way, helping to define the next gen-

eration of data acquisition standards to make your job easier,while continuing to support your ongoing requirements.

S

P.S. If you wish to submit an article of relevance for future issues of Transactions, please submit to my

attention via mail (P.O. Box 4047, Stamford, CT 06907), FAX (203-359-7700), or e-mail ([email protected]).

About OMEGA


OMEGA’s Transactions in Measurement & Control series, as well as our legendary set of handbooks and

encyclopedias, are designed to provide at-your-fingertips access to the technical information you need

to help meet your measurement and control requirements. But when your needs exceed the printed

word—when technical assistance is required to select among alternative products, or when no “off-the-shelf”

product seems to fill the bill—we hope you’ll turn to OMEGA. There is no advertising or

promotional materials in the Transactions series. There will be none.

Our people, our facilities, and our commitment to customer service set the standard

for control and instrumentation. A sampler of our comprehensive resources and

capabilities:

• OMEGA’s commitment to leading-edge research and development and

state-of-the-art manufacturing keeps us firmly at the forefront of technology.

OMEGA’s Development and Engineering Center, located on our Stamford, Conn.,

campus, is home to OMEGA’s design and engineering laboratories. All product

designs are tested and perfected here prior to marketing. This facility houses

OMEGA’s metrology lab and other quality control facilities. The testing that takes place here assures

that you receive the best products for your applications.

• On the manufacturing side, our Bridgeport, N.J., vertically integrated manufacturing facility near

Philadelphia houses advanced thermocouple wire production equipment along with a host of other com-

puterized CNC milling machines, injection molding equipment, screw machines, braiders, extruders,

punch presses and much, much more.

• If our broad range of standard products don’t quite match your needs, OMEGA is proud to offer the most

sophisticated and extensive custom engineering capabilities in the process measurement and control industry.

Whether you need a simple modification of a standard product or complete customized system, OMEGA can

accommodate your special request. Free CAD drawings also are supplied with customized product orders or a

new design built to your specifications at no obligation.

• We believe in active versus reactive customer service. To complement our current business and

manufacturing operations, OMEGA continues to strive toward new levels of quality by pursuing ISO 9000

quality standards. This systematic approach to quality strengthens OMEGA’s competitive edge. Our

calibration services and quality control test center are trustworthy resources that help satisfy our customers’

needs for accuracy on an initial and ongoing basis.

• The company’s technical center welcomes many corporate groups of engineers and scientists who

turn to OMEGA for training. Our 140-seat auditorium, equipped with the latest in multimedia presentation

technologies, provides an ideal learning environment for training tailored to your company’s needs—from

basic refreshers to in-depth courses.

In short, it is our commitment to quality instrumentation and exceptional customer service that remains

the cornerstone of our success. OMEGA’s priority is clear: we exist to facilitate solutions to your needs.

For more information about Transactions or OMEGA Technologies, look us up on the Internet at

www.omega.com.

Exceeding Your ExpectationsO

Today, digital computers andother microprocessor-baseddevices have replaced analogrecording and display tech-

nologies in all but the simplest dataacquisition applications. And whilecomputers have had an undeniablypositive impact on the practice ofdata acquisition, they speak only abinary language of ones and zeroes.

Manufacturing processes and naturalphenomena, however, are still bytheir very nature analog. That is, nat-ural processes tend to vary smoothlyover time, not discontinuouslychanging states from black to white,from on to off.

To be meaningfully recorded ormanipulated by a computer then,analog measurements such as pres-sure, temperature, flow rate, andposition must be translated into dig-ital representations. Inherently digi-tal events, too, such as the trippingof a motor or a pulse generated by apositive displacement flowmeter,must be made interpretable as atransistor-to-transistor logic (TTL)level changes in voltage. Hence, theorigination and ongoing develop-ment of input/output (I/O) systems

(Figure 1-1) for converting analog anddigital information about real-worldprocesses and events into the lan-guage of computers.

Resolution & Aliasing Most sensors for measuring tempera-ture, pressure, and other continuousvariables provide a continuouslyvarying electrical output to repre-

sent the magnitude of the variable inquestion. To make this signal inter-pretable by a microprocessor, it mustbe converted from a smooth contin-uous value to a discrete, digital num-ber (Figure 1-2).

This analog-to-digital (A/D) con-version process poses two primarychallenges: one of quantization andone of sampling in time (Figure 1-3).Quantization refers to the uncertain-ty introduced upon conversion of ananalog voltage to a digital number.Measurement transducers or trans-mitters typically provide continuous-ly varying signals between 0-10 V dc,±5 V dc, 0-100 mV dc, or 4-20 mA dc.Thermocouples and resistance tem-perature devices (RTDs) are othercommon low voltage inputs.

When this analog value is repre-

sented as a digital number, however,this essentially continuous resolutionis limited to discrete steps. This reso-lution of an A/D conversion often isstated in terms of bits—the more bitsthe finer the resolution. The numberof bits determines the number of divi-sions into which a full-scale inputrange can be divided to approximatean analog input voltage. For example,

8-bit resolution of a 0-10 V input sig-nal means that the range is dividedinto 28 = 256 steps. This yields a step,or interval, size of 10 V/256 = 0.039 V.Thus, a 10-V input is equal to the digi-tal number 255 and a 0-V input corre-sponds to 0. Each 0.039-V change inthe input is indicated by adding orsubtracting 1 from the previous num-ber. (For example, 9.961 V is digitallyrepresented by 254.)


Resolution & Aliasing

Analog-to-Digital

Digital-to-Analog

DATA ACQUISITIONAnalog I/O Functionality

1

TAnalog I/O Functionality

Figure 1-1: Functional Diagram for Data Acquisition & Control

Physical World

Sensing Signal Conditioning

Analog-to-Digital Conversion

Display & Decision Making

Actuation Signal Conditioning

Digital-to-Analog Conversion

Figure 1-2: The Analog-to-Digital Interface

Waveform Numbers

V

t

0 1 0 1

0 1 1 0

1 0 1 1

1 1 1 0

0 0 0 1

Digital data acquisition systemsnot only quantize data in terms ofmagnitude; time, too, is parceled intodiscrete intervals (Figure 1-3). In gen-eral, there is no information aboutthe behavior of the process betweendata points gathered. Special precau-tions, then, must be taken to ensureno meaningful data is lost and inter-polation between recorded pointsremains a valid assumption.

The Nyquist theorem defines thenecessary relationship between thehighest frequency contained in a sig-nal and the minimum required sam-pling speed. Nyquist stated that thesample rate must be at least twicethe highest frequency componentcontained within the input signal. So,to sample a 1-Hz sine wave, the sam-ple rate should be at least 2 Hz. (Buta rate of 8-16 Hz would be much bet-ter for resolving the true shape ofthe wave.)

The primary implications of ignor-ing the Nyquist criterion include notonly missing high frequency informa-tion but of introducing aliasing; if thesample rate is not fast enough, thepresence of totally nonexistent fre-quencies may be indicated (Figure 1-4). It is aliasing that makes a heli-copter’s rotors or a car’s wheelsappear to turn slowly backwardswhen seen in a movie. Low-pass, or

anti-aliasing filters can be used tolimit the measured waveform’s fre-quency spectrum so that nodetectable component equals orexceeds half of the sampling rate.

Designing or specifying a devicefor A/D conversion consists of aseries of trade-offs. As will be amplydemonstrated in the next section,more resolution (more bits) meansmore accurate conversion but moreexpensive hardware. Similarly, slowersample rates mean cheaper A/Dconversion, but the Nyquist criterionmust still be satisfied.

A/D ConversionContinuous electrical signals areconverted to the digital language ofcomputers using analog-to-digital(A/D) converters. An A/D convertermay be housed on a PC board withassociated circuitry or in a variety ofremote or networked configurations.In addition to the converter itself,sample-and-hold circuits, an amplifi-er, a multiplexer, timing and synchro-nization circuits, and signal condi-tioning elements also may be onboard (Figure 1-5). The logic circuitsnecessary to control the transfer ofdata to computer memory or to aninternal register also are needed.

When determining what type ofA/D converter should be used in agiven application, performanceshould be closely matched to therequirements of the analog inputtransducer(s) in question. Accuracy,signal frequency content, maximumsignal level, and dynamic range allshould be considered.

Central to the performance of anA/D converter is its resolution, oftenexpressed in bits. An A/D converter

essentially divides the analog inputrange into 2N bins, where N is thenumber of bits. In other words, res-olution is a measure of the numberof levels used to represent the ana-log input range and determines theconverter’s sensitivity to a change inanalog input. This is not to be con-

Analog I/O Functionality1


Figure 1-3: A/D Conversion Compromises

Sampling Issues

Qua

ntiz

atio

n Is

sues

v

t

Figure 1-4: Aliasing Due to Slow Sample Rate

Sample Period

Alias Actual Signal

Signal Period

Figure 1-5: Analog Input Flow Diagram

Analog Input

Signals

1

2

3

4

Signal Conditioning

Digital Output

Mul

tipl

exer

Amplifier

Sample/Hold

A/D Converter

fused with its absolute accuracy!Amplification of the signal, or inputgain, can be used to increase theapparent sensitivity if the signal’sexpected maximum range is less thanthe input range of the A/D convert-er. Because higher resolution A/Dconverters cost more, it is especiallyimportant to not buy more resolu-tion than you need—if you have 1%accurate (1 in 100) temperature trans-ducers, a 16-bit (1 in 65,536) A/D con-verter is probably more resolutionthan you need.

Absolute accuracy of the A/Dconversion is a function of the ref-erence voltage stability (the knownvoltage to which the unknown volt-age is compared) as well as thecomparator performance. Overall,it is of limited use to know theaccuracy of the A/D converteritself. Accuracy of the system,together with associated multiplex-er, amplifier, and other circuitry istypically more meaningful.

The other primary A/D converterperformance parameter that must be

considered is speed—throughput fora multi-channel device. Overall, sys-tem speed depends on the conver-sion time, acquisition time, transfertime, and the number of channelsbeing served by the system:• Acquisition is the time needed by

the front-end analog circuitry toacquire a signal. Also called aperturetime, it is the time for which the con-verter must see the analog voltage inorder to complete a conversion.

• Conversion is the time needed toproduce a digital value correspond-ing to the analog value.• Transfer is the time needed tosend the digital value to the hostcomputer’s memory.Throughput, then, equals the numberof channels being served divided bythe time required to do all threefunctions.

A/D Converter OptionsWhile all analog-to-digital convert-ers are classified by their resolutionor number of bits, how the A/D cir-

cuitry achieves this resolution variesfrom device to device. There are fourprimary types of A/D convertersused for industrial and laboratoryapplications—successive approxima-tion, flash/parallel, integrating, andramp/counting. Some are optimized

for speed, others for economy, andothers for a compromise amongcompeting priorities (Figure 1-6).Industrial and lab data acquisitiontasks typically require 12 to 16 bits—

12 is the most common. As a rule,increasing resolution results in highercosts and slower conversion speed.• Successive approximation: Themost common A/D converter designused for general industrial and labora-tory applications is successive approx-imation (Figure 1-7). This design offersan effective compromise among reso-lution, speed, and cost. In this type ofdesign, an internal digital-to-analog(D/A) converter and a single compara-tor—essentially a circuit that deter-mines which of two voltages is high-er—are used to narrow in on theunknown voltage by turning bits in theD/A converter on until the voltagesmatch to within the least significantbit. Raw sampling speed for successiveapproximation converters is in the 50kHz to 1 MHz range.

To achieve higher sampling speeds,a redundancy technique allows a fastinitial approximate conversion, fol-lowed by a correction step thatadjust the least significant bit afterallowing sufficient settling time. Theconversion is therefore completedfaster at the expense of additionalhardware. Redundancy is usefulwhen both high speed and high reso-lution are desirable.• Flash/parallel: When higher speedoperation is required, parallel, orflash-type A/D conversion is called

Analog I/O Functionality 1


Figure 1-7: A/D Conversion by Successive Approximation

Analog Input

Digital Output

Reference VoltageD/A Converter

Output Register

N bits

Control Register

Comparator

Gate

Successive approximation

Integrating

Ramp/counting

Flash/parallel

DESIGN SPEED RESOLUTION NOISE IMMUNITY COST

Medium

Slow

Slow

Fast

10-16 bits

12-18 bits

14-24 bits

4-8 bits

Poor

Good

Good

None

Low

Low

Medium

High

Figure 1-6: Alternative A/D Converter Designs

for. This design uses multiple com-parators in parallel to process sam-ples at more than 100 MHz with 8 to12-bit resolution. Conversion isaccomplished by a string of com-parators with appropriate referencesoperating in parallel (Figure 1-8).

The downside of this design is thelarge number of relatively expensivecomparators that are required—forexample, a 12-bit converter requires4,095 comparators.• Integrating: This type of A/D con-verter integrates an unknown inputvoltage for a specific period of time,then integrates it back down to zero.This time is compared to the amountof time taken to perform a similarintegration on a known referencevoltage. The relative times requiredand the known reference voltagethen yield the unknown input volt-age. Integrating converters with 12 to18-bit resolution are available, at rawsampling rates of 10-500 kHz.

Because this type of design effec-tively averages the input voltageover time, it also smoothes out sig-

nal noise. And, if an integration peri-od is chosen that is a multiple of theac line frequency, excellent com-mon mode noise rejection isachieved. More accurate and morelinear than successive approxima-tion converters, integrating convert-ers are a good choice for low-levelvoltage signals.• Ramp/counter: Similar to succes-sive approximation designs, countingor ramp-type A/D converters useone comparator circuit and a D/Aconverter (Figure 1-9). This designprogressively increments a digitalcounter and with each new countgenerates the corresponding analog

voltage and compares it to theunknown input voltage. When agree-ment is indicated, the counter con-tains the digital equivalent of theunknown signal.

A variation on the counter methodis the ramp method, which substi-tutes an operational amplifier orother analog ramping circuit for theD/A converter. This technique issomewhat faster.

Multiplexing & Signal ConditioningAs shown in Figure 1-5, A/D convert-ers seldom function on their own butmust be considered in a systems con-text with associated circuitry for sig-nal conditioning, multiplexing, ampli-fication, and other functions. Everyapplication will dictate a unique mixof add-ons that may be implementedin a variety of physical configura-tions—on a PC I/O board, inside aremote transmitter, or at a local ter-mination panel. • Multiplexing: In many industrialand laboratory applications, multipleanalog signals must be converted todigital form. And if speed is not thelimiting factor, a single A/D convert-er often is shared among multipleinput channels via a switching mech-anism called a multiplexer. This iscommonly done because of the rela-tively high cost of converters.Multiplexers also allow amplificationand other signal conditioning circuitry



Figure 1-8: A/D Conversion by Flash/Parallel Technique

Analog Input

Voltage

Digital Output

Vmax

R

1N-1

02

3

2N-1-1

R

R

R

R

Vmin

Comparators

Encoder

-+

-+

-+

-+

Figure 1-9: A/D Conversion by Counting/Ramp Technique

Analog Input

Clock

Digital Output

Reference Voltage

D/A Converter

Counter

Comparator

Gate

to be time-shared among multiplechannels. Software or auxiliary hard-ware controls the switch selection. • Sample-and-hold: It is importantto acknowledge that a multiplexer

does reduce the frequency withwhich data points are acquired, andthat the Nyquist sample-rate criteri-on still must be observed. During atypical data acquisition process, indi-vidual channels are read in turnsequentially. This is called standard, ordistributed, sampling. A reading of allchannels is called a scan. Becauseeach channel is acquired and convert-ed at a slightly different time, howev-er, a skew in sample time is createdbetween data points (Figure 1-10).

If time synchronization amonginputs is important, some dataacquisition cards offer “burst”mode operation or simultaneous“sample-and-hold” circuitry. Burstmode, or pseudo-simultaneoussampling, acquires each channel atthe maximum rate of the board,then waits a user-specified amountof time before sampling again.

True simultaneous sample-and-hold systems can sample all channelswithin a few nanoseconds of eachother, eliminating phase and timediscontinuities for all but the fastestprocesses. Essentially, a switched

capacitor on each channel tracks thecorresponding input signal. Beforestarting the A/D conversion process,all switches are opened simultane-ously, leaving the last instantaneous

values on the capacitors. • Signal scaling: Because A/D con-verters work best on signals in the1-10 V range, low voltage signals mayneed to be amplified before conver-sion—either individually or aftermultiplexing on a shared circuit.Conversely, high voltage signals mayneed to be attenuated.

Amplifiers also can boost an A/Dconverter’s resolution of low-levelsignals. For example, a 12-bit A/Dconverter with a gain of 4 can digi-tize a signal with the same resolutionas a 14-bit converter with a gain of 1.It’s important to note, however, thatfixed-gain amplifiers, which essen-tially multiply all signals proportion-ately, increase sensitivity to low volt-age signals but do not extend theconverter’s dynamic range.

Programmable gain amplifiers(PGAs), on the other hand, can beconfigured to automatically increasethe gain as the signal level drops,effectively increasing the system’sdynamic range. A PGA with threegain levels set three orders of magni-tude apart can make a 12-bit convert-

er behave more like an 18-bit con-verter. This function does, however,slow down the sample rate.

From a systems perspective,amplifier performance should be onpar with that of the A/D converteritself—gain accuracy should be spec-ified as a low percentage of the totalgain. Amplifier noise and offset erroralso should be low.• Other conditioning functions:Other A/D signal conditioning func-tions required will vary widely fromapplication to application. Amongthe options:• Current-to-voltage conversion: A

4-20 mA current signal can bereadily converted to a voltage sig-nal using a simple resistor (Figure1-11). A resistor value of 250 ohmswill yield a 1-5 V output.

• Filtering: A variety of physicaldevices and circuits are availableto help separate desired signalsfrom specific frequencies of unde-sirable electrical noise such as ac

line pick-up and other electro-magnetic/radio frequency inter-ference (EMI/RFI). If the signal ofinterest is lower in frequency thanthe noise, a low-pass filter can beused. High-pass and notch-bandfilters are designed to target lowfrequency interference and specificfrequency bands, respectively.

• Excitation: Voltage supplied by thedata acquisition card or discrete sig-nal conditioner to certain types of



Figure 1-11: Conversion of 4-20 mA to 1-5 V

4-20 mA Current signal

1-5 V Voltage signal

Ground

+

250 Ω

Figure 1-10: Alternative Methods for Eliminating Time Skew Among Multiplexed Channels

Distributed Sampling

1 2 3 4 1

Skew Skew

Time

Conversion Time

1 2 3 4 1 2 3 4 1 2 3 4

No Skew

Simultaneous Sample-and-Hold

Burst Mode

transducers such as strain gages.• Isolation: Used to protect person-

nel and equipment from high volt-ages. Isolators block circuit over-loads while simultaneously pass-ing the signal of interest.

Single-Ended & Differential InputsAnother important considerationwhen specifying analog data acquisi-tion hardware is whether to use sin-gle-ended or differential inputs(Figure 1-12). In short, single-endedinputs are less expensive but can beproblematic if differences in groundpotential exist.

In a single-ended configuration,the signal sources and the input tothe amplifier are referenced toground. This is adequate for highlevel signals when the difference inground potential is relatively small.A difference in ground potentials,however, will create an error-caus-ing current flow through the groundconductor otherwise known as aground loop.

Differential inputs, in contrast,connect both the positive and neg-ative inputs of the amplifier to bothends of the actual signal source.Any ground-loop induced voltageappears in both ends and is rejected

as a common-mode noise. Thedownside of differential connections

is that they are essentially twice asexpensive as single-ended inputs; aneight-channel analog input board canhandle only four differential inputs.

D/A ConversionAnalog outputs commonly are usedto operate valves and motors inindustrial environments and to gen-erate inputs for electronic devices

under test. Digital-to-analog (D/A)conversion is in many ways the con-

verse of A/D conversion, but tendsto be generally more straightforward.Similar to analog input configura-tions, a common D/A converteroften is shared among multiplexedoutput signals. Standard analog out-put ranges are essentially the sameas analog inputs: ±5 V dc, ±10 V dc,0-10 V dc, and 4-20 mA dc.

Essentially, the logic circuitry for



Figure 1-12: Single-Ended & Differential Analog Input Configurations

High

LowJumper

Remote ground

Analog ground

0

1

2

3

4

5

6

7

High

LowJumper

Remote ground

Analog ground

0 Hi

1 Hi

2 Hi

3 Hi

0 Lo

1 Lo

2 Lo

3 Lo

Single-Ended Inputs Differential Inputs

Figure 1-13: Weighted Value & Single Value Resistor Networks for D/A Conversion

2R

2R

2R 2R 2R

R R

R

Io

-+

Voltage Output

DN-1D2D1D∅DN-1

DN-2

D∅

R

Weighted Value Resistor Single Value Resistor

2R R

Voltage Output

2N-1 R-+

an analog voltage output uses a dig-ital word, or series of bits, to dropin (or drop out, depending onwhether the bit is 1 or 0) a series ofresistors from a circuit driven by areference voltage. This ladder ofresistors can be made of eitherweighted value resistors or an R-2Rnetwork using only two resistor val-ues—one if placed in series (Figure1-13). While operation of theweighted-value network is moreintuitively obvious, the R-2Rscheme is more practical. Becauseonly one resistor value need beused, it is easier to match the tem-perature coefficients of an R-2R lad-der than a weighted network,resulting in more accurate outputs.Plus, for high resolution outputs,very high resistor values are neededin the weighted-resistor approach.

Key specifications of an analogoutput include:• Settling time: Period required for a

D/A converter to respond to afull-scale setpoint change.

• Linearity: This refers to thedevice’s ability to accuratelydivide the reference voltage intoevenly sized increments.

• Range: The reference voltage setsthe limit on the output voltageachievable.Because most unconditioned ana-

log outputs are limited to 5 mA of

current, amplifiers and signal condi-tioners often are needed to drive afinal control element. A low-pass fil-ter may also be used to smooth outthe discrete steps in output. T



References & Further Reading• The Data Acquisition Systems Handbook, Omega Press LLC, 1997.• New Horizons in Data Acquisition and Computer Interfaces, Omega Press

LLC, 1997.• Omega® Universal Guide to Data Acquisition and Computer Interfaces,

Omega Press LLC, 1997. • Analog I/O Design: Acquisition, Conversion, Recovery, Patrick Garrett,

Reston Publishing Co., 1981.• Analog Signal Processing and Instrumentation, Arie F. Arbel, Cambridge

University Press, 1980.• Analog-To-Digital and Digital-To-Analog Conversion Techniques, David

Hoeschele, John Wiley & Sons, 1994.• Analog-To-Digital Conversion: A Practical Approach, Kevin M. Daughtery,

McGraw Hill, 1995.• Automation Systems for Control and Data Acquisition, Lawrence T. Amy,

ISA, 1992.• Data Acquisition and Control, Microcomputer Applications for Scientists

and Engineers, Joseph J. Carr, Tab Books Inc., 1988.• Data Acquisition and Process Control Using Personal Computers, Tarik

Ozkul, Marcel Dekker, 1996.• Instrument Engineers’ Handbook, Third Edition, Bela Liptak, Chilton Book

Co., 1995. • Process/Industrial Instruments & Controls Handbook, Fourth Edition,

Douglas M. Considine, McGraw-Hill Inc., 1993.


In contrast to analog transduc-ers that sense continuous vari-ables such as pressure and tem-perature, many transducers pro-

vide an output that is one of twostates: high or low, open or closed.A pressure might be too high or a

temperature too low, triggering clo-sure of a switch. Outputs, too, arenot strictly analog—solenoid valvestypically are opened or closed,many pumps and heaters are simplyturned on or off. Pulse signals areanother form of digital I/O, withone rotation of a turbine flowmeteror tachometer corresponding to asingle, countable event. Digital I/Oalso can be used for parallel com-munications among plug-in expan-sion cards, and to generate clockand other timing signals.

Already in the binary language ofcomputers, these types of digital,or discrete, inputs and outputs(I/O) are much easier for micro-

processor-based data acquisitionsystems to deal with than analogsignals. Similar to analog-to-digitalconverters used for analog I/O, dig-ital I/O is designed to deal directlywith transistor-to-transistor logic(TTL) level voltage changes. TTL

typically sets the low voltage levelbetween 0 and 0.8 V and the highvoltage level between 2.0 and 5.0 V.Voltage levels between 0.8 and 2.0 Vare not allowed. A voltage change,then, from the high range to thelow range (or vice versa) represents

a digital change of state from highto low, on to off, etc.

And because acquiring an analogsignal is more complex than acquir-ing a digital one, analog I/O channelsalso are more expensive. Hence, ifdigital I/O is adequate, do not both-er with analog.

Digital InputsMany types of digital input signalsfrom switch closures, relay contacts,or TTL-compatible interfaces can beread directly by digital I/O cards(Figure 2-1). Other types of inputsmay require some signal condition-ing, most likely to reduce higher-level voltage changes to TTL levels.A variety of signal conditioningmodules are available to provideisolation and other digital condi-tioning functions.

The most common type of digitalinput is the contact closure (Figure2-2). Essentially a sensor or switch ofsome type closes or opens a set ofcontacts in accordance with someprocess change. An applied electricalsignal then determines whether thecircuit is open or closed. Current flowsif the circuit is closed, registering a “1”in a transistor at the computer inter-face. Conversely, an open circuit

Digital Inputs

Digital Outputs

Pulse I/O

DATA ACQUISITIONDigital I/O Functionality

2

IDigital I/O Functionality

Figure 2-1: Signal Processing Requirements for Digital and Analog Signals

Signal conditioning Signal conditioning

Multiplexing

A/D-D/A conversion

Sensors & Actuators

Computer

Digital signals

Analog signals

Digital signals

Figure 2-2: Digital Input Applied to a Contact Closure

Digital InputRemote Switch Remote Contact

R

+5 V

R

retains a high voltage (and no current),registering a “0” at the transistor.

Another type of digital inputuseful in data acquisition applica-tions is the hardware trigger. Thisallows an external event—a highreactor temperature, perhaps, or alow tank level—to control data col-lection. If during routine operationdata is only being acquired forarchival storage on a once-per-sec-ond basis, a hardware trigger can beused to boost the data acquisitionrate during an upset until normalconditions are restored.

Digital OutputsAt its simplest, a digital output pro-vides a means of turning somethingon or off. Applications range fromdriving a relay to turning on an indi-cator lamp to transmitting data toanother computer. For latching out-puts, a “1” typically causes the associ-ated switch or relay to latch, while a“0” causes the switch to unlatch.Devices can be turned on or off,depending on whether the externalcontacts are normally open or nor-mally closed.

Standard TTL level signals can be

used to drive 5-V relay coils; a protec-tive diode is used to protect the digi-tal output circuitry (Figure 2-3).Because data acquisition boards cantypically supply only 24 mA of drivingcurrent, they are intended primarily todrive other logic circuits, not final con-trol elements. Scaling may be neededso that logical voltage levels are suffi-cient to cause switching in largerrelays. Outputs intended to drive larg-er solenoids, contactors, motors, oralarms also may require a boost.

Pulse I/OA somewhat separate class of digitalI/O is pulse inputs and outputs,which typically is associated withfrequency, counting, or totalizationapplications. Pulse inputs might beused to count the rotations of a tur-bine flowmeter; pulse outputs mightbe used to drive a stepping motor.

Pulse inputs are handled in muchthe same way as digital logic inputs,but the output of the sensing circuitis normally connected to a counterrather than a specific bit position inthe input register. Successive pulsesincrement or decrement thecounter. Add an elapsed time mea-sure and a frequency or pulse ratecan readily be determined. Similarto an analog-to-digital converter, acounter is characterized by its num-ber of bits—an N-bit countercan accumulate up to 2N discreteevents. Thus, a 16-bit counter cancount to 216 = 65,536. T

Digital I/O Functionality 2


Figure 2-3: Digital Output Applied to a Relay

Digital Output

Remote Switch

Remote Relay

+5V



Omega Press LLC, 1997. • Automation Systems for Control and Data Acquisition, Lawrence T. Amy,

ISA, 1992.• Data Acquisition and Control, Microcomputer Applications for Scientists

and Engineers, Joseph J. Carr, Tab Books Inc., 1988.• Data Acquisition and Process Control Using Personal Computers, Tarik

Ozkul, Marcel Dekker, 1996.• Instrument Engineers’ Handbook, Third Edition, Bela Liptak, Chilton Book

Co., 1995. • Process/Industrial Instruments & Controls Handbook, Fourth Edition,

Douglas M. Considine, McGraw-Hill Inc., 1993.


Although the microprocessorand digital network tech-nologies have fundamental-ly reinvented the ways in

which today’s data acquisition sys-tems handle data, much laboratoryand manufacturing information isstill communicated the “old” way,via analog electrical signals. And afundamental understanding of howanalog signal transmission worksmust first begin with a discussion ofelectrical basics.

To understand the ways in whichan analog signal is transmitted overa circuit, it is first important tounderstand the relationships thatmake analog signal transmissionpossible. It is the fundamental rela-tionship between voltage, current,and electrical resistance (Figure 3-1)that allow either a continuouslyvarying current or voltage to repre-sent a continuous process variable.

While charge flow is electric cur-rent, voltage is the work done inmoving a unit of charge (1 coulomb)from one point to another. The unitof voltage is often called the poten-tial difference, or the volt (V). TheInternational System of Units (SI)unit for electrical flow is the

ampere (A), defined as one coulombper second (c/s).

A signal source of voltage, V, willcause a current, I, to flow through aresistor of resistance, R. Ohm’s law,which was formulated by theGerman physicist Georg Simon Ohm(1787-1854), defines the relation:

V = IR

While most single-channel ana-log signal transmissions use directcurrent (dc) variations in current orvoltage to represent a data value,frequency variations of an alternat-ing current (ac) also can be used tocommunicate information. In theearly 19th century, Jean BaptisteJoseph Fourier, a French mathe-matician and physicist, discoveredthat ac signals could be defined interms of sine waves. A sine wave isdescribed by three quantities:amplitude, period, and frequency.The amplitude is the peak value ofthe wave in either the positive ornegative direction, the period isthe time it takes to complete onecycle of the wave, and the frequen-cy is the number of complete

cycles per unit of time (the recip-rocal of the period).

Analog Signal TypesMost data acquisition signals can bedescribed as analog, digital, or pulse.While analog signals typically varysmoothly and continuously overtime, digital signals are present at dis-crete points in time (Figure 3-2). Inmost control applications, analog sig-nals range continuously over a speci-fied current or voltage range, such as4-20 mA dc or 0 to 5 V dc. While dig-ital signals are essentially on/off (thepump is on or off, the bottle is thereor isn’t), analog signals represent con-tinuously variable entities such astemperatures, pressures, or flowrates. Because computer-based con-trollers and systems understand onlydiscrete on/off information, conver-sion of analog signals to digital repre-sentations is necessary (and dis-cussed in Chapter 1).

Transduction is the process ofchanging energy from one form intoanother. Hence, a transducer is adevice that converts physical ener-gy into an electrical voltage or cur-rent signal for transmission. There

AAnalog Signal Transmission

Figure 3-1: A Basic Electric Circuit

V

I

+

-

R

5

0

Digital Analog

VoltsVolts

Time Time

10

0

-10

Figure 3-2: Digital and Analog Signal Representations

Analog Signal Types

Noise & Grounding

Wire & Cable Options

DATA ACQUISITIONAnalog Signal Transmission

3

Analog Signal Transmission 3


are many different forms of analogelectrical transducers. Commontransducers include load cells formeasuring strain via resistance, andthermocouples and resistance tem-perature detectors (RTDs) for mea-suring temperature via voltage andresistance measurement, respec-tively. Transmission channels may bewires or coaxial cables.

For noise-resistant transmissionover significant distances, the rawtransducer signal is often convertedto a 4-20 mA signal by a two-wire,loop-powered transmitter. The bot-tom value of a process variable’srange, for example, a temperature, istypically designated as 4 mA, makingit easy to distinguish transmitterfailure (0 mA) from a valid signal. Ifthe current source is of good quality,current loops tend to be less sensi-tive to noise pickup by electromag-netic interference than voltage-based signals.

Noise & GroundingIn transmitting analog signals acrossa process plant or factory floor, one

of the most critical requirements isthe protection of data integrity.However, when a data acquisitionsystem is transmitting low level ana-log signals over wires, some signaldegradation is unavoidable and willoccur due to noise and electricalinterference. Noise and signal degra-dation are two basic problems inanalog signal transmission.

Noise is defined as any unwantedelectrical or magnetic phenomenathat corrupt a message signal. Noisecan be categorized into two broadcategories based on the source—internal noise and external noise.While internal noise is generated bycomponents associated with the sig-nal itself, external noise results whennatural or man-made electrical ormagnetic phenomena influence thesignal as it is being transmitted.Noise limits the ability to correctlyidentify the sent message and there-fore limits information transfer.Some of the sources of internal andexternal noise include:• Electromagnetic interference (EMI);• Radio-frequency interference (RFI);

• Leakage paths at the input terminals;• Turbulent signals from other

instruments;• Electrical charge pickup from

power sources;• Switching of high-current loads in

nearby wiring;• Self-heating due to resistance

changes;• Arcs;• Lightning bolts;• Electrical motors;• High-frequency transients and

pulses passing into the equipment;• Improper wiring and installation;• Signal conversion error; and • Uncontrollable process disturbances.

Signal leads can pick up two typesof external noise—common modeand normal mode. Normal modenoise enters the signal path as a dif-ferential voltage and cannot be dis-tinguished from the transducer sig-nal. Noise picked up on both leadsfrom ground is referred to as com-mon-mode interference.

Typical ranges for data signals andnoise are shown in Figure 3-3.Whether the noise is detrimental tothe proper performance of the sys-tem depends on the ratio of the total

0 10-4 10-3 10-2 10-1 1 10 102 107

Frequency (Hz)

Figure 3-3: Signal, Noise, and Filtering Frequencies

Controllable Disturbances

Useful Information

Analog & Digital Filtering

Noise

All-Analog Filtering

Uncontrollable Disturbances

Stray Electrical Pickup

Analog Filter Rejection

Analog Filter Rejection

Control Range

Input Range

Control Range

Digital Filter Rejection

Measurement Noise

Figure 3-4: A Ground Conductor

UtilityAC LineReturn

Ground

Earth ground

Outlet (front view)

Connection between ground and return



signal power to the total noise level.This is referred to as the signal-to-noise ratio. If the signal power islarge in comparison to the noise sig-nal, the noise can often times beignored. However, with long-distancesignals operating with limited signalpower, the noise may disrupt the sig-nal completely.

Current-driven devices have beenmost widely accepted in processingplants, with a common current rangeof 4-20 mA. Low-level current signalsare not only safe, but are not as sus-ceptible to noise as voltage signals. Ifa current is magnetically coupledinto the connecting wires in thetransmission of the signal from a cur-rent source, no significant change inthe signal current will result. If the

transducer is a voltage-driven device,the error adds directly to the signal.After current transmission, voltagesignals can be easily rederived.

Even though internal and externalinterference can be problematic insending analog signals, analog signaltransmission is widely and success-fully used in industry. The effects ofnoise can be reduced with carefulengineering design, proper installa-tion, routing techniques of wires andcables, and shielding and grounding.

One of the ways in which engineershave tried to minimize the effects ofnoise is to maximize the signal-to-noise ratio. This involves increasingthe power of the signal being sent.Although this works in some cases, ithas its limitations. By increasing the

signal, nonlinear effects becomedominant, as the signal amplitude isincreased—it enhances the signal andthe noise in the same proportion.

Proper grounding also is essentialfor effective operation of any mea-surement system. Improper ground-ing can lead to potentially dangerousground loops and susceptibility tointerference. To understand the prin-ciples involved in shielding andgrounding, some terms must first beunderstood. A ground is a conduct-ing flow path for current between anelectric circuit and the earth. Groundwires are typically made with materi-als that have very low resistance.Because current takes the path ofleast resistance, the ground wiresconnected from the system provide

Figure 3-5: Incorrect Grounding of Signal Circuit

Signal Source

Signal Ground

Signal Ground Returnee

Zw

Ground current flows in return line

System Ground

+

+-

-

V1

Figure 3-6: Correct Grounding of Signal Circuit

Signal Source

Signal Ground

Signal Return Only

(No path for current to circulate)

System Ground

+

-

V1

Analog Signal Transmission 3


a stable reference for making voltagemeasurements. Ground wires alsosafeguard against unwanted com-mon-mode signals and prevent acci-dental contact with dangerous volt-ages. Return lines carry power or sig-nal currents (Figure 3-4). A groundloop is a potentially dangerous loopformed when two or more points inan electrical system are grounded todifferent potentials.

There are many different ground-ing techniques designed to not onlyprotect the data being transmitted,but to protect employees andequipment. There are two ways inwhich all systems should be ground-ed. First of all, all of the measuringequipment and recording systemsshould be grounded so that mea-surements can be taken with respectto a zero voltage potential. This notonly ensures that potential is notbeing introduced at the measuringdevice, but ensures that enclosuresor cabinets around equipment donot carry a voltage. To ground anenclosure or cabinet, one or moreheavy copper conductors are runfrom the device to a stable groundrod or a designated ground grid. Thissystem ground provides a base forrejecting common-mode noise sig-nals. It is very important that thisground is kept stable.

The second ground is for the signalground. This ground is necessary toprovide a solid reference for themeasurement of all low-level signals.It is very important that this groundis grounded separate and isolatedfrom the system ground. If a signalreturn line is grounded at the signalsource and at the system ground, adifference in potential between thetwo grounds may cause a circulatingcurrent (Figure 3-5). In this case, thecirculating current will be in series

with the signal leads and will adddirectly to the signal from the mea-suring instrument. These groundloops are capable of creating noisesignals 100 times the size of the orig-inal signal. This current can also bepotentially dangerous. In a single-point ground configuration, mini-mal current can flow in the groundreference. Figure 3-6 shows that bygrounding the wire at the signal endonly, the current has no path, elimi-nating the ground loop.

For off-ground measurements, theshield or the ground lead is stabilizedwith respect to either the low-levelof the signal or at a point betweenthe two. Because the shield is at apotential above the zero-referenceground, it is necessary to have prop-er insulation.

Wire & Cable OptionsAnother important aspect to consid-er in analog signal transmission is aproper wiring system, which caneffectively reduce noise interference.Analog signal transmission typicallyconsists of two-wire signal leads orthree-wire signal leads. In systems

that require high precision and accu-racy, the third signal lead, or shield, isnecessary. In the three-wire configu-ration, the shield is grounded at thesignal source to reduce common-mode noise. However, this does noteliminate all of the possibilities ofthe introduction of noise. It is crucialto prevent the noise pickup by pro-tecting the signal lines. For example,in the case where the noise and sig-nal frequency are the same. In thisscenario, the signal cannot be isolat-

ed/filtered from the noise at thereceiving device.

Generally, two-wire transmissionmediums are used to carry an analogsignal to or from the field area. A wirecarrying an alternating current andvoltage may induce noise in a pair ofnearby signal leads. A differential volt-age/noise will be created since thetwo wires may be at different distancesfrom the disturbing signal. There aremany different wiring options that areavailable to reduce unwanted noisepickup from entering the line. Fourtypes of wires are fundamental in dataacquisition—plain pair, shielded pair,twisted pair, and coaxial cable.

Figure 3-7: Coaxial Cable Construction

Magnetic field

Wire (+)

Electric field

Cylindrical conductor (-)



While plain wire can be used, it isgenerally not very reliable in screen-ing out noise and is not suggested. Ashielded pair is a pair of wires sur-rounded by a conductor that doesnot carry current. The shield blocksthe interfering current and directs itto the ground. When using shieldedpair, it is very important to followthe rules in grounding. Again, theshield must only be grounded at onesource, eliminating the possibility ofground-loop currents.

Twisted-pairs help in elimination ofnoise due to electromagnetic fields bytwisting the two signal leads at regularintervals. Any induced disturbance inthe wire will have the same magnitudeand result in error cancellation.

A coaxial cable is another alterna-tive for protecting data from noise. Acoaxial cable consists of a centralconducting wire separated from anouter conducting cylinder by an insu-lator. The central conductor is positivewith respect to the outer conductorand carries a current (Figure 3-7).Coaxial cables do not produce exter-nal electric and magnetic fields andare not affected by them. This makesthem ideally suited, although moreexpensive, for transmitting signals.

Although noise and interference

cannot be completely removed inthe transmission of an analog signal,with good engineering and proper

installation, many of the effects ofnoise and interference can be sub-stantially reduced. T

References & Further Reading• Analog Signal Processing and Instrumentation, Arie F. Arbel, Cambridge

University Press, 1980.• Basic Circuit Analysis, David R. Cunningham, and John A. Stuller,

Houghton Mifflin Co., 1932.• Circuits: Principles, Analysis, and Simulation, Frank P. Yatsko, and David M.

Hata, Saunders College Publishing, 1992.• Data Acquisition and Control, Microcomputer Applications for Scientists

and Engineers, Joseph J. Carr, Tab Books Inc., 1988.• Data Communications, A Beginner’s Guide to Concepts and Technology,

Scott A. Helmers, Prentice-Hall, Inc., 1989.• Digital and Analog Communication Systems, K. Sam Shanmugam, John

Wiley & Sons, 1979.• Fundamentals of Transducers, Stan Gibilisco and R. H. Warring, Tab Books

Inc., 1985.• Instrument Engineers’ Handbook, Third Edition, Bela Liptak, Chilton Book

Co., 1995. • Introduction to Signal Transmission, Electrical and Electronic Engineering

Series, William R. Bennett, McGraw-Hill, 1970.• Microprocessors in Industrial Measurement and Control, Marvin D. Weiss,

Tab Books Inc., 1987.• Modern Digital and Analog Communication Systems, B.P. Lathi, Holt,

Rinehart, & Winston, 1983.• Process/Industrial Instruments & Controls Handbook, Fourth Edition,

Douglas M. Considine, McGraw-Hill Inc., 1993.• Signals & Systems Made Ridiculously Simple, Zohey Z. Karu, Zi Zi Press,

1995.• Signals, The Telephone and Beyond, John R. Pierce, W. H. Freeman and Co., 1981.

Industrial networks that transmitdata using digital signals often arean integral part of a data acquisi-tion or process control solution.

A basic understanding of the networktechnologies that are available for var-ious applications is required to makethe best implementation decisions—decisions that can have a profoundeffect on the ability to adapt to ever-changing technologies.

For example, the type of net-work(s) or network products select-ed for a data acquisition applicationcan greatly affect cost/benefit eval-uations for future projects. Untilrecently, fiber optics were an expen-sive option that seemed like overkillfor most applications. But with theinformation load now likely to flowbetween nodes on an enterprise net-work, fiber looks attractive.

Network technologies come in abewildering array of options, andcertain segments of this technologyare changing at an incredible rate.The average user might spend weeksresearching the various ways to buildor improve a network, just to findthat once a decision is made andproducts are purchased, the nextwave of bigger, better, faster technol-ogy is now available.

A well-designed, integrated solu-tion to data transmission will yield acompetitive advantage to any indus-trial enterprise. Users in all aspects ofa business should be able to obtainplant and business data from anyphysical node, local or remote.“Gluing” pieces of new and/or exist-ing networks together is becomingmore feasible with the use of bridg-ing, routing, and media conversion

technologies that link local area,wide area, and industrial networkstogether. And with Internet and wire-less technologies, data transmissionover large geographic areas isincreasingly feasible.

The OSI Network ModelNearly all digital network descrip-tions start with the OSI (OpenSystems Interconnect) model (Figure4-1). It explains the various “layers” ofnetwork technology. To the casualuser, this model is a little abstract,but there is not a better way to beginto understand what is going on.

Sometimes it helps to understandeach layer by examining the technol-ogy it represents. The ApplicationLayer is the most intuitive because itis what the user sees. It represents

the problem the user wants the sys-tem to solve. Internet browsers ande-mail programs are good examples.They allow the user to input and readdata while connected between aclient PC and a server somewhere on

the Internet. In an industrial applica-tion, a program on a programmablelogic controller (PLC) might control asmart valve.

The Presentation Layer performsformatting on the data going to andfrom the application. This layer per-forms such services such as encryp-tion, compression, and conversion ofdata from one form to another. Forexample, an application (one layerup) might send a time stamp format-ted in 12-hour time: 01:30:48 p.m. Amore universal representation is13:30:48, in 24-hour time, which canbe accepted or presented at the nextnode’s application in the form that itneeds. One advantage to presenta-tion services is that they help elimi-nate overhead, or embedded ser-vices, in application programs.

The Session Layer establishes theconnection between applications. Italso enforces dialogue rules, whichspecify the order and speed of datatransfer between a sender and areceiver. For example, the session


Physical Layer Options

Network Topologies

Fieldbus & Device Networks

DATA ACQUISITIONDigital Signal Transmission

4

IDigital Signal Transmission

Figure 4-1: The Seven-Layer OSI Network Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

User Programs

Data Formatting

Application Dialogue

Addressing

Routing

Bridging

Media

Application

Presentation

Session

Transport

Network

Data Link

Physical

layer would control the flow of databetween an application and a print-er with a fixed buffer, to avoid bufferoverflows. In the time stamp exam-ple, once the data is presented in24-hour time format, an identifierand length indicator are appendedto the data string.

The Transport Layer is essentiallyan interface between the processorand the outside world. It generatesaddresses for session entities andensures all blocks or packets of datahave been sent or received. In thetime stamp example, an address foreach session entity (sender andreceiver) and checksum are append-ed to the block generated by thesession layer.

The Network Layer performsaccounting, addressing, and routingfunctions on messages receivedfrom the transport layer. If the mes-sage is lengthy, this layer will breakit up and sequence it over the net-work. This layer also uses a networkrouting table to find the next nodeon the way to the destinationaddress. In the time stamp example,a node address and sequence num-ber are appended to the messagereceived from the session layer.

The Data Link Layer establishesand controls the physical path ofcommunication from one node tothe next, with error detection. Thislayer performs media access control(MAC) to decide which node can usethe media and when. The rules usedto perform these functions also areknown as protocols. Ethernet andtoken ring contention are examplesof protocols. In the time stampexample, a header and trailer areappended to the message receivedfrom the transport layer for flaggingthe beginning and end of the frame,type of frame (control or data),

checksum, and other functions.The Physical Layer is perhaps the

most conspicuous layer from a costpoint of view. It is relatively easy tounderstand the labor and material

costs of pulling cables, along with aphysical infrastructure (conduits,raceways, and duct banks) for main-taining cable integrity. This layer doesnot add anything to the messageframe. It simply converts the digitalmessage received from the data linklayer into a string of ones and zeroesrepresented by a signal on the media.One example is RS-485, where a bina-ry 1 is represented by a Mark, or Offstate, and a binary 0 is represented bya Space, or On state. A Mark is a neg-ative voltage between terminals onthe generator, while a Space is a pos-itive voltage on those terminals.

Physical Layer OptionsThere are a number of implementa-tions of the physical layer. Networkdevices allow a wide range of connec-tivity options. Some networks arewell defined using the OSI model,where cables, bridges, routers, servers,modems, and PCs are easily identified.Sometimes only a few devices arelinked together in some kind of pro-

prietary network, or where the net-work services are bundled in a black-box fashion with the device.

The most common serial dataexchange interfaces are RS-232,

RS-422, and RS-485 for connectingtwo or more devices together. Allthree interfaces use data terminalequipment (DTE) and data communi-cation equipment (DCE) terminology(Figure 4-2). The DTE is the compo-nent that wants to communicate withanother component somewhere else,such as a PC communicating withanother PC. The DCE is the compo-nent actually doing the communicat-ing, or, performing the functions ofthe generator and receiver discussedin the standards. A modem is a com-mon example of a DCE.

The interfaces between DTE andDCE can be categorized by mechan-ical, electrical, functional, and pro-cedural aspects. Mechanical specifi-cations define types of connectorsand numbers of pins. Electricalspecifications define line voltagesand waveforms, as well as failuremodes and effects. Functional spec-ifications include timing, data, con-trol and signal grounds, and whichpin(s) the functions are to use. The



Figure 4-2: Serial Data Transmission

DCE Data Communications Equipment

Modem Modem

DTE Data Terminal

Equipment

DTE Data Terminal

Equipment

procedural interface specifies howsignals are exchanged.

RS-485 is another serial datatransmission method. Officially, it isEIA 485, or “Standard for ElectricalCharacteristics of Generators andReceivers for Use in BalancedDigital Multipoint Systems” by theElectronics Industry Association(EIA). This standard defines a methodfor generating ones and zeroes asvoltage pulses. Remember, for all thedata handling, framing, packeting,routing and addressing performed bythe upper layers, it still comes downto pushing ones and zeroes oversome physical media.

What is important to know aboutRS-485 is that it allows multiplereceivers and generators, and it speci-fies cable characteristics in terms ofsignaling speeds and lengths. A typicalcable is a shielded twisted copper pair,which is adequate for the typical sig-

naling rate of 10 million bits per sec-ond (Mbps). This standard only definesthe electrical characteristics of thewaveforms. Note that RS-485 does notspecify any media control functions—that is strictly up to the device con-nected to the generator (usually achip). RS-485 is generally good forcable lengths up to 2,000 feet.

One example of a simple serialnetwork might be a series ofrecorders connected over an RS-485link to a PC that receives dataacquired by each recorder. The man-ufacturer sells a plug-in card thatinstalls in each recorder, with wiringinstructions. Each network card isdaisy-chained to the others over aseries of shielded twisted pair cablesthat ultimately terminate on a net-work interface card in the PC. Thereis no real need to know and under-stand the network layers in thisarrangement, except to understand

the limitations on RS-485 (distance,shielding, data rate, etc.).

By title, the RS-422 standard is TIA/EIA 422 B, “Electrical Characteristicsof Balanced Voltage Digital InterfaceCircuits” by the TelecommunicationsIndustry Association (in associationwith the EIA). It is similar to RS-485;the main differences being the risetimes and voltage characteristics ofthe waveform. RS-422 generallyallows cable lengths up to 1.2 kilome-ters at up to 100 thousand bits per sec-ond (kbps). At 10 million bps (Mbps),cable lengths are limited to around 10meters (Figure 4-3). In the presence ofcable imbalance or high commonmode noise levels, cable lengths maybe further reduced in order to main-tain a desired signaling rate.

RS-232C is perhaps the most com-mon form of serial data exchange. Itis officially known as EIA/TIA 232 E,“Interface Between Data TerminalEquipment and Data Circuit-Terminating Equipment EmployingBinary Data Interchange,” again byTIA in association with the EIA. The“E” suffix denotes a later version thanthe common “C” version. Whatmakes this standard different fromRS-422 and RS-485 is that it definesthe mechanical as well as the electri-cal interfaces.

RS-232 is good for signal rates upto 20 kbps, at distances up to 50feet. A zero (space) and a one (mark)are measured in terms of a voltagedifference from signal common (+3 Vdc = 0, -3 V dc = 1). The most com-mon mechanical interfaces are theD-sub 9 and D-sub 25 connectors.

Interchange circuits (pins) in RS-232devices fall into four categories: signalcommon, data circuits (transmitteddata, received data), control circuits(i.e., request to send, clear to send, DCEready, DTE ready), and timing circuits.

Digital Signal Transmission 4


Figure 4-3: Cable Length vs. RS-422 Data Rate

RS-422 Data Signaling Rate (bit/s)

10k 100k 1M 10M

Cabl

e Le

ngth

(met

ers)

10k

1.2k1k

100

10

The standards described above allare used in serial communicationsschemes designed for longer dis-tances. There is one common parallelinterface, known as the GeneralPurpose Interface Bus (GPIB), or IEEE-488. Up to 15 devices can be intercon-nected, usually personal computersand scientific equipment or instru-ments. It provides a high data signalingrate, up to 1 Mbps, but it is limited inlength. The total bus length permittedis 20 meters, with no more than 4meters between devices.

The IEEE-488 bus is a multi-drop,parallel interface with 24 linesaccessed by all devices. The lines aregrouped into data lines, handshakelines, bus management lines, andground lines. Communication is digital,and messages are sent one byte at atime. The connector is a 24-pin con-nector; devices on the bus usefemale receptacles while intercon-necting cables have mating maleplugs. A typical cable will have maleand female connectors to allowdaisy-chaining between devices.

An example of an IEEE-488 imple-mentation is a measurement systemdesigned to assess the performanceof a chemistry sample sink. The sinkperforms sample conditioning (pres-sure, flow, and temperature control)and chemical analysis (pH, dissolvedoxygen, and conductivity) on watersamples. The sink is instrumentedwith pressure sensors, resistance tem-perature detectors (RTDs), thermo-couples, and reference junctions. A30-point scanner is used to multiplexdata from all of the sensors. The scan-ner is connected to a desktop or lap-top PC using the GPIB interface. Datais acquired, stored, displayed, andreduced using application programson the PC, efficiently and reliablyunder IEEE-488.

The media used to implement thephysical layer is usually a set of cop-per wires. Unshielded twisted pair(UTP) cable is the most affordable. It islightweight, easy to pull, easy to ter-minate, and uses less cable tray spacethan shielded twisted pair (STP).However, it is more susceptible toelectromagnetic interference (EMI).

STP is heavier and more difficultto manufacture, but it can greatlyimprove the signaling rate in a giventransmission scheme (Figure 4-4.).

Twisting provides cancellation ofmagnetically induced fields andcurrents on a pair of conductors.Magnetic fields arise around otherheavy current-carrying conductorsand around large electric motors.Various grades of copper cables areavailable, with Grade 5 being thebest and most expensive. Grade 5copper, appropriate for use in 100-Mbps applications, has more twistsper inch than lower grades. Moretwists per inch means more linearfeet of copper wire used to makeup a cable run, and more coppermeans more money.

Shielding provides a means toreflect or absorb electric fields thatare present around cables. Shieldingcomes in a variety of forms fromcopper braiding or copper meshes toaluminized mylar tape wrappedaround each conductor and againaround the twisted pair.

Fiber optics are being used moreoften as user applications demandhigher and higher bandwidths. Theterm “bandwidth” technically meansthe difference between the highestand lowest frequencies of a transmis-sion channel, in hertz (Hz). Morecommonly, it means the capacity oramount of data that can be sentthrough a given circuit.

A bandwidth of 100 Mbps is stan-dard using fiber optic cables. Whenfirst introduced, fiber was considered

only for special applications becauseit was expensive and difficult to workwith. In recent years, the quest forgreater bandwidth combined witheasier-to-use fiber have made itmore common. Tools and training forinstalling and troubleshooting fiberare readily available.

There are three basic fiber opticcables available: multimode stepindex, multimode graded index, andsingle mode. Multimode fibers usu-ally are driven by LEDs at each endof the cable, while single modefibers usually are driven by lasers.Single mode fibers can achievemuch higher bandwidths than multi-mode fibers, but are thinner(10 microns) and physically weakerthan multimode. Equipment costsfor transmitting and receiving singlemode fiber signals are much higher(at least four times) than for multi-mode signals.



Figure 4-4: Shielded Twisted Pair (STP) Cable Construction

Twisting Attenuates Magnetic Fields Shield Attenuates Electrical Fields

+

-

-

+

One distinct advantage of fiberoptic cables is noise immunity. Fiberoptic cables can be routed indis-criminately through high noise areaswith impunity, although fire ratingsshould be observed. Cables thatpass through multiple spaces in aplant should be rated for heating/ventilation/air conditioning (HVAC)plenums where they can withstand

fires per National Fire ProtectionAssociation (NFPA) requirements.

Network TopologiesThe term topology refers to themethod used to connect compo-nents on a network. The most com-mon topologies are ring, bus, and startopologies (Figures 4-5, 4-6, and 4-7),but they can take on the appearancesof each other and still maintain theircharacteristics. For example, a tokenring network segment can be wired ina star configuration, where compo-nents are cabled back to a hub wherethe ring is “inside” the hub. Thisallows a common wiring closet for agiven building or area with home-run cable pulls for each component.A hub offers the advantages of cen-

tralized maintenance and configura-tion control.

• Token Buses & RingsThe MAC functions of token ringsand buses are similar. ARCnet, devel-oped by the Datapoint Corp. in the’70s, is a token passing protocol thatcan be implemented in a bus or startopology using coaxial or UTP cables.

A “token” is passed around the bus orring. Whichever node has the tokenis allowed to communicate on themedia. ARCnet runs at 2.5 Mbps, atthe following lengths:• 400 feet with 10 nodes using UTP;• 2,000 feet with a practical limit of

up to 100 nodes using RG-62 coax-ial cable in a coaxial star configu-ration (using a hub or hubs); and

• 1,000 feet with 10 nodes per 1,000foot segment using RG-62 coaxialcable.ARCnet uses active and passive

hubs in the star configuration, withnetwork cards on the devices thathave switches for setting node num-bers. The lowest numbered node isthe master controller, giving permis-sion to communicate to each node

by number. ARCnet is available in a20-Mbps version.

The IBM token ring protocol, stan-dardized via IEEE-802.5, runs at 4 or 16Mbps. Nodes on the ring connect to amulti-station access unit (MAU), atype of hub. MAUs can be connectedtogether in a main ring, with seg-ments, or lobes, from each MAU con-nected in a star configuration todevices with network interface cards.The length of the ring is limited to 770meters and the maximum number ofnodes allowed on a ring is 260 usingSTP cable. STP cable (150 ohm) is usedmost often, but UTP (100 ohm) cablecan be used if passive filtering is pro-vided for speeds up to 16 Mbps.Bridges can be used to connect rings.

Jitter is an interesting problem thatcan arise on token ring networks,where nodes that are supposed to besynchronized with the master nodereceive distorted waveforms due tocable attenuation. The result is thateach node operates at a slightly dif-ferent speed. Jitter restricts the num-ber of nodes allowed on the ring (72at 16 Mbps on UTP). Jitter suppressorsare available that can help alleviatethis problem.

Repeaters are available for extend-ing the ring. Using phase-locked-loop(PLL) technology, a repeater canextend the main ring an additional800 feet at 16 Mbps on Category 5copper UTP.

Using a media converter, or fiberoptic transceiver, conversion betweencopper and single mode or multi-mode fiber is possible, thus extendingmain ring lengths or lobe lengths upto 1.25 miles.

• Ethernet, or CSMA/CDThe most common bus topology inbusiness applications is Ethernet.Ethernet originally was developed by



Figure 4-5: Ring Network Topology

the Xerox Corp., and later improved inassociation with Digital EquipmentCorp. (DEC) and Intel. IEEE-802.3, basedon the Ethernet standard, specifies aCarrier Sense Multiple Access withCollision Detection (CSMA/CD)media access control protocol. Thisnetwork standard provides func-tionality at the first two OSI net-work layers—the physical and datalink layers.

This MAC protocol allows indepen-dent transmission by all nodes on anetwork segment. A node sends mes-sages down the segment with data,addressing, and control bits. All othernodes “see” the message, but only thenode with the destination address willacknowledge and receive the mes-sage. When the sending node is usingthe segment (transmitting), all othernodes with messages to send “hear”the carrier and do not send. This typeof protocol is known as “listen beforesend.” If two nodes attempt to send atthe same time, a collision is detected,and both nodes stop sending, wait asmall, random amount of time, andresend. Where there is a lot of net-

work traffic, many collisions can occurand overload the system.

The Ethernet CSMA/CD standardcan be implemented in five differentways:• 10Base5—standard “thick” coaxial

cable;• 10Base2—“thin” coaxial cable;• 10BaseT—UTP;• 100BaseT—UTP; and• 10BaseFL or FOIRL—fiber optics.

The XBaseY nomenclature isinterpreted as follows: X = the sig-

naling rate in Mbps, and Y = someindication of the media type. The“Base” portion of the term meansthe signal uses the cable in a base-band scheme (as opposed to abroadband, multi-frequency, multi-channel modulating scheme).

10Base5 is a standard Ethernetimplementation that uses RG-58coaxial cable segments up to 500meters long. Transceivers areclamped on to the segment, with aminimum distance between trans-ceivers of 2.5 meters. A transceivercable (or spur) connects the trans-ceiver to the network node ordevice; this cable can be up to 50meters in length. The maximumnumber of transceivers allowed is100 per segment, and both ends ofeach segment are terminated with a50-ohm resistor. Transceivers can beconnected to hubs for fanning outto network nodes.

The 10Base2 “ThinNet” does not usea long coaxial segment with trans-ceivers. Instead, modular repeaters areused with ThinNet modules in a starconfiguration. Thin coaxial cable isterminated with “T” connectors onnetwork nodes, with multiple nodesconnected in series on a segment,



Figure 4-6: Bus Network Topology

Figure 4-7: Star Network Topology

Hub

with one end of the cable home-runback to the repeater. The maximumlength of a series segment is 185meters, with a minimum distancebetween nodes (T-connectors) of 0.5meters. No more than 30 nodes areallowed on a segment. 50-ohm ter-minators are required at each end ofa segment.

The 10BaseT implementation usesUTP cable, with best results fromCategory 5 copper. Nodes are con-nected to a hub in a star configura-tion. The maximum length of onesegment is 100 meters.

The 10BaseFL implementation uses50, 62.5, or 100-micron multimode fiberoptic cable. Fiber transceivers are avail-able for connecting to network inter-face cards that do not support fiber.

Repeaters can be used to connect10Base5 segments together up to2,500 meters, and components andsmaller networks can be linkedtogether with interface devices tojoin the various media types into alarger network.

The IEEE-802.3 standard coversseveral CSMA/CD implementations,

with lengths up to 925, 2,500, and3,600 meters. The number of nodesallowed follows the hardware andperformance requirements of thenetwork operating system. Morenodes and higher traffic mean morepotential for collisions. Network per-formance might drop when you needit the most, such as during a planttransient when data flow and controlelement actuations are high.

Ethernet or CSMA/CD is estab-lished for 1, 10 and 100 Mbps, with 10Mbps currently being the most com-

mon. Some vendors are offering “fast”Ethernet at 100 Mbps, compatiblewith existing 10 Mbps implementa-tions. 100BaseT is essentially 10BaseT,only 10 times faster. They both use thesame MAC layer. At higher speeds,implementations should account forround-trip delays for collisions.

Finally, a wireless Ethernet imple-mentation is available. It supportsdata rates up to 3 Mbps throughopen spaces up to 3,000 feet andindoors from 200 to 600 feet, andline of site is not required indoors.Wireless repeaters and bridges areavailable to link cells together.

Decisions regarding local area net-work (LAN) topologies and MAC pro-tocols can affect the way a network ismanaged for years to come. A newfacility can be wired with state-of-the-art network components andpremise wiring devices, but the net-work administrator or plant engineeris usually faced with an existing infra-structure, where options are con-strained by installed media. For exam-ple, an old, abandoned 4-20 mA dccurrent loop STP cable can always beused to implement 10BaseT Ethernetand save considerable project funds.Or, a new fiber optic cable can be pur-chased with multimode fibers for this



Figure 4-8: A Remote File Request According to OSI

User Application

Client File Transfer Protocol

Transmission Control Protocol

Internet Protocol

Logical Link Control

Media Access Control

NODE ONE

MEDIA

File Server

Server File Transfer Protocol

Transmission Control Protocol

Internet Protocol

Logical Link Control

Media Access Control

NODE TWO

Figure 4-9: Dual-Ring Redundant FDDI Network

Intact Broken

year’s low bandwidth project and stillhave spare, single-mode fibers forfuture, high bandwidth applications.

Token passing and CSMA/CD pro-tocols make interesting comparisons.Token passing offers a predictableaccess window with consistent per-formance on large, busy networks,but nodes need to wait for the tokenand large rings can result in signifi-cant delays. CSMA/CD allows a nodeto transmit immediately when thenetwork is quiet, but performancecan be unpredictable and dependson the probability of collisions.

Upgradeability and growth are twoareas that should be considered foreach new segment or modification toa given segment. The challenge is tokeep an eye on network demands asnew applications come along, andselect the right amount of bandwidthand upgradeability for the dollar.

• Moving Up the LayersSo far, the physical layer has beendescribed, with implied functions atthe data link and network layers. Twodevices can communicate by way ofa simple RS-485 link or they can bepart of a larger LAN with complexmedia access controls.

The data link’s only function is topass data from one node to the next.The data link layer can connect twonodes or devices with a “bridge.” Earlybridges only connected two segmentsof a network with the same protocol(such as CSMA/CD or token passing).Smart bridges are available now thatcan connect differing protocols, withselective forwarding of data packets.Various levels of data link perfor-mance exist depending on the size ofthe network (wide vs. local area net-work; WAN vs. LAN) and the proto-col/media selections.

The network layer routes incoming

data for another node onto anappropriate outbound path. Thephysical device that performs thisfunction is naturally called a router.Since bits still need to be pusheddown some physical media, a routerinherently performs the physical anddata link functions in order to go upand down the OSI model, althoughrouters are not sensitive to the datalink and physical layer details, allow-

ing them to connect different typesof networks. A router takes an incom-ing packet, looks at the destinationaddress, determines the best route,and provides the required addressing.One common implementation of thisfunction is Transmission ControlProtocol/Internet Protocol (TCP/IP).On Novell networks, this function iscalled Sequenced Packet Exchange/Internetwork Packet Exchange(SPX/IPX). Novell, Inc. is perhaps the

largest and most popular provider ofnetwork operating system (NOS) soft-ware. Novell provides CSMA/CD andtoken passing options.

The TCP and SPX protocols oper-ate at the transport layer, and the IPand IPX layers operate at the networklayer. Thus, routers send and receivepackets between nodes with “IPaddresses.” The downside is thatrouters can only exchange data

between nodes using the same net-work layer protocol.

Gateways operate at the networklayer or above; they are used to con-nect dissimilar networks or systemsvia network protocol conversion.

The term “stack” refers to piecesof the OSI model that are bundledtogether. In client-server models, theTCP/IP software is sometimesreferred to as a stack.

For example, on a PC with a net-



Figure 4-10: Profibus Applications

Area Controller

PC/VME

VME/PC

Field DevicesDrives I/O Valves Trans-

mittersField

Devices

Factory Level

Bus Cycle Time

< 1000 msec

Cell Level

Bus Cycle Time

< 100 msec

Field Level

Bus Cycle Time

< 10 msec

Profibus-FMS

Profibus-DP Profibus-PA

MMS, TCP/IP Backbone

CNCHost

PLCDCS

work interface card, a word proces-sor program (application), operatingsystem (Windows NT), and redirec-tor are at work (redirectors inter-cept network application I/Orequests alongside a device’s oper-ating system). In this example (Figure4-8), a user wants to open a filestored on a remote file server onanother network by clicking on theG drive in the list of available drives(including the local A, B, and C dri-ves). The redirector sees the I/Orequest, recognizes its external

location, and sends it to the net-work interface card. On its way, therequest goes through the TCP/IPstack where addressing and proto-cols at the transport and networklayers are handled. The networkinterface card receives the requestand holds it until it can get on thenetwork and pass it on (push thebits) to the next node. The requestis sent from node to node, usingappropriate IP addresses and proto-cols until it gets to the server, where

it is processed, and the reply is sentback essentially the same way.

• BackbonesIn larger enterprises, some sort ofinformation backbone, or data high-way, allows high speed, high qualityinternetworking with good reliability.Backbones connect LANs, WANs andother forms of network segmentstogether in one large network. Thedecision to build a backbone dependson the present and future needs ofthe enterprise. But once it is imple-

mented, growth becomes less painful.One common backbone is the FiberDistributed Data Interface, or FDDIstandard. This backbone is also avail-able in a copper version (CDDI).

FDDI is considered an implementa-tion of a “metropolitan” LAN, orMAN. It is a dual-ring topology thatcan span up to 200 kilometers at 100Mbps. It is commonly used to connectLANs together where higher band-widths are required. Two rings of fibercarry data. All nodes attach to at least

the primary ring, with some or allattached to the secondary ring. Theidea is that if one ring breaks, theother automatically picks up the load.If both rings break at the same point,they automatically join together inone long ring (Figure 4-9).

FDDI hubs are available that bringtogether various FDDI devices, LANs,and Ethernet devices over single-mode or multimode fiber or copper.FDDI is a good choice for linkingbuildings together on a campus orindustrial site.

Another implementation of anenterprise backbone, either stand-alone for one particular link or inconjunction with other technologies,is microwave technology. Operatingat frequencies between 2 and 40gigahertz (GHz), transmitters andreceivers use long, line-of-site trans-mission paths. The economics arereasonable when compared withtrenching and laying cable. The high-er the tower, the longer the link (onthe order tens of miles). Data ratesare typically 1.544 Mbps. Leasing tele-phone lines is usually the alternativeto microwave.

Satellite technology is just begin-ning to come into play for industrialand commercial network implemen-tations. Remote transmitters for sta-tionary platforms (such as wellheads) or mobile platforms (as in thetrucking industry) are being used totransmit real-time data via uplinks toa communications satellite and backdown to ground stations. Telephonecompanies can provide switched ser-vices between the satellite systemand a given enterprise network.

Fieldbus & Device NetworksWhile the network technologiesdescribed so far are typical for busi-ness information systems, they are



Figure 4-11: Protocol Architecture of Profibus

Application (7)

User

Laye

r

(3)-(6)

Data Link (2)

Physical (1)

FMS DP PA

Fieldbus Message Specification (FMS)

FMS Device Profiles

DP-Profiles

DP-Extensions

DP Basic Functions

Not Used

EN 50 170 DIN E 19245 part 4 Profibus Guidelines

PA-Profiles

IEC Interface

IEC 1158-2

Fieldbus Data Link (FDL)

RS-485/Fibre Optic

converging with industrial instru-mentation and control systems.Industrial plants have usedinformation technology (IT) foryears, but with open IT standards,faster computers, emerging soft-ware, and demand for integratedinformation by all segments of anindustrial enterprise (accounting,engineering, operations, mainte-nance, and management), industrialnetworks are merging right intoenterprise-wide IT solutions.

PLCs and distributed control sys-tems (DCSs) are offering Ethernet/TCP/IP connectivity so that real-time information on plant processesis readily accessible by any worksta-tion on the network (LAN or WAN),in a client-server relationship. Aplant manager can watch a graphicdisplay of plant operations in onewindow while scanning accountingdata in another. Object Linking andEmbedding (OLE) and Dynamic DataExchange (DDE) can merge datastreams, such as unit cost (fromaccounting) and production totals(from the plant floor) in onespreadsheet. Internet and intranetbrowsers running Java or HTMLapplications access data acquisitionand control systems using standardLAN/WAN technologies.

Meanwhile, on plant and factoryfloors, industry has developed itsown range of fieldbus or device-level networks for linking controldevices with increasingly intelligentinstrumentation. As the terms haveevolved over the past several years,device networks typically embracethose optimized for discrete manu-facturing automation, while field-bus connotes more process-orient-ed instrument networks. On thediscrete side, the emphasis is onhigh-speed transmission of typical-

ly smaller information packets.Process fieldbuses, on the otherhand, sacrifice speed for moresecure transmission of larger infor-mation packets. Intrinsically safecapabilities also are prerequisite formany process applications.

An exhaustive comparison of the20-some device and fieldbus proto-cols currently available is beyond thescope of this volume. However, anexamination of two leading alterna-tives, the Profibus family of automa-tion networks and the Foundationprocess automation protocol, illus-trates the essential concepts of bothtypes of networks.

• The Profibus FamilyProfibus is a fieldbus and device net-work technology used primarily inEurope, but gaining worldwideacceptance. The Profibus family fol-lows an open network standard(EN 50 170) with hundreds of vendorssupporting a common, interchange-able interface and protocol.

• Profibus PA—Process Automation.Allows sensors and actuators to beconnected on one common bus line,even in intrinsically safe areas.• Profibus DP—Factory Automation.Optimized for high speed andinexpensive connectivity (plug andplay). Designed for communicationbetween automation control sys-tems and distributed I/O at thedevice level.• Profibus FMS—Automation forGeneral Purposes. Designed for alarge number of applications andcommunications at the cell level.

Profibus is basically a serial bus sys-tem with which digital controllers canbe networked together with masterand slave devices over a pair of wires.Masters, or active stations, control thedata communication on the bus. Theycan send messages without an exter-nal request by holding bus accessrights by way of a token. Slaves, orpassive stations, are peripheraldevices that may include input/out-put (I/O) modules, valves, drives, and



Figure 4-12: Foundation Fieldbus Protocol Stack

User Data

4 0 to 251

1 4 to 255

5 to 256

8-273 11 1

25-15

FMS PCI

FAS PCI

DLL PCI

User Encoded Data

FMS PDU

FAS PDU Frame Check Sequence

Preamble Start Delimiter DLL PDU End

DelimiterFieldbus

******

Protocol Control Information Protocol Data Unit There may be more than 1 octet of preamble if repeaters are used.

User Application

Fieldbus Message Specification

Fieldbus Access Sublayer

Data Link Layer

Physical Layer

**

**

**

***

*

*

transmitters. Slaves do not have busaccess rights; they can only acknowl-edge received messages or send mes-sages when the master requests one.

The Profibus protocol varies witheach member of the family (Figure 4-11).

Profibus DP uses the physical anddata link layers (1 and 2), coupled toa user interface, for fast and effi-cient data transmission. Layers 3through 7 are not defined. A DirectData Link Mapper (DDLM) providesaccess between the user interface

and the data link layer. Applicationfunctions are defined in the userinterface. The physical layer isdefined via the RS-485 or fiber optictransmission technologies.

Profibus FMS defines the physi-cal, data link, and application layers.Layers 3 through 6 are not defined.The application layer combines theFieldbus Message Specification

(FMS) and a Lower Layer Interface(LLI). FMS runs the application pro-tocol as well as supporting commu-nication services. The LLI imple-ments communications and pro-vides device-independent access tothe Fieldbus Data Link (FDL) at layer2. Layer 1 is implemented again inRS-485 or fiber optics.

Profibus PA is an extension of theProfibus DP protocol for data trans-mission. Layer 1 (physical) in thisstandard uses IEC 1158-2, which pro-

vides intrinsic safety and “power onthe bus” for field devices. 1158-2modulates a steady-state 10 mA dc“basic current” (the bus power) byManchester encoding a ±9 mA dc sig-nal on top of the basic current.

Manchester encoding refers to asignaling technique that producesbinary ones and zeroes by transition-ing between high and low signals

(+9 mA dc to -9 mA dc in the 1158-2case) over an elevated steady-statevoltage or current. A high-to-low tran-sition is a one, and a low-to-high tran-sition is a zero. Thus, every bit periodhas a transition in it, allowing thereceiver to synchronize easily with thetransmitter. One disadvantage is thatit requires twice the bandwidth asstraight binary encoding.

Profibus DP and Profibus PA canbe integrated via a segment cou-pler. And because Profibus DP andProfibus FMS use the same trans-mission technology and a uniformbus access protocol, they can beoperated simultaneously on thesame cable.

• Foundation FieldbusThe Fieldbus Foundation is a world-wide consortium of manufacturersand industry groups that havedesigned and manufactured anotheropen fieldbus technology calledFoundation. Software and hardwarespecifications have been written bydesign and marketing teams, andmany products are becoming avail-able that conform to this standardacross many different vendors.

The Foundation protocol (Figure4-12) uses layers 1, 2, and 7 (physical,data link, and application layers).Layers 2 and 7 are considered bundledtogether in a communication stack.

Foundation and Profibus look sim-ilar at first. On the physical layer, themain differences are in signalingmethods. Foundation offers IEC 1158-2 signaling, like Profibus.

Foundation also offers a 31.25-kbpsoption, as well as 1.0 and 2.5-Mbpsoptions. The 31.25-kbps version usesa Manchester encoding scheme(±10 mA dc into 150 ohm load) thatcan use existing 4-20 mA twistedpair wiring. With an intrinsically safe



Figure 4-13: Typical Enterprise Network

Mainframe

Microwave

Fieldbus

Internet

PLC

Token Ring Ethernet

FDDI Backbone

(IS) barrier, the 31.25-kbps optionsupports intrinsic safety. The dcsupply voltage ranges from 9 to 32volts, but may be restricted to thebarrier rating. Stubs or spurs areallowed, from 1 meter to 120 metersdepending on the number ofdevices on the bus.

The 1.0 and 2.5-Mbps options bothoffer voltage mode signaling. The1.0-Mbps option also allows currentmode signaling. The voltage modemethod delivers ±60 mA dc into a75-Ohm equivalent load, while thecurrent mode signaling methodmodulates the fieldbus signal on topof a 16-kHz ac power signal.

The Foundation communicationstack is comprised of layers 2 and 7 inthe OSI model. Layers 3 through 6 arenot used. The Data Link Layer, or DLL,controls message transmission onthe bus through a Link ActiveScheduler (LAS). Link Masters andBridges are used to control andextend the bus.

The Fieldbus Access Sublayer (FAS)works with the Fieldbus MessageSpecification (FMS) to form an applica-tion layer, just underneath the userapplication. The FAS services are

described via Virtual CommunicationRelationships (VCRs). The VCR is like aspeed-dial feature on a telephone,used for communicating quickly andeasily to another fieldbus device. ThreeVCR types, Client/Server, ReportDistribution, and Publisher/Subscriberare used for operator messages, eventnotification and trend reports, and datapublishing, respectively.

The Fieldbus Message Specification(FMS) communicates via data objectsand function blocks over the bus.FMS can only use the client/serverVCR type.

User applications in the Fieldbusprotocol use standardized functionblocks for I/O engineering unit con-version, and common functions likePID control. T



References and Further Reading• The Data Acquisition Systems Handbook, Omega Press LLC, 1997.• New Horizons in Data Acquisition and Computer Interfaces, Omega Press


Omega Press LLC, 1997.• Business Data Communications, Fourth Edition, D. A. Stamper,

Benjamin/Cummings, 1994.• Computer Networks, Second Edition, A. S. Tanenbaum, Prentice Hall, 1988.• Data Communications - Video and Workbook, by Industrial Training Corp.

for ISA, 1993.• Data Communications, Computer Networks and Open Systems, Fourth

Edition, F. Halsall, Addison-Wesley, 1996.• Distributed Systems - Concepts and Design, Second Edition, G. Coulouris,

J. Dollimore, and T. Kindberg, Addison-Wesley, 1994.• Guide to Networking and Internetworking Terms, P. Simoneau, American

Research Group, 1994.• How Networks Work, F. J. Derfler, Jr., and L. Freed, Ziff-Davis Press, 1993.• Technical Overview: Foundation Fieldbus, Fieldbus Foundation, 1996.

Previous chapters of this vol-ume have acquainted youwith the technology of howinput/output (I/O) signals are

generated, conditioned, linearized,and transmitted to a host computeror controlling system. In this chapter,we will look at the different kinds ofdata acquisition hardware that areavailable to help you perform all ofthese functions.

Before starting, a brief goodnews/bad news observation: Thegood news is that modern micro-computer technology has made itrelatively simple to install, config-ure, and start up a high-perfor-mance data acquisition system.Experienced users will be happy toknow that many of the problemsencountered a few years ago aregone. New users will never realizehow difficult it once was to bring upa data acquisition system. The badnews is that these same technologyadvances have made it even moredifficult than ever to select amongthe many options. The hardwaredecisions facing a user today requirea great deal of study, analysis, andconsideration. Every system willwork, but some are better than oth-ers for each particular application.

Some things to consider:Connect to the Internet or an

intranet? Today, users can obtainreal-time data over the Internet or anin-house intranet, using a standardbrowser, an Ethernet connection ormodem, and a PC or network com-puter (NC). A network computer hasno moving disk to store programs.Instead, it downloads various soft-ware packages in the form of

“applets” to the NC when needed.This means that your real-time datacan be viewed by anyone in theworld (if they have the properaccess). The argument for using NCsis powerful. It could affect yourchoice of hardware.

What PC interface? Virtually allobjections to using PCs on the plantfloor have been answered, and PCs

are now performing everything fromreal-time control to high-speeddata acquisition. One major issue tobe considered is the various busarchitectures available for PCs.While we will discuss the variouscomputer buses below, you maywant to explore this topic in moredetail to determine the effect itmay have in your application, nowand in the future.

Is there software support? Mostdata acquisition hardware is compati-ble with most popular industrial soft-ware, or it comes with a softwarepackage of its own. Free or not, it is agood idea to investigate the software

and hardware’s capabilities. (For moredetails on software selection criteria,see Chapter 6.)

Selecting a SystemIt is possible to select a wide rangeof different products and configura-tions that will acquire and processdata equally well. These range fromfield-mounted individual devices—such as DIN-rail signal conditionersor smart transmitters—to plug-indata acquisition systems that can beinstalled in a PC.

Virtually all these data acquisitiondevices and systems have the samebasic specifications and options ontheir data sheets:• Signal conditioning: The signalfrom any sensor must be convertedfrom its analog form into a digitalsignal that the host computer orcontroller can understand. This canbe done by a transducer or transmit-ter in the field, a local signal condi-tioner, or—if close enough—by theanalog-to-digital (A/D) converter ona data acquisition board. In addition,signals from nonlinear sensors, suchas differential pressure flowmetersand thermocouples, must be lin-earized. If the environment is partic-ularly noisy, these conversionsshould be done as close to the sen-sor as possible.• Number of analog input channels:Analog inputs usually are specified assingle-ended or differential inputchannels. Single-ended (SE) channelsall reference the same ground point.Differential inputs (DE) have differentreference points for each input, andtherefore need two channels. In gen-eral, SE channels are appropriate


Plug-in Cards

Standalone Components

Communication Devices

DATA ACQUISITIONData Acquisition Hardware

5

PData Acquisition Hardware

Commercially available data acquisition systems

and components come in a wide variety of

configurations and form factors.

when the input signals are greaterthan 1 V, the signal source is no morethan 15 feet away, and all inputs sharea common reference. DE inputs havebetter noise immunity and help pre-vent ground loops.• Sampling rate: This determineshow fast an analog signal is convertedto digital. In general, select a samplingrate that is twice as fast as you need.The operating software of most PCsystems causes skew to occur. Skewcan approach or even exceed thesampling interval. When this happens,patterns in the data may no longer beaccurate or reliable.• Resolution: This determines thesmallest value change the system candetect. Most systems have 12 or 16-bit resolution; a 12-bit systemresolves 1 part in 4,096, while a 16-bitsystem resolves 1 part in 65,536. Theresolution in engineering units isdetermined by dividing the measure-ment span by 4,096 (or 65,536). Forexample, if you are measuring a volt-age signal of 0-1,000 V with 16-bitresolution, the smallest change thesystem can see is ±0.2 V (or ±0.015 V).• Accuracy: This is a function ofmany variables in the system, includ-ing A/D nonlinearity, amplifier non-linearity, gain and offset errors, drift,and noise. (See further discussion inthe “High-Accuracy Data Acquisition”section that follows.)• Analog inputs: Typical voltageinputs are 0-1 V, 0-10 V, ±50 mV, ±100mV, ±0.5 V, ±5 V, and ±10 V. Otherinputs include 0-20 mA, 4-20 mA,frequency, and resistance. Most ofthe big four process control mea-surements—flow, temperature, pres-sure, and level—fit one or more ofthese input ranges. Most data acqui-sition products have selectable orprogrammable analog inputs thataccept the signal, and send it to a 12-

or 16-bit A/D converter.• Analog outputs: While not strictlypart of data acquisition, many dataacquisition products offer analogoutputs on the same board. In almostall cases, a data acquisition board haseither 2 or 4 outputs. If more areneeded, a dedicated analog outputboard may be required.• Digital I/O: Most digital I/O canbe selected for either input or out-put. In most cases, it is possible toconfigure a data acquisition boardwith digital inputs, outputs, or bothin any combination. Boards thatclaim to be “multifunctional” havethe most digital I/O.

In addition to comparing basicspecs, examine how the product isdesigned and built. Accuracy is oftendependent upon the amount of noisepicked up by the system. Moderndata acquisition products minimizethis problem in several ways:• Surface mount technology (SMT):Using SMT shortens the length oftraces on the board, which improveselectrical noise resistance.• ASICs: Application specific inte-

grated circuits (ASICs) combine thefunctions of several componentsinto one. They also eliminate theconnections between those individ-ual devices, increasing speed,improving accuracy, and reducingexposure to noise.• Reduced board size: With today’sSMT and ASIC technology, manydata acquisition boards are beingmade in a smaller size (form factor).Once again, reducing the size andnumber of traces boosts speed andminimizes noise.• Isolated inputs: Input isolationprotects the board from damage ordisruption from an abnormal signal

or noise on the input line. Finally, you have to decide what

type of data acquisition system touse. There are two major choices:• Plug-in data acquisition cards:These plug directly into the chassisof your computer. Signals from sen-sors or signal conditioners are wiredto the computer and connected tothe data acquisition board.• Standalone systems: Self-con-tained I/O systems acquire data in

Data Acquisition Hardware5


Typical ISA-bus data acquisition board plugs into a standard PC and offers a variety of speed, resolu-

tion, and other feature options.

the field, and send it to the comput-er via a network or communicationsmethod of some kind.

Let’s look at both types.

Plug-in CardsData acquisition cards that plug intothe chassis of a desktop PC, a PC-compatible industrial computer, oran Apple Macintosh computer havemade measurement and controlextremely economical for the typicallab or industrial user.

A plug-in data acquisition cardwith 16 analog input channels in the

$1,000 to $2,000 price range repre-sents a cost of $62.50 to $125.00 permeasurement point. This comparesto $350 per point (or more) for a con-ventional “front-end” or communica-tions-based system.

In the past, high-speed dataacquisition systems required expen-sive, high-end computer platformssuch as VAX/VMS minicomputers orUnix RISC workstations to handlelarge amounts of real-time data andhave enough computing power leftover to perform control and opera-tor interface functions. Such systems

were expensive to buy, install, main-tain, and update.

Today, a PC has the same amount ofcomputing power as yesterday’s RISCworkstations. Desktop and panel-mounted industrial PCs are availablewith up to 300-MHz Pentium IIprocessors, enormous amounts ofmemory, fast disk drives, high-speednetworking capability, and powerfuloperating systems. Mac-based sys-tems have similar capabilities.

Previously, hardware and softwarefor high-end systems were not inter-changeable among products from

different vendors. Plug-in dataacquisition boards for a RISC work-station were scarce and very expen-sive. Conversely, plug-in data acqui-sition boards for PCs, compatibles,and Macs are plentiful, relativelyinexpensive, and compatible witheach other.

Once, setting up a PC-based dataacquisition system was very difficult.Hardware and software were primi-tive, so installation and configurationrequired setting jumpers and DIPswitches, allocating operating systemresources, changing config.sys set-

tings, changing IRQ levels, writingsoftware drivers, and performinghardware calibration.

The trend today is toward plug-and-play systems that configure,install, and calibrate themselves. Plug-and-play software selects and assignssystem resources automatically.

Many modern data acquisitionboards do not require periodic cali-bration. Those that do often comewith autocalibration functions built in.

Plug-in boards offer a great deal ofsoftware compatibility, virtuallyeliminating the need to write driversor special software to interface themto a system. If you already have soft-ware, check to be sure that the dataacquisition hardware you are consid-ering is compatible with your system.You can also check with your soft-ware supplier to see what data acqui-sition boards it supports.

A typical plug-in data acquisitionboard offers these features:• Analog inputs: 8 DE or 16 SE;• Resolution: 12- or 16-bit;• Input range: mV and V;• Sampling rate: 30,000-330,000

samples/second; • Analog outputs: 2;• Digital I/O: 8-16; and• Base price: $199 to $1,200.

There are hundreds of data acqui-sition board configurations availablefor PCs and Macs. Many other boardsare available with more inputs, high-er resolution, and faster scanningspeeds. Now, let’s look at some ofthe higher performance boards.

• High-Speed Data AcquisitionA typical plug-in data acquisitioncard has a sampling rate of 30,000 to250,000 samples/second. A high-speed data acquisition card, on theother hand, operates at 330,000 to 20million samples/second. Applications

Data Acquisition Hardware 5


This 12-bit data acquisition system provides eight analog inputs, two analog outputs, and 24 digital

I/O—all on a single low-cost PC card.

for such high sampling speeds includewind tunnel testing, auto crash test-ing, video processing, ultrasonic imag-ing, and waveform analysis.

At such speeds, the data acquisi-tion card must be able to communi-cate with its computer via directmemory access (DMA). This allowsthe card to transfer large quantitiesof data directly to the computer’smemory. In some cases, the card mayhave dual-channel DMA.

A high-speed data acquisitionboard often has a considerableamount of first-in-first-out (FIFO)memory. Typically, a high-speed dataacquisition board can store 64,000samples. Some can store more than amillion samples, either on the boardor by using additional memory ondaughter boards.

In most cases, the board does notattempt to acquire 20 million sam-ples every second, because it wouldsoon overflow on-board memory,host computer memory, and diskspace. Instead, the board waits for anexternal trigger signal that says an“event” is occurring, and then itbegins to acquire data at high sam-pling rates. Trigger sources can beanalog or digital inputs.

A “typical” high speed data acqui-sition board—above 330,000 sam-ples/second—has these features:• Analog inputs: 8 DE or 16 SE;• Resolution: 12-bit;• Input range: mV and V;• Sampling rate: 330,000 to

1,000,000 samples/second;• Analog outputs: 2;• Digital I/O: 8; and• Base price: $595 to $2,399.

Often, as speed increases resolu-tion drops to 12-bit. Of course, 14-bitand higher resolutions are availablein high speed data acquisition boardsat a cost.

One ultra high-speed data acqui-sition board for radar, ultrasonic,seismic, and related applicationsacquires data at sample rates up to500 million samples/sec. Usinginterleave techniques, rates of up to1 billion samples/second may beachieved. Boards are available for PCand VME-based systems.

For applications that require-highspeed data acquisition, plug-inboards that operate at 330,000 to1,000,000 samples per second arereadily available at reasonable prices.The only sacrifice may be in accuracyand resolution.

• High-Accuracy Data AcquisitionFor most applications, plug-in dataacquisition boards with standard 12-or 16-bit resolution are adequate.Some applications, however, requiregreater accuracy and resolution.Higher resolution means an A/Dconverter with more bits. Each bit-size increase means that the comput-er can resolve twice as much data:for example, a 16-bit data acquisition

board can resolve to 1 part in 65,536;increasing to 18 bits quadruples thisto 1 part in 262,144.

Data acquisition boards are avail-able with resolutions of 18 bits, 20bits, and higher. At the higher resolu-tion, much smaller voltage changescan be identified. With a voltage sig-nal of 0-1,000 V, the smallest changethe system can see with a 20-bit A/Dconverter is ±0.0038 V (compared to±0.015 at 16 bits).

Accuracy also is affected by manyother variables. Some of the specifi-cations to consider are:• Code width: This is the smallestvoltage an A/D converter candetect. It is a function of resolution,range and gain:

code width = range/(gain x resolution)

For example, a 12-bit data acquisitionboard with a range of 20 V and a gainof 100 can detect a 48.8 microvolt µVchange in signal (0.0000488 =20/(100 x 4,096)). A 16-bit data acqui-



Designed to use external signal conditioning cards and a notebook computer, this unique data

acquisition system provides a good mix of portability, flexibility, and economy.

sition board detects a 0.030 µVchange, and so on. It is a commonmisconception that accuracy is thesame as code width. • Settling time: This is the timerequired by the programmable gainamplifier (PGA) to respond to theinput signal. Most data acquisitionboards use a multiplexer to switch sig-nals to the PGA. If data is sampledfaster than the PGA can respond,errors will occur. For maximum accu-racy, select a data acquisition boardwith a PGA that can respond to thesampling rate needed. • A/D conversion time: This is thetime required by the A/D converterfor each sample. A high precision

A/D converter might require 2.5 ms,making the maximum sampling rate afairly slow 400 Hz.• Computing accuracy: It is possibleto calculate the theoretical accura-cy of any data acquisition board, butthe computation is relatively com-plex, taking into account a dozenboard performance parameters.I&CS magazine offers the procedureas an Excel worksheet, which canbe found on the Internet at:www.chiltonco/ics/calcexam.xls.

The file can be downloaded directlyinto your computer.

Perhaps the most important con-sideration is that high speed and highaccuracy are often mutually exclu-sive. To obtain maximum accuracy,speed must be sacrificed. This isbecause:• Accurate A/D converters operate

more slowly than low-precisiondevices.

• Data acquisition components musthave adequate time to settle andconvert to obtain the accuracythey are capable of providing.

• Sampling interval may need to beincreased to eliminate softwareskew.

• A 20-bit digital value requires 3bytes of information, 50% morethan a 12- or 16-bit value, so ittakes 50% more time just to movethis data from point to point inthe system.

• Specialty Data Acquisition BoardsA wide variety of specialty data acqui-sition boards also are available. Insome cases, these are standard dataacquisition boards that have beenmodified to fit a specific application

by adding one or more functions, suchas high input isolation, timer interface,or specialty software.• Thermocouple and RTD input:These boards permit direct connec-tion of field wiring from tempera-ture sensors without additionalexternal signal conditioning. All nec-essary linearization, signal condi-tioning, and noise filtering are doneon the data acquisition board itself.Multi-purpose temperature boardsusually also accept inputs fromstrain gages or similar low-level mVsignals. Boards also are available toaccept thermocouples only (or RTDsonly). These boards accept all com-mon thermocouple types, plus mVand voltage signals. A typical boardwill linearize the thermocouple sig-nal and compute the cold junctioncompensation. Options includeautomatic calibration, averaging, andvoltage-to-frequency (V/F) samplingto reduce noise.• Sensor direct-connect: Similar tothe thermocouple and RTD cards,direct-connect cards allow field-wired sensors to connect directlyto the data acquisition board.Sometimes called universal sensorinput boards, they accept raw,unconditioned signals from thermo-couples, RTDs, thermistors, straingages, and linear variable displace-ment transducers (LVDTs), as well asresistance, variable reluctance, fre-quency, and weight sensors. Whenusing such a board, take care to min-imize the length of sensor field wires,because the low-power signals aresusceptible to electrical noise. • Load cell interface: This type ofdata acquisition board performs adual function. First, it inputs fieldwiring signals from load cells andstrain gages All signal conditioning,filtering, and data conversions are



High-speed data acquisition boards can exceed one million samples per second for applications such

as transient recording and vibration analysis.

done on the board. Second, it sup-plies the excitation voltage neededby 6-wire, 350-ohm load cells andstrain gages. With optional software,the board can make a PC into a high-speed check weigher, load scale, ortruck scale. • Sample-and-hold (S/H): Availableas an option on some data acquisi-tion boards or as an accessory, thesample-and-hold function allows anumber of channels to be sampledvirtually simultaneously, while stillallowing the A/D converter to beshared among multiple channels. TheS/H function “holds” the data untilits buffer is emptied and the data issent to computer memory. In somecases, the simultaneous S/H func-tion is activated by a trigger. Whenadded as an external accessory, theS/H board essentially acts as a front-end to a plug-in data acquisitionboard, and minimizes skew problems.

• Bus Architecture OptionsPlug-in data acquisition cards aredesigned primarily for two lines ofcomputers: PCs and compatibles,and the Apple Macintosh. Amongthese, several types of bus architec-tures are available. Selecting theproper bus architecture is impor-tant—the wrong choice can limit orthwart expansion plans.

Following is a description of thevarious buses available (Figure 5-1):• ISA: The first Industry StandardArchitecture (ISA) bus was intro-duced with the original 8088-basedPC/XT in 1980. It can address 8- and16-bit devices, and has DMA capabil-ity. When the 16-bit 80386 PC/ATwas introduced, it kept the original8-bit bus, but added a 16-bit connec-tor for 16 data lines and 24 addresslines. This did not increase bus speed,but it allowed the bus to address

more memory. Neither of the ISAbuses can act as a bus master. Manyplug-in boards list both PC/XT andPC/AT compatibility, and many mod-ern computers still have one or moreISA expansion slots. • EISA (Extended ISA): Another pro-gression of ISA, this generation addedsupport for intelligent bus-masterexpansion cards. This is importantbecause if the data acquisition board

cannot take control as a bus master, itmay not be able to transfer data anyfaster than an ordinary ISA board.With bus mastering, the data acquisi-tion board makes the DMA transfer atthe highest possible rate. EISA sup-ports 8-, 16-, and 32-bit data transfersat speeds up to 33 Mbps.• MCA: The ill-fated Micro ChannelArchitecture is a feature of IBM’sPS/2 computer. Its proprietary archi-tecture thwarted PC-compatiblemanufacturers, so it was never a suc-cess outside IBM. Few plug-in dataacquisition boards are available forthe MCA system. • VESA: The Video ElectronicsStandards Association developed theVESA bus primarily to maximizethroughput for video graphics in the80386/486 environment. It adds a

local bus for faster access to theprocessor, and supports limited databursts up to 132 Mbps. Like the MCAbus, it enjoys only limited successoutside its industry, and few plug-indata acquisition boards are availablefor it. Those that are available typi-cally are intended for use in thevideo industry. • PCMCIA: The Personal Comp-uter Memory Card InternationalAssociation (PCMCIA) developedthis interface for small computers,such as notebooks and laptops. Itspopularity skyrocketed, and todayvirtually every portable PC has PCM-CIA slots. It supports 8-, 16-, and 32-bit data transfers. Plug-in data acqui-sition boards are available that oper-ate at speeds to 100 kHz. • PCI: The Peripheral ComponentInterconnect (PCI) bus is a processor-independent bus that supports up to64-bit addressing, bus mastering, andburst transfer rates up to 1,056 Mbps(132 Mbytes/second). The standard isstill evolving, with developmentssuch as the CompactPCI standard forindustrial applications, and hybridversions that work with VME, PMC,Multibus, and STD. Virtually everyPentium PC being shipped today hasa combination of ISA/PCI orEISA/PCI buses. • PC/104: Intended for embeddedcomputer applications, PC/104 cardsare very compact at 3.6 x 3.8 in.PC/104 also allows card modules toself-stack without backplanes orcard cages, and has pin-and-socketbus connectors designed for highreliability in harsh environments. It is,essentially, a miniaturized version ofthe standard PC bus architecture.Also in continual development, thelatest generation (PC/104-Plus) oper-ates at the same speed as PCI. • USB: The Universal Serial Bus (USB)



This specialized data acquisition board is

designed to handle strain gage inputs for

load-cell applications.

is around the corner. Currently beingoffered in most commercial PCs, it isessentially an upgrade of the venera-ble RS-232 port. It will allow up to 127USB devices to connect to a PC. USBperipherals will be automaticallydetected and configured when con-nected and could include virtuallyevery peripheral device now beingused, including keyboards, monitors,touchscreens, modems, and dataacquisition devices. It operates at 12Mbps. Almost no industrial dataacquisition boards are availabletoday, but many companies havethem in development.

Buses for Apple computers are notnearly as complicated and diverse asPC buses, but specs vary from modelto model. Until recently, Apple buseswere based on two basic architectures:NuBus and Processor Direct Slot (PDS).Both are 32 bits wide. The NuBus isfound in almost every Macintosh com-puter, while PDS provides very high-

speed communications.It is more interesting to look at the

latest Macintosh to see what direc-tion Apple is going. Its latest comput-er, the Power Macintosh G3 has a 233or 266 MHz PowerPC processor andtwo processor buses: a standard 66

MHz system bus and a 133 MHz “back-side” bus for 64-bit data transfers.

The G3 also comes with three 12-in.PCI expansion slots that are compatiblewith PCI 2.1 cards, two high speed DMAserial ports, and a built-in 10BaseTEthernet connector, presumably forInternet/intranet connections.

Anyone considering plug-in dataacquisition boards for either PC orMac systems can be confident thatboth the computer processing hard-ware and the data acquisition boardsare capable of handling just aboutany data acquisition job imaginable.

Standalone ComponentsIn the previous section, plug-in dataacquisition boards for PCs and Macswere examined. In this section, wewill look at communications-based,or front-end, data acquisition sys-tems—standalone data acquisitionsystems that collect data and send itto a processor over a data communi-

cations scheme. Data communica-tions involve everything from a sim-ple RS-232 connection to a high-speed network (Chapter 4).

One major advantage of communi-cations-based systems is that they areprocessor independent. A “box” of

standalone data acquisition hardwarecan transmit its data to a PC, pro-grammable logic controller (PLC),RISC workstation, midrange or main-frame computer, or even the Internet.A standalone system can gather onedata point—such as an infrared sen-sor—or thousands of analog and dig-ital measurement points.

Unlike a plug-in data acquisitionsystem based on a commercialPC, standalone systems often aredesigned to be located anywhere onthe plant floor. Ruggedized I/O mod-ules, racks, power supplies, and sealedNEMA enclosures are readily availableto house a data acquisition system,often at a lower cost than a similarlyruggedized industrial computer.

A plug-in data acquisition systemis limited by the number of cards thatcan fit into the available slots of a PCor Mac. Also, when a data acquisitionsystem gets overly large on a PC, itbegins to take more and moreprocessor time for DMA access,slowing even the extremely powerfulprocessors. A standalone data acqui-sition system, on the other hand, canmore easily grow to be as large asnecessary. You can even start small,building and expanding the system asrequirements grow.

Finally, standalone systems areportable from system to system.They operate with industry standardcommunications, so the I/O can beeasily transferred from a PC to a Macto a PLC as needed.

A plug-in board works with onetype of processor only. Even moredifficult, all the field wiring termi-nates at the data acquisition board.To change computers, you may face amonumental rewiring task, evenwhen moving to a bigger and fastercomputer of the same type.

While a plug-in data acquisition



ISA

EISA

MCA

VESA

PCMCIA

PCI

PC/104

USB

NuBus

PDS

BUS BUS DEFINITION NUMBER OF BITS MAXIMUM SPEED ACRONYM (millions of bits/ second, Mbps)

Industry Standard Architecture

Extended ISA

Micro Channel Architecture

Video Electronics Standards Association

Personal Computer Memory Card International Association

Peripheral Component Interconnect

(ISA in a smaller, more rugged form factor)

Universal Serial Bus

(Macintosh bus)

Processor Direct Slot (Macintosh bus)

8, 16

8, 16, 32

16, 32

32

8, 16, 32

32, 64

32

32

32

1.6

33

33

132

132

132, 1056

132

12

33

132

Figure 5-1: Computer Bus Architectures

board represents the most efficientand most economical solution tomany data acquisition applications,

standalone systems have more capa-bility and flexibility. Remember, plug-in boards for PCs and Macs are a rela-tively new product—they are notmore than 15 years old. Standalonesystems have been evolving for 35years or more, since the first supervi-sory control and data acquisition(SCADA) systems of the 1960s. Withhundreds of manufacturers world-wide, and tens of thousands of differ-ent devices and models, finding front-end data acquisition equipment to dothe job is a much easier task.

• Connecting to Field I/OWhen considering a standalone sys-tem, a major question that has to beanswered is: How will the field I/Obe connected? This involves a choiceamong several systems: DIN-rail sys-tems, rack-mounted systems, andfield-mounted devices.

A DIN-rail system is a convenient

way to connect sensor field wiring tosignal conditioners, converters,power supplies, terminal blocks,

relays, modems, power conditioners,surge suppressors and other devicesin the field. The DIN rail is either 32-or 35-mm wide, and provides a stur-dy platform on which dozens ofdevices can be mounted. A singlepower supply, surge suppressor, andcommunication module can serve arow of modules.

DIN modules are small, rugged,and designed for high-densityinstallation. For example, high-den-sity signal conditioners from onemanufacturer can be packed soclose together they can accommo-date 52 analog signals in one foot ofDIN rail, at a cost of about $100 perpoint. DIN devices often are offeredwith universal foot mountings thataccommodate 32- and 35-mm DINrails and the Allen-Bradley rail.Adapters are available that allowany flat surface device to mount ona DIN rail. Both schemes allow mix-

ing and matching of I/O devicesfrom several different systems.

Depending on the plant environ-ment, the DIN rails can be mountedin cabinets, racks or NEMA enclo-sures. Field I/O wiring is brought tothe enclosure, and typically connectsto termination modules or directlyto the signal conditioners.

In most cases, DIN rail devices donot perform any processing of fielddata other than signal conditioningand conversion. Instead, they serveprimarily as a termination and distri-bution point for field I/O. For dataacquisition purposes, DIN devicesgather sensor data and distribute itto downstream equipment throughRS-232/RS-485 links, various fieldbusor device networks, or other types ofcommunications systems.

Rack-mounted data acquisitionsystems have been around since thebeginning of the industrial controlindustry. This solution involves I/Omodules, subsystems, and processorsthat are installed in a standard indus-trial 19-in. rack that’s housed in aNEMA free-standing or wall-mount-ed enclosure. Subsystems andprocessors typically mount horizon-tally across the enclosure, while I/Oboards and processor cards mountvertically in a card chassis.

The card chassis usually has abuilt-in power supply for all the I/Oand processor cards that plug into it.It may also include a backplane, as inthe case of VMEbus and VXIbus, tointerconnect the cards and proces-sors. In other systems, a large powersupply mounted at the bottom ofthe enclosure provides conditioned,noise-free power to every module.

Rack-mount systems can accom-modate PLC I/O, data-logging sys-tems, remote I/O for a computer,field networks, and just about every



Discrete signal conditioners are available to perform almost any combination of manipulations

conceivable. These, for example, convert raw thermocouple inputs into RS-485 digital signals.

type of front-end or I/O on themarket today.

Historically, mV sensor signalshave been wired to a transmitter ortransducer that converts the signalinto a higher voltage or a 4-20 mAcurrent loop signal that is less sus-ceptible to electrical interference(see Chapter 3). While this is still aperfectly acceptable technique,newer field-mounted devices arebecoming available. One of thehottest topics in I/O today is thedigital instrument network, whichinvolves connecting sensors to dataacquisition systems or host comput-ers via a digital signal (see Chapter 4).Field-mounted transmitters of yes-terday have become extremely pow-erful digital devices that do muchmore than convert data. “Smart”transmitters and I/O devices cannow mount at or near the sensorlocation in junction boxes or sealedenclosures, and perform a wide vari-ety of signal conditioning and pro-cessing functions locally.

• Signal ConditionersSignal conditioners are available tohandle virtually every sensor on themarket today, including mV, V, mA,and resistance signals from RTD, ther-mocouple, potentiometer, straingage, load cell, LVDTs, and frequencysensors, to name the most common.

In many cases, the signal condition-ing consists of signal isolation, filter-ing, amplification, and linear conver-sion of signals ranging from ±10 mV to±40 V to a more usable voltage.

For thermocouple inputs, signalconditioners linearize the incomingsignal and perform cold junctioncompensation to increase accuracy.A thermocouple signal conditionerusually accepts inputs from types J,K, T, E, R, S, B, and N thermocouples.

RTDs may be 2- or 3-wire. For sensorsthat require an excitation voltage,such as a strain gage or load cell, thesignal conditioner supplies it.

DIN-rail signal conditioners arenarrow devices that often accommo-date only one signal at a time andhave a selectable output from 0-5 Vor ±10 V, 4-20 or 0-20 mA, and RS-232or RS-485 communications. WithRS-485, up to 32 modules can beconnected to a single computercommunications port.

Signal conditioners in rack systemsare full-size I/O boards that pluginto the chassis. Sensor signals termi-nate at the I/O board or onto termi-nal panels. A typical analog inputboard accepts 8, 16, or 32 channels.Unlike multifunction PC boards, racksystems tend to specialize. For exam-ple, all digital I/O goes to a digitalI/O card, thermocouples go to a ther-mocouple input board, and so on.

Field-mounted signal conditionershave the same capabilities as DIN-railand rack-mount devices, but arepackaged in tougher enclosures.Some are in “hockey puck” packages,others in I/O “bricks.” Both can bemounted near the sensor in benignenvironments, or housed in junctionboxes or small enclosures if the envi-ronment is more difficult.

• Transmitters A traditional analog transmitter takesa sensor signal, amplifies and condi-tions it, and sends it on its way as a 4-20 mA signal that is relatively noiseimmune and intrinsically safe if nec-essary. In a two-wire system, powerfor the transmitter’s electronics istaken from the current loop. In afour-wire transmitter, power is pro-vided from an external source on asecond set of wires.

As you might expect, today’s tech-

nology has gone far beyond this point.Many of today’s transmitters are“smart” devices that communicate toa host PC or PLC via a digital signalsuperimposed on the 4-20 mA current,or by RS-232, RS-485, or other digitalnetworks (Chapter 4). Embeddedmicroprocessors perform diagnosticsand self-calibration, and accept pro-gramming and setpoint informationfrom a host processor. Smart transmit-ters also can be configured remotelyfor any number of tasks, thereby cut-ting down on the number of differenttransmitters needed for spares. Powerfor the transmitter is supplied by the4-20 mA current loop, the phone linein a modem-equipped device, or bythe RS-232 line.

Some smart transmitters accepttwo sensor inputs, using one as theprimary and the other as a hot back-up. If the primary sensor fails, thetransmitter switches over instantlyand notifies the host. More and moresystem responsibility is being off-loaded to smart transmitters. In onesystem, the transmitter does not senddata at all, unless it has changed fromthe previous value. In another, outputsfrom three transmitters are evaluatedby a two-out-of-three voting schemeto determine which one has seen afalse input (for example, flame-out ina flare stack application).

• Alarm ModulesWith computer processors availableat all levels of a data acquisition sys-tem, the need for alarm modules hasdiminished. When independent localalarm indication is needed, however,alarm modules are an ideal solution.

DIN-rail mounted alarm modulestypically accept an ac or dc voltage orcurrent input signal, or a temperaturereading from a thermocouple or RTD,and compare the input with high and



low setpoints. The limit alarm can befield configured for high or low, fail-safe or non-fail-safe operation.

Alarm modules have one or morerelay outputs for sounding audiblealarms or illuminating a panel annun-ciator. When the operator investi-gates, the alarm module has LED indi-cators that show input status.

• Remote I/OMany PLCs and computer-based con-trol systems use remote I/O to col-lect data in a far-off location, from awork cell where sensors are concen-trated, or from various convenientlocations in a plant. The remote I/Osystem serves as a data concentrator,minimizing the amount of physicalcabling needed to directly connecteach field I/O point.

In some cases, the I/O for a PLC isassembled into a cabinet by the ven-dor—complete with analog and digi-tal I/O, termination cards, signal con-

ditioners, and other devices. Theremote system communicates with itshost over a communications network.

Computer-based control systemscan use remote I/O made up from

digital and analog I/O boards from awide variety of vendors, sent over anyof several data communicationsschemes. In many cases, a large con-trol system spread over a plant floormay be taking data from entirely dif-ferent types of data acquisition equip-ment. Information coming from atesting lab might be originating in ana-lyzers and instruments that use theIEEE-488 standard; real-time data froma batch reactor may consist of dozensof 4-20 mA flow, pressure, and tem-perature transmitter signals coming inover a fieldbus network; and informa-tion from the inventory control mate-rial-handling system could be motioncontrol data from resolvers, encoders,and limit switches gathered by a PLCand made available on a PLC commu-nications network. • Front end: A front end is simply adedicated I/O system that is setapart from the host computer orcontroller. The two systems are phys-

ically separated, with all or most ofthe I/O concentrated in the frontend. A front end typically connectsto the computer via one or moreparallel cables that support a 32- or

64-bit interface. The front end mayconnect to multiple remote I/Ounits, networks, serial I/O, or PLCdata highways.

Communications DevicesNo-fuss data communications havecontributed much to the world ofdata acquisition. From telephonemodems to wireless networks, abroad variety of specialized datacommunication devices are availableto suit every communication need.

• ModemsUsing telephone wires, twisted pairs,or fiber optic cable, modems trans-mit and receive serial, digital data.You may be familiar with the current56 kbps (actually, it’s 57,600 baud)controversy on the Internet, withtwo competing, incompatible suppli-ers claiming that they can achievesuch data rates over commercialphone lines. Such a modem has beenavailable for years in the industrialworld, but it requires special isolatedcircuitry. Achieving higher data ratesfor serial data often requires fiberoptics. Examples of modems avail-able for data acquisition include:• Limited distance modem: Low-cost ($86) general purpose modemscan be used to transmit non-criticaldata up to 12 miles. As distanceincreases, transmission speed decreas-es. At 12 miles, speed is 1.2 kbps. Atone mile, speed is 19.2 kbps, and atone half mile, 57.6 kbps. It requiresfour wires (two twisted pairs) for fullduplex operation. To achieve thehighest speed of 57.6 kbps, specialhigh-speed, optically coupled cir-cuits are required.• Fully isolated modem: When datais important, it must be protectedagainst noise, common mode voltageproblems, and electrical transients.



General purpose modems are a low cost option for sending digital information at rates up to 57.6 kbps.

Optical isolation and dc-to-dc trans-former coupling create an isolationbarrier of 1000 V rms (root meansquared). An isolated modem has thesame transmission speed as a limiteddistance modem. • Signal-powered fiber optic modem:In particularly nasty environmentswith much electrical noise, a fiberoptic modem can be used. Signals onfiber optic cable are immune to EMIand RFI. Simple, low cost ($127) fiberoptic transmitter/receivers can beplugged into the RS-232 ports atboth ends of the link. The transmit-ters take their power from the RS-232port, and transmit data at 19.2 kbpsfor up to 2.2 miles.• High-speed fiber optic modem:This device transmits RS-232, RS-422,or RS-485 data at speeds up to5 Mbps 1.2 miles over a fiber opticlink. As above, the transmitter/receiver plugs into the data port atboth ends. For more distance, arepeater can be installed.

• Wireless SystemsBefore the advent of reliable radiodata transmission for reachingremote locations, communicationsrequired hard wiring. Wires had to bestrung on utility poles or under-ground. This is neither cheap norconvenient. Today, transmitting databy radio-frequency (RF) links isbecoming widely acceptable. RF linksare available in several varieties:• Microwave: Requiring line-of-sightbetween antennas, microwave sys-tems can transmit data up to 32miles. Such a system consists of anoperator base station that can com-municate with one or more remoteterminal units (RTUs). Each RTU has amicrowave two-way radio and anantenna. Data acquisition equipmentat the remote site connects to the

RTU via RS-232 or RS-485 ports.Typical cost of an RTU or base sta-tion is $3,000 to $5,000. • Unlicensed link: These narrow-band units operate in the 900-MHzband, and work at distances to 1 mile.With antennas or repeaters, dis-tance can reach 90 miles at speedsto 115 kbps with a response time of5-15 milliseconds between units.• Spread spectrum: When interfer-ence is present in the 900 MHz bandfrom telephones or other 900-MHzdevices, spread spectrum radios “fre-quency hop” from 902 to 928 MHz toget away from the offending interfer-ence. At distances under one mile, line

of sight is not always necessary forcommunications. Spread-spectrumradios can achieve full duplex uncom-pressed data rates of 115.2 kbps. • Licensed RF: If data acquisitionneeds cannot be met by 900-MHzdevices, devices that operate onlicensed frequencies can connect upto 10,000 remote units. The limitedavailability of frequencies keeps

these systems out of metro areas.Utilities in outlying areas are bigusers of licensed systems.• Trunked or cellular: For hilly,mountainous, or city settings whereline-of-sight systems are impracti-cal, trunked radio is a solution. Theradio signal is transmitted throughrepeaters by the same serviceproviders used by taxicabs. Cellulartelephones also can be used to trans-mit data in such situations.

• Protocol ConvertersAs noted above, any data acquisitionsystem might mix and match compo-nents and transmit data over multiple

interfaces. Protocol converters help agreat deal in the interfacing of varioushardware. Some of the wide variety ofconverters available include:• IEEE-488 to RS-232/422: Providesa transparent interface between twodissimilar devices. For example, itallows an IEEE 488 instrument tocommunicate with an RS-232 printeror plotter. Conversely, a PC with an



Automatic dialing systems are designed to alert people—or supervisory systems—about remote

alarm conditions via standard phone lines.

RS-232 port could communicate withan IEEE-488 device. Such convertersare available with one port fordevice-to-device communications,or with multiple ports.• IEEE-488 to Mac: Connects to SCSIport on Macintosh computers,allowing the computer to control upto 14 IEEE-488 devices.• RS-232 to RS-422: Converts anRS-232 signal to RS-422, with com-plete signal isolation. It allows anRS-232 device to get onto an RS-422multi-drop system. RS-422 supportsup to 32 devices.• RS-232 to RS-485: Similar to above,this converter lets an RS-232 deviceget onto an RS-485 multi-drop sys-tem. RS-485 supports up to 64 devices.

• Auto DialersAn auto-dialer monitors conditionsat a remote site and, if an alarm con-dition develops, alerts people orother equipment. It is a small, self-contained data acquisition systemthat monitors one or more key digitalor analog sensors. In almost all cases,it has automatic dial-out capabilitythat will continue to call a series ofphone numbers until someoneanswers. It then delivers a prepro-grammed voice alarm message.

At the low end, systems withvoice-only outputs start at about$395. One such a system monitorsone temperature and three contactclosure inputs. When an alarm condi-tion develops, it dials multiple phonenumbers and delivers a prerecordedvoice message. It continues to calluntil someone properly acknowl-edges the message.

As features are added, costs andcapabilities increase:• Additional analog and contact

closure inputs allow more systemsto be monitored.

• Digital speech recording allowsmultiple messages to be recordedand assigned to various alarms.

• An RS-232/485 interface allows the

system to call up a computer orcontroller to deliver an alarm mes-sage and transmit data.

• A built-in data-logger functionstores data about the event forlater analysis.

• Local digital and analog outputsallow alarm and control functions tobe programmed. T



References & Further Reading• The Data Acquisition Systems Handbook, Omega Press LLC, 1997.• New Horizons in Data Acquisition and Computer Interfaces, Omega

Press LLC, 1997.• Omega® Universal Guide to Data Acquisition and Computer Interfaces,

Omega Press LLC, 1997. • Data Acquisition and Control, Microcomputer Applications for Scientists

and Engineers, Joseph J. Carr, Tab Books Inc., 1988.• Data Communications, A Beginner’s Guide to Concepts and Technology,

Scott A. Helmers, Prentice-Hall, Inc., 1989.• “Flying Your Plant by Radio Control,” Dan Herbert, Control, August, 1996.• “How to Improve Industrial Data Communication,” Michel E. Maes and

James R. Steffey, Control Engineering, March 1996. • Instrument Engineers’ Handbook, Third Edition, Bela Liptak, Chilton

Book Co., 1995. • “Microcontrollers and the Design Engineer,” Julie Anne Schofield, Design

News, June, 1997.• Modern Digital and Analog Communication Systems, B.P. Lathi, Holt,

Rinehart, & Winston, 1983. • PC Bus Performance Brief: PC Bus Overview, Intel Corporation, 1997.• “PC-Based Equipment Can Offer High-Speed Data Acquisition,” Jeffrey R.

Payne and Bradford A. Menz, Plant Engineering, November, 1996. • PDS Expansion Interface Q&A, Developer Support Center, Apple Corp. • Process/Industrial Instruments & Controls Handbook, Fourth Edition,

Douglas M. Considine, McGraw-Hill Inc., 1993.• “Radio Days,” Paul Studebaker, Control, April, 1997. • “A Road Map to Successful Data Acquisition Board Selection,” Chad

Chesney, I&CS, October, 1997.• “Temperature Transmitters Go Upscale,” Dan Herbert, Control, March, 1997. • What is PC/104?, PC/104 Consortium, 1994.• “Wireless Modem Offers Fast, Accurate Data Transmission,” Peggy Piper,

Control, May, 1997.

Prior chapters have covered themechanics of acquiring datafrom various sources andtranslating this data into bits

and bytes. This chapter will focus ontools available to translate data intoworthwhile information. These toolsare primarily programming languagesand off-the shelf software packagesrunning under Microsoft Windowsand Macintosh operating systems onpersonal computers.

The steps required to select a dataacquisition software program are:1) determine the features and perfor-

mance necessary to accomplishthe task at hand;

2) determine the available resourcesof time, money and expertise; and

3) select the tools to perform thetask.Selection of tools requires investi-

gation of available software pro-grams and languages, and of operat-ing systems under which these pro-grams and languages will run.

Various versions of MicrosoftWindows have come to dominatethe operating system market for PC-based data acquisition systems. Manydata acquisition software packagesare designed to run on more thanone operating system, but theWindows operating system is almostalways one of the available choices.

At one time the technical perfor-mance of Windows was significantlyinferior to other more “real-time”systems. More recent versions ofWindows, however, have largelyameliorated the real-time problem,either through the operating systemitself or through the use of variousoperating system extensions. These

extensions improve the real-timeperformance of Windows and allowfor deterministic scan times.

As in the office, the OS/2 andMacintosh platforms seem destinedto recede in market share, invest-ment, and new software applications.The relative technical merits of PCoperating systems were an interest-ing area for discussion several yearsago, but these considerations havereceded into the background asmuch of industry has standardizedon the Windows platform.

One big difference remainsbetween office and industrial oper-ating systems. There remains astrong market for low-end, embed-ded applications in the industrialarena. There is no analogy for this inoffice computer systems. Original

equipment manufacturers (OEMs)with product runs in the tens of

thousands need a low cost solutionthat addresses the task at hand withno wasted overhead. This marketcontinues to be addressed byDOS/386 machines, often in com-pact form factors such as PC-104 orSTD. OEMs with embedded hard-ware/software solutions often con-tinue to use just enough horsepowerto solve the task at hand.

Development ConsiderationsOnce the operating system isselected, the next decision to bemade is whether to write customsoftware or to buy an off-the-shelfpackage. Writing software is muchlike cooking dinner, and buying anoff-the-shelf package is like eatingat a restaurant. If you are good inthe kitchen, it may be better to

cook. The dinner may be less expen-sive—although a bit more work—


Development Considerations

Component Architectures

Off-the-Shelf Offerings

DATA ACQUISITIONPresentation & Analysis

6

PPresentation & Analysis

Off-the-shelf software packages provide the data acquisition systems developer with a range of

graphical user interface and signal processing tools.

and customization will be easier.The risks of cooking dinner (writing

software) are the very real possibilitiesof a bad cook (programmer), or badingredients (wrong programming plat-form), or both. Furthermore, it is verydifficult to determine if the dinner isbad until the cooking is finished. Onlythe most experienced master chefs(programmers) can determine theprobability of a bad meal (program)early on in the task. After the cooking(programming) is done, it is too lateand too expensive to start over. Eventhe best chef may not be able to res-cue a poorly prepared dinner.

The restaurant (off-the-shelf) is amore dependable and initially moreexpensive approach. Much dependson the reputation and reliability of therestaurant (software vendor). The bestrestaurants consistently deliver highquality meals, and stand behind theirofferings if things go wrong.

Custom software requires theinvolvement of at least two peopleat all stages of software develop-ment. The application expert mustunderstand the problem and transferthat knowledge to the programmer.The programmer must then write thesoftware and review the completedprogram with the application expertto close the loop. Training an appli-cation expert to be a programmer israrely feasible because the learningcurve is steep and is measured inmonths or even years.

Off-the-shelf software usually canbe configured by an applicationexpert. Training is required, but typical-ly is measured in weeks. This greatlyreduces development time becausecommunication problems and delaysbetween the experts and the program-mers are eliminated. Expenses also arelower because fewer people arerequired to complete the project.

Figure 6-1 shows some of the relativemerits of writing code versus the off-the-shelf approach.

The majority of applications arewell suited to off-the-shelf pro-grams. The chief drawback to theseprograms has been the high cost ofsoftware licenses. A full-featureddata acquisition program used tocost in the neighborhood of $2,000,but prices today are closer to $1,000and continue to fall. These lowprices mean that custom software’sprice advantage is only apparent inrare situations where the softwarecost is high relative to the total sys-tem cost and where product runsare in the thousands.

Off-the-shelf programs scorehigh in customer acceptance.Clients are much more apt to pur-chase a data acquisition system withname-brand software. In fact, tomost clients the software is the sys-tem because software is the frontend seen day in and day out.

The drawback (from a systemsintegrator’s point of view) to cus-tomer acceptance of off-the-shelfsoftware is the necessity of comply-ing with client demands for theirspecific brands of software. This canmake development and support

costs prohibitively high, especially ifmarket share is split among manysoftware vendors.

Off-the-shelf programs are attheir best in the related areas ofsupport cost, modification cost, andrisk of obsolescence. Assuming theright software vendor is chosen, auser can expect regular upgrades.These upgrades should accomplishthe following while protecting theinvestments made in existing appli-cation software:• Fix bugs: All software has bugs.Good vendors listen to their clientsto find these problems and promptlyissue bug fixes, or at leastworkarounds.

• Incorporate new features: Eachnew revision should add importantproduct features.• Provide customer support: Upgradesfor most data acquisition softwarecan now be purchased as part of anannual support agreement. Theseagreements generally require anannual investment of 10-20% of theinitial cost of the software. Thisinvestment yields regular upgradesand comprehensive customer sup-port including phone hotlines,Internet, e-mail, and newsletters.• Maintain backward compatibility.



Initial cost

Cost of subsequent copies

Customization

Cost to modify

Cost to support

Customer acceptance

Risk of unusable program

Risk of obsolescence

Speed

Use of system resources

ATTRIBUTE WRITE IT BUY IT

Low

None

Infinite

High

High

Low

High

High

High

Low

High

Relatively high (but falling)

Restricted (but improving)

Low if desired changes are part of upgrades, very high or impossible otherwise

Low if upgrades address bugs in a timely manner

High

Low if right vendor is selected

Low if right vendor is selected

Low

High

Figure 6-1: Cost of Writing vs. Buying Software

One area where off-the-shelf soft-ware has traditionally been weak isflexibility for customization. This ischanging rapidly as most packagesnow offer developer toolkits for cre-ation of custom features. Thesetoolkits allow a user to program spe-cial functions in a popular program-ming language such as C++. Thesefunctions can then be seamlesslyintegrated into the application pro-gram. Special software functionsmust be developed to comply withtoolkit specifications. This reducesthe cost of support because thetoolkit specifications standardizesoftware customization.

When is the write-your-ownapproach effective? An OEM with aplanned product run of thousandsof units would probably choose towrite a data acquisition program,especially if the cost of softwaremade up a large percentage of totalproduct cost. The high cost of sub-sequent copies of off-the-shelfsoftware would tend to outweighall other factors. Another situationnecessitating home-grown softwarewould be an unusual applicationnot addressed by commerciallyavailable programs.

Many of these types of applica-tions involve high-speed data acqui-sition. In these applications, systemresources must be concentrated onspeed and not on the overheadrequired to support off-the-shelfprograms. Another compelling rea-son to write your own software is tominimize the use of system resourcessuch as processing power and mem-ory. Many OEMs find that custom-written programs can perform quiteadequately on DOS/386 machineswith minimal memory. This can sig-nificantly reduce the total cost ofthe data acquisition system.

On the negative side, obsoles-cence is a major problem. This isbecause home-grown software mustincorporate the latest advances inthe field in order to remain competi-tive. Large software firms with thou-sands of customers can afford towrite such features into the nextrelease of their programs, but firms

not in the software business tend toput their development dollars intotheir primary products instead ofancillary software programs.

Another problem with customsoftware is high cost for modificationand customer support. Custom pro-grams usually are authored by a smallgroup of programmers, or even by justone person. A firm can become highlydependent on a very small group ofemployees, and if these employeesare no longer available, support canbe extremely expensive and difficult.Even when the original author ordevelopment team is available, pricesmay rise dramatically as they grasptheir negotiating leverage.

Component software claims toaddress many of the problems associ-ated with support and modificationof custom programs. The next sectionwill address the problems andpromise of this new methodology.

Component ArchitecturesSoftware written with object-ori-ented languages, also referred to as

component software, tends to beself-documenting and lends itself tostandardization. Component soft-ware uses programming languagessuch as Visual Basic, C++, or Java tocreate blocks of software. Eachblock of software performs a specif-ic function. These function blockscan be encapsulated together to

create procedure blocks. Functionblocks are then linked to each otheror to procedure blocks to create asoftware program.

Blocks of software usually can betested independent of one another.Once a block is written and tested, itcan be put into a library for later use.Blocks written by one programmer canbe used by another programmer to cre-ate a complete software program.Software blocks can also be down-loaded from various Internet addresses.

A data acquisition software pro-gram written with component soft-ware thus becomes a collection oflinked blocks of software. Becausethe operation of each block of codehas been tested and verified, pro-gramming only requires the correctlinking of these blocks.

As Figure 6-2 illustrates, compo-nent software can overcome many ofthe problems associated with cus-tom software. This figure wouldseem to indicate that componentsoftware is the ideal solution, com-bining the flexibility of custom soft-

Presentation & Analysis 6


High initial cost

High cost to modify

High cost to support

Low customer acceptance

High risk of obsolescence

DRAWBACKS OF CUSTOM SOFTWARE COMPONENT SOFTWARE SOLUTION

Software blocks written by others for similar applications can be reused; much of the initial programming effort is eliminated

New component software blocks can easily be added to existing programs

Can be controlled if original program blocks are tested properly

"Brand name" development environments such as Visual Basic increase customer acceptance

Risk is lessened because new features can be added by adding new blocks of software (assuming the original platform is still viable)

Figure 6-2: Advantages of Component Software

ware with the customer acceptanceof off-the-shelf programs. This is trueto some extent but only if the devel-opment platform remains viable anddiscipline is followed when the soft-ware is written and modified.

Long-term viability of the softwaredevelopment platform is critical. It ismore important to pick a strong brandname with staying power than to strivefor the latest features. If the selectedplatform vendor goes out of business,the software developer is in trouble.All programs developed with the dis-continued software platform immedi-ately become obsolete. Bugs are notfixed, industry advances are not imple-mented, and most importantly, pro-grammers begin to discontinue devel-opment of new software blocks.

The remaining problem with com-ponent software is the necessity ofmaintaining a disciplined approachto software development and sup-port. Relatively small blocks of soft-ware code are inherently easier tosupport than one large monolithicprogram, but these small blocks ofcode must still be written in a man-ner to allow subsequent modifica-tion and support. Software blocksmust be large enough to performimportant functions but not so largeas to become unwieldy. The blocksmust be developed in a prescribedmanner so as to allow communica-tions with other blocks. Judgmentand discipline still are required todevelop maintainable software.

• Objects & ContainersThe latest advances in object-orienteddevelopment for the Microsoft envi-ronment include objects based onMicrosoft’s Component Object Model(COM), and its distributed extension,DCOM. Formerly know as ActiveX“controls,” these objects are created

with development systems such asVisual Basic, Visual C++, BorlandDelphi, and Microsoft InternetExplorer. Once an object is created it iscompiled and tested within the devel-opment system. Each ActiveX objectfollows a prescribed specification,namely 32-bit Object Linking andEmbedding (OLE). This allows ActiveXobjects to run within OLE control“containers,” and to communicate witheach other and with other OLE-com-pliant software.

OLE control containers act asplatforms for COM/DCOM-basedobjects. These objects run insidethese containers, communicatingwith each other and with otherobjects within the container. AnyCOM/DCOM development systemcan act as a container. Other contain-ers of interest to users of data acquisi-tion systems include many of the lead-ing data acquisition software packages,

especially on the high end, as well ascommercially available word proces-sor and spreadsheet programs.

One object within a container

constitutes a working program, albeitnot a very effective one. It is onlywhen these objects are combinedthat useful applications are createdto solve real-world problems. Thestandalone characteristic of anobject is nonetheless importantbecause it allows each object to befully tested in isolation. This greatlysimplifies software debugging.

Each object is a block of code,much of which is invisible to the user.Users typically work with only thecompiled code and are not allowedaccess to the source code. Usersaccess the object through itsProperties, Events, and Methods.Methods are the least accessed ofthese parameters. Most objectdevelopers intend for users to inter-act with the object through itsProperties and Events. An object canappear as a push-button, an analogor digital display, or as a much more

complicated device.Properties are used to define the

visible attributes of the object. Forexample, a gauge might have the



Trending packages help unearth information buried in historical data.

Property of color. This would allow auser to change the color of the gauge.The power of the user to customizethe object is determined by the num-ber of Properties. Users want a largenumber of Properties to increase theirability to customize the object.

On the other hand, the developerwants to minimize the number ofProperties. The object grows in com-plexity and size as more Propertiesare created. The developer can alsolose money by creating too manyProperties. If the Property of color isnot available to the user and if theuser cannot use the standard color,the user must purchase anotherobject from the developer.

Events are the user-accessible itemsin the object that allow interactionbetween the object and the outside

world. For example, an Event for thegauge might be a change in the inputvalue. This would move the needle onthe gauge. Another Event would allowa user to set high and low alarm set-points for the gauge. Like Properties,Events create usefulness for the objectbut also add complexity and cost.

Leading vendors of data acquisi-tion software now are selling ActiveXobjects. These objects are sold insets and are meant to address partic-ular classes of applications. They aredeveloped either internally by thevendor, or developed by third partiesand private-labeled by the vendor.

Many of these vendors will sell theseblocks over the Internet.

In theory, a user will soon be ableto purchase a low-cost containerand fill it with the needed objects.These objects can then be intercon-nected to create the program. Theuser could then apply this softwareprogram to an application, orbecome a developer by selling thisapplication to others.

• Semi-Custom SoftwareA compromise approach to customsoftware development and ofteninflexible off-the-shelf packages arecommercially available, graphicallyoriented programming environments.These programming environmentstypically consist of a group of func-tion blocks, each dedicated to a data

acquisition or a data manipulationtask. Drawing tools and libraries ofstandard symbols are also providedto create presentation screens.

Users connect the function blocksto each other to create applications,and also fill-in-the-blanks withineach block to configure the softwarefor their specific application. Screensare then drawn using the provideddrawing tools and library symbols.Finally, screens are linked to functionblock fields to create animation andlive data displays.

One popular program offers func-tion blocks for communications,

comparison, data conversion, digitalsignal processing, data I/O, logic,math, string manipulations, wave-form generation and statistical calcu-lations. Free-form blocks also areincluded. The power of this type ofprogram is evident. Virtually anyapplication can be addressed ifenough system resources are avail-able and if high-speed data acquisi-tion is not a requirement.

The line between these types ofprograms and non-custom, off-the-shelf programs is sometimes murky.Costs tend to be similar, althoughsemi-custom software yields a widerrange of functions for the money.Figure 6-3 illustrates some of themain differences.

Semi-custom development envi-ronments are a middle-of-the-roadapproach between writing your ownsoftware and buying an off-the-shelfpackage dedicated to a specific task.One of the most important differ-ences is in the use of hardwareresources. Many limited function,off-the-shelf programs can run in aDOS environment on a bare-bones386 machine. This can be very attrac-tive to an OEM needing an embed-ded, low-cost solution.

Semi-custom programs by necessi-ty operate only in graphical environ-ments. These types of environmentsdemand the use of a high-poweredPC. Throw in a 17-in., high-resolutioncolor monitor, and you have a high-end solution to what may be a low-end problem.

The chief advantage of the semi-custom approach is high flexibilitycompared to off-the-shelf programs,coupled with relative ease of usecompared with writing code. If a userhas many different types of dataacquisition applications, or if the dataacquisition task cannot be clearly



Cost per range of functions

Cost of customizing for application

Ease of use

Customer acceptance

Risk of obsolescence

Speed

Use of hardware resources

ATTRIBUTE SEMI-CUSTOM OFF-THE-SHELF

Low

Moderate

Moderate

Moderate

Moderate

Slow

High

High

Low

Very easy

High

Low

Moderate

Moderate

Figure 6-3: Semi-Custom Development vs. Off-the-Shelf Packages

defined, semi-custom software mayoffer the optimal solution.

Off-the-Shelf OfferingsIf the decision has been made to gowith off-the-shelf software function-ality, it is important to remember thatmany data analysis programs do notoffer data acquisition. This necessi-tates the purchase of a separate dataacquisition program, and the integra-tion of this program with the dataanalysis and presentation program.

Another scenario dictating theseparation of data acquisition anddata analysis is when budget con-straints do not allow for the pur-chase of an integrated data acquisi-tion and analysis program. The lowcost option in this case is to procurea data acquisition software programonly capable of acquiring data, stor-ing data, and exchanging this datawith an application software pro-gram. These data are then madeavailable for access by any programthrough standard data exchange for-

mats such as DDE. These data-acqui-sition-only programs have limitedfunctionality and correspondinglylow costs. They are often suppliedwith data acquisition hardware andbundled into the hardware price.

A data-acquisition-only programcan interface to a spreadsheet pro-gram via DDE or OLE. The spread-sheet program then can display thesedata in either a tabular or graphicalformat. Data also can be analyzedusing the standard functions avail-able with the spreadsheet program.This approach can be used for anytype of data analysis and presenta-tion program.

Drawbacks to this approachinclude slow rates of data exchange,and a lack of tight integrationbetween the programs. Upgrades toone of the programs may cause diffi-culties with the other despite thepresence of industry standard dataexchange formats. Finally, vendors ofdata-acquisition-only software tendto be small in size compared with

vendors of data acquisition/analy-sis/display software—this heightensthe risk of obsolescence.

If these drawbacks are not critical,then it is possible to select the bestdata analysis and presentation pro-gram without regard to data acquisi-tion functionality. If a user must havean integrated data acquisition/analy-sis/presentation solution, thenselection of the best available soft-ware must be made from a more lim-ited range of offerings.

The task of acquiring data acquisi-tion software consists of five primarysteps, ideally followed in the ordergiven below:1) Define the required data acquisi-

tion, analysis, and presentationfeatures.

2) Determine if custom, semi-cus-tom, or non-custom software con-tains the required features.

3) Select the custom, semi-customor off-the-shelf software optionbased on the application and theavailable expertise.

4) If the custom option is selected,purchase the required tools andlocate the right programmer(s) andthe right application expert.

5) If the semi-custom option isselected, purchase the develop-ment environment and locate theright configuration expert andthe right application expert. Thiswould ideally be an applicationexpert with configuration trainingand/or experience.If the non-custom option is select-

ed, purchase the off-the-shelf pro-gram and locate the right applicationexpert. Have this application expertlearn how to use the program.

If the custom option is selected,features are only limited by the abilityof the programmer and the resourcesavailable. If the off-the-shelf, non-cus-



Graphical programming environments allow users to develop their applications by connecting lines

between icons and filling in configuration tables.

tom option is selected, a careful studymust be made of available productsto determine the software programbest suited for the application.

The first step is to determine whatdata needs to be collected, and whattype of analysis and presentation isrequired for the data. Options rangefrom simple plotting of an analogvariable versus time to extremelysophisticated mathematical analysisof multiple analog and discrete val-ues. Data analysis techniques can bedivided into three main areas asshown in Figure 6-4. Each of thesetypes of data analysis software will

be examined in detail in sections tofollow, starting with the most widelyused application: process monitoring.

• Process MonitoringProcess monitoring is the simplestand most widely used of the dataanalysis techniques. Typical applica-tion examples include:• Monitoring the setpoint, the out-

put, and the process variable ver-

sus time to aid in PID loop tuning;• Plotting one analog variable versus

a second analog variable;• Alarming variables when alarm

points are reached, or when a vari-able exhibits certain behaviors suchas a rapid change in value;

• Filtering variables to eliminatenoise problems;

• Plotting historical data, includingoverlaying current data on histori-cal data for analysis; and,

• Comparing sets of measured vari-ables to desired results (whethermanually entered or automaticallygenerated).

As its name implies, process moni-toring is typically used in processindustries to analyze the batch andcontinuous production of variousproducts. At the most basic level,software-based systems replacefunctions previously performed bypanel instruments. This is done bycreating a numerical and/or agraphical representation of thepanel instrument. For example, a

display of analog data is an emula-tion of a digital panel meter. Bar-graph meters become bar-graph dis-plays. A chart recorder is depictedas a graph with different coloredtrend lines replacing multi-coloredinks. If correctly implemented,these techniques allow an operatoronly familiar with panel instrumentsto become instantly familiar withdata displayed on a CRT.

Panel instrument replacement is abasic function of virtually all processmonitoring software. The softwarealso adds many other capabilities thatbuild on these basic functions.Displayed data can be stored for lateranalysis or record-keeping purposes.New methods of data visualizationcan be implemented to create physi-cal representations of plant processes.

For example, the liquid in a vesselcan be shown to rise and fall basedon the value of the correspondinganalog variable. Liquids and gases canbe shown to fill not only vessels, butalso transfer lines. The status of vari-ous on/off devices can be shown bychanging the color of the item, or byanimating the display. A pump rotorcan be shown to rotate if the pump ison. The range of display options islimited only by the imagination. Themain drawback of this flexibility isthe possible incoherence of the datadisplay. Good screen design isrequired to ensure clear understand-ing by plant operating personnel.

Another powerful technique avail-able with process monitoring soft-ware is the emulation of specializedlaboratory instruments such asoscilloscopes. The software can beused along with the PC and the dis-play to create a virtual instrumentcapable of equaling or outperform-ing its real world counterpart. Inaddition, the virtual instrument can



Off-the-shelf packages include prewritten objects that simulate instruments such as bar graphs,

gauges, and panel meters.

simultaneously emulate many dif-ferent types or instruments, result-ing in substantial cost savings.

The process monitoring featuresdescribed above emulate the func-tions of panel and laboratory instru-ments. Sophisticated mathematicaldata manipulation software opens upan entirely new realm of data analy-sis and presentation.

• Mathematical ManipulationIn-depth analysis of data oftenrequires the derivation of mathemat-ical relationships, either among dataor between data and other parame-ters. Many software programs areavailable to perform these types offunctions. Most of these programsare not capable of data acquisitionand only perform data analysis andpresentation. These data analysis andpresentation programs are designedto work in conjunction with dataacquisition programs. The cost ofthese programs is usually less thanfor programs combining data acquisi-tion with data analysis and presenta-tion. They tend to be widely usednot only for the analysis of collecteddata, but also for off-line analysis ofentered or stored data.

This class of software programscan perform calculations on eitherreal-time data (concurrent process-ing) or stored data (post processing).Multiple channels of data can beplotted against time, frequency, orother data. Zooming is supportedalong multiple axes. Plotted wave-forms can be analyzed to yield mini-mum, maximum, and average values.Successive waveforms can be aver-aged to reduce the effects of noise. Data can be stored to disk or otherstorage medium, either as raw dataor in a processed format using datacompression. For example, an analog

variable might be sampled once asecond and analyzed for variability.Only variations greater than one per-cent would trigger data storage.

Most of these programs supportstandard mathematical calculations ofarithmetic, trigonometric, calculus,logic, correlation, and basic statisticalfunctions. These functions can be per-formed on single data points or agroup of data points (waveforms).

One of the most popular mathe-matical data manipulation tech-niques is fast Fourier transform(FFT). FFT is used to convert time-domain data to the frequencydomain. This allows frequency spec-trum analysis of data. One leadingprogram calculates the amplitudeand phase for multiple channels ofdata. The difference in phase angleis calculated to provide the transferfunction, impedance, and frequencyresponse. The power spectral densi-ty is specified along with the rela-tive power for specified frequencybands. These calculated data canthen be displayed on graphs ineither linear of log format.

Another popular mathematicaldata manipulation technique is curvefitting. Software programs are avail-able to plot data on two (curves) orthree axes (shapes), and to analyzethese curves and shapes to derivemathematical expressions. Typicalcurve-fitting mathematical functionsare linear, polynomial least squaresup to the tenth order, cubic spine,

and point-to-point. User-definedexpressions can be compared tocurves to determine the degree offit. Curve-fitting equations, both

derived and user-defined, can beranked in terms of best fit.

For each set of data points thestandard deviation and the correla-tion coefficient can be calculated inaddition to the curve-fitted equa-tion. Equations can be stored, andthe recalled equations can be plot-ted against real-time or other storeddata. Most curve-fitting softwareprograms can also generate curvesand shapes from tabular data.

Some common applications forcurve-fitting software include the gen-eration of calibration curves forflowmeters, optimization of processcontrol parameters, and the genera-tion of logistic dose response curves(concentration versus time and dose)for pharmacological research. Built-inkinetic equation functions can be usedto study reactions and to describe bio-chemical responses. Kinetic functionsalso can be used to study the disap-pearance of a reactant (decay) and theappearance of a product (formation)over time. These kinetic models candetermine the actual order of the for-mation or the decay.

Another related type of mathe-matical data manipulation is peakfitting. Peak-fitting software pro-vides nonlinear peak separation andanalysis, primarily for spectroscopy,chromatography, and electrophore-



Process monitoring

Mathematical manipulation

Statistical analysis

FEATURE DESCRIPTION

Analog and discrete data are collected and displayed in a graphical format. Data is usually displayed on a graph versus time, or another variable

Data is analyzed using various high-level mathematical techniques such as fast Fourier transform (FFT), curve fitting, filtering, etc.

Data is analyzed using various statistical techniques including X-bar and R-bar charts, frequency histograms, Chi-square analysis, etc.

Figure 6-4: Primary Data Analysis Techniques

sis applications. Overlapping peakscan be located and separated to aidin analysis. Noisy data can beprocessed using advanced datasmoothing and manipulation tech-niques. Deconvolution techniquescan be used to remove the datasmearing often induced by dataacquisition instruments.

Functions specific to spectroscopyapplications include the identifica-tion of different nonlinear spectralapplication line shapes includingthe Gaussian, the Lorentzian, andthe Voigt. Information reported foreach peak includes amplitude, area,center, and width data. Overall areais determined by integrating thepeak equations.

Chromatography applications aresupported by equations such as theExponentially-Modified Gaussian,the Haarhoff-Van der Linde, the NLC,and the Giddings. Data reportedinclude column efficiency, resolu-tion, first moment, second moment,and center as well as peak width at

base and half maximum, and asym-metry at base and 10% of maximum.Electrophoresis applications can beanalyzed by separation of up to 100peaks, even if they overlap.

New software programs are con-stantly being released to addressspecific types of mathematicalanalysis applications. Another area

where data acquisition software iswidely used is for statistical analysis.

• Statistical AnalysisStatistical analysis software is wide-ly used in industrial applications forquality acceptance, quality control,and statistical process control (SPC).These types of software programsanalyze data to derive informationwith respect to the statistical rela-tionships between or among datapoints. The most common support-ed functions include the calculationof the mean, the standard deviation,the range, the moving average, themoving range, and the cumulativesum. Other supported statistical

functions include Chi-square analy-sis, Pareto analysis, distributionanalysis, non-parametric analysis,correlation, single variable regres-sion, multiple linear regression, andrandom number generation.

Many different types of controlcharts can be generated including X-bar, R-bar, histograms, and probabil-ity plots. These functions and chartsare used in a wide variety of indus-trial applications.

A typical SPC application wouldbe to measure the weight of a boxof cereal. The actual weight wouldbe compared to the desired weightand also to the boundary limit highand low weights on an X-bar chart.The software program would pro-duce alarms to show a single unac-ceptably low or high weight; unac-ceptable weights could also beanticipated by alarming certainconditions such as successiveweights trending towards a bound-ary limit, successive erratic weightsalternating between high and lowlimits, or a number of sequentialmeasurements close to a high or alow limit. Anticipation of an alarmcondition could allow plant opera-tions personnel to correct a prob-lem before it occurs, thus avoidingunacceptable product.

Many of these software programswere originally written for applica-tions not involving data acquisition.Data was expected to be enteredmanually or imported from othersoftware programs. Support of datainterfaces is often minimal.Fortunately, there are hardware andsoftware methods available toaddress this issue.

• Data InterfaceOnce a decision has been made asto what type of data analysis and



Object-oriented software packages allow software blocks to emulate the functions of a traditional

instrument such as the trend recorder shown here.

presentation is required, the nextstep is to ensure a compatible inter-face with data acquisition instru-ments. Ideally, software selectionwould be made first based on thefeatures and options offered by theselected software vendor. The dataacquisition hardware would then beselected based on compatibilitywith the software.

This approach is not always viablefor two reasons. One, the data acqui-sition hardware may already exist ormay already have been purchased.Two, the type of data to be acquiredmay be specialized in nature andrequire the use of certain instru-ments with a limited number of datainterface options.

Software programs acquire data by

interfacing to data acquisition hard-ware, either plug-in cards or stand-alone systems. Communication toplug-in cards is via the computer bus,and communication to standalonesystems is via industry standard serialdata exchange formats like RS-232, RS-422/485, and IEEE-488. The selectedsoftware must be compatible withthe data acquisition hardware, or abridge must be used.

The bridge may be either a softwareor a hardware solution. A hardwaresolution accepts inputs from fielddevices and instruments and convertsthese inputs into outputs compatiblewith the selected software. A softwaresolution processes input data andmakes these data available to the datapresentation and analysis programvia a standard dynamic data transferformat such as DDE. T




LLC, 1997.• Omega Universal Guide to Data Acquisition and Computer Interfaces,

Omega Press LLC, 1997.• The Software Connection, 97.1 & 97.2, Rockwell Software, 1997.

Once data has been acquired,there is often a need tostore it for current andfuture reference. Today,

alternative methods of data storageembrace both digital computermemory and that old traditionalstandby—paper. Covering in anydetail the scores of instruments onthe market today for recording, log-ging, printing, and storage of data isfar beyond the scope of this section.The aim here is to identify and definemajor categories and offer theprospective user some practicalguidelines to a selection best suitedfor the application.

Broadly speaking, there are twoprincipal areas where recorders ordata loggers are used: • Process measurements for such

variables as temperature, pressure,flow, pH, humidity; and,

• Scientific and engineering appli-cations such as high-speed test-ing (e.g., stress/strain), statisticalanalyses, and other laboratory oroff-line uses where a graphic ordigital record of selected vari-ables is desired.Instrumentation for each of these

fields can have many operating prin-ciples and features in common. Eacharea, however, may place moreemphasis on the importance of suchfactors as cost, method of use (e.g.,benchtop vs. panel mounting), porta-bility, degree of accuracy, speed ofresponse, and number of points(channels) to be measured.

For some time now, digital com-puter systems have had the ability toprovide useful trend curves on CRTdisplays—displays that could be ana-

lyzed and printed out. Today, a simi-lar approach permits digital inputs torecorders and data loggers to bestored electronically on-board orsent to a remote PC, distributed con-trol system (DCS) workstation, orfull-color printer.

There is now a plethora of dataacquisition hardware and softwarethat can work with PCs—making itquite plausible to classify them asrecorders or, alternatively, data log-gers. These are referred to as PC-based solutions.

There remains a continuing role forconventional standalone recordersthat still use paper and pen or print-er, but here, too, such productshave gained greatly in capabilitiesthrough advances in electronics andnew technologies such as inkjet andlaser printing.

New technology has also madepaperless, or videographic, recorders

a practical reality, with attendantsavings in materials and maintenance.This mushrooming class of instru-ments mimics conventional trendrecorder displays; it also has thecapabilities of a data logger and can

export data to a PC or DCS worksta-tion, in real-time or periodically forlater analysis.

The traditional data logger, whoseoriginal basic function was to printout digital values of a measured vari-able versus time, also has benefitedfrom new technology that has addeddisk or card memory storage andfaster logging capabilities. Some canprovide real-time recording via net-work connection to a host system.

In recent years, conventional ana-log recorders have had the functionsof a data logger added to their capa-bilities. Called “hybrid recorders,”they combine analog trend represen-tations with digital information onthe same chart paper.

Scores of specialized softwarepackages are offered not only toimplement the PC-based functionsmentioned above but also to per-form complex mathematical opera-

tions or signal conditioning on inputand output data.

For high-speed recording, the tra-ditional analog oscilloscope has alsobeen enhanced by a digital storageversion (DSO), which finds an impor-


Trend Recorders

Data Loggers

Videographic Recorders

DATA ACQUISITIONRecording, Printing & Storage

7

ORecording, Printing & Storage

Figure 7-1: Typical Strip-Chart Recorder Trace

0 1 2 3 4 5 6 7 8 9 10 11

3

4

Measured Variable (y coordinate)

Tim

e (x

coo

rdin

ate)

tant application in test and analysisof today’s many electronic prod-ucts—both consumer and industrial.

The moral of this overview? Thereare no longer sharp distinctionsbetween many of the products thatserve the “front end” of a data acqui-sition system. In fact, the breakneckevolution of computer and network-ing technologies will continue tomeld alternative offerings for processmonitoring, data acquisition, and testand measurement applications.

Definitions & ClassificationsThe first terminology distinction todraw is that between a recorder anda data logger. Acknowledging thatsharp lines no longer exist, considerthe fundamental ways in which eachtype of recording instrument func-tions to store, analyze, and recorddata. A traditional recorder acceptsan input and compares it to a full-scale value. The recording pen is thenmoved to a point on the scale thatrepresents a useful value of theinput—say, 500°F with a chart rangeof 0 to 1000°F—at a time that is givenby the calibrations on the recorder’smoving chart (Figure 7-1).

On the other hand, a data loggeraccepts an input which is fed intoan analog-to-digital converter andstored digitally, or printed out as aseries of time-stamped values. Today,many such instruments have someform of on-board intelligence whichprovides the user with diverse capa-bilities for signal conditioning as wellas data storage and analysis.

As an alternative to a recorder, adata logger can have the ability toaccept a greater number of inputs(referred to as channels), with betterresolution and accuracy. Only arecorder, however, can provide atruly continuous trend display of the

variable’s change with time.The term graphic recorder more

aptly describes the majority of

recorders used in data acquisitionsystems. It is defined as an instru-ment which, in some automaticfashion, draws a graph. Such a graphmay relate two or more variables totime, or to each other (the latter,typified by an X-Y recorder).“Graphic” is also used widely todayin reference to CRT displays, manyof which do not qualify as graphs.Graphic coordinates hearken backto basic geometry, where curves areplotted on a rectangular coordinatematrix. The strip-chart recorder, socommon in a great variety of formstoday, had its origin in this basicconcept (Figure 7-1). Note that thevertical coordinate (time) is the Xaxis of a graph and the horizontalcoordinate (measured variable) isthe Y axis. The heavy vertical line inthe center of the chart is the curveplotted by the recorder (oftencalled the chart record or trace).

Recorders can have adjustable

chart speeds—fixed, once config-ured—to suit the application. Thus, at4 (o’clock) in Figure 7-1, an operator

can see that the temperature was625°F. He or she can also note that thetemperature has remained rathersteady. The trend is often as impor-tant, or more important, than the pre-cise value. Thus, this type of instru-ment is often called a trend recorder.

With an X-Y recorder, the timecoordinate X is replaced by someother variable, such as stress, plottedversus strain. Other versions of thistype may have two different vari-ables, Y and Y’ plotted vs. X or havetime (t) also as one of the variables.

Circular-chart coordinates (Figure7-2) were developed early in recorderhistory to utilize the convenience ofa spring-wound clock and synchro-nous motor movements. Dependingon application needs, the circularchart speed can be one rotation in 24hours, one rotation in a week, oranother arbitrary time interval. Notethe advantage of a circular chart ingiving at a glance a complete history

Recording, Printing & Storage7


Paper-based recording devices are available in a wide range of configurations.

of one or several variables over aperiod of time—albeit not with ahigh-accuracy readings.

Chart sizes are referred to bytotal diameter, such as 10 inch or 12inch, although full-scale pen move-ment is often much less. Becausemany plant operators are used toreading circular chart recorders,suppliers still find a strong marketfor them.

From a commercial catalogingstandpoint, graphic recorders aregenerally classified by their commonusages or physical appearance, suchas strip chart, circular chart, or spe-cial purpose (X-Y, event, or oscillo-graphic). A further broad distinctionis whether the instrument is flatbedfor benchtop mounting or vertical,for panel or rack mounting.

Data loggers, too, can be classifiedas benchtop units; others aredescribed as panel-mount printersand still others are “vertical” bench-top units. Smaller, handheld unitsalso are available.

As with any method of classifica-tion, such names leave gray areas. Forexample, some flatbed types utilizestrip charts and some are vertical.Many data loggers are bench-top,but some are vertical and panel-mounted. And videographic, orpaperless recorders can provide

screen displays that look like a stripchart or a circular chart.

Trend RecordersVertical strip-chart recorders headthis category, embracing many typesthat are commonly designed for per-manent installation and are normallymounted vertically—usually in panelcut-outs for process measurementsbut also on relay racks as for labora-tory measurements. Size is one dis-tinction of this class, with chartwidths ranging from 100 mm to 10inches (250 mm) or more.

Other terms used to describe ver-tical strip-chart recorders includeelectronic, hybrid, and multi-point.Certain models can record up to 32channels, although common designsfor process measurements normallydo not exceed 12. Instruments in thisclass are designed to plot variablesover long periods of time, utilizingpaper take-up rolls or trays for Z-foldpaper. Chart speeds, programmablein most cases, normally range from1 cm/min to 30 cm/min.

This category contains many varia-tions on the basic principle of strip-chart recording shown in Figure 7-1.One extensive catalog reference cov-ers 29 different models; a majority ofthese are for the measurement ofprocess variables, but there are also

numerous designs customized for sci-entific and laboratory applications.

The impact of the microprocessoris obvious in many strip-chartrecorder models, with digital storageof data points a common advancedfeature. Another result of new tech-nology is the ability to handle a vari-ety of variables, such as flow, alongwith temperature and pressure.

Another result of microprocessortechnology is the hybrid recorder,which provides high-speed recordingof up to 30 recording channels aswell as digital measurement data. Onthese units, the digital information issometimes printed on the left marginof the chart in up to six colors.Typically, some form of keyboard onthe front panel (or accessible withthe door open) is used for program-ming of chart speed, alarm setpoints,full-scale range, and other features.



Flatbed recorders offer a portable, flexible

alternative to permanently mounted units.

Figure 7-2: Typical Circular Chart Recorder Trace

Time

2

3

Hole to Accept Chart HubPrinted Line for Manual Entry Of Date

Hole to Accept Chart Positioning and Drive Pin

20 30 40 45 50 55 60 65

19

Some of the features of strip-chartprocess recorders can include:• 1 to 4-pen continuous writing and

up to 24-point dot printing;• Memory card for recorder setup

and data storage; and,• Digital display, horizontal bar

graph, and digital totalizer foreach channel.In large case, 250-mm recorders,

additional features may include:• Interactive, menu-driven program-

ming;• Trend printing with user-assigna-

ble colors; and, • Ability to print up to 60 analog

input channels. Flatbed and portable versions also

are available, more suited for labora-tory or test uses. Some include signalinput modules which plug into com-partments on the recorder so that,for example, they can accommodatethe direct input from a thermocou-ple. Some models have as many as sixsuch plug-in input modules and arereferred to as modular recorders.

One recorder with a capabilityto handle 1,600 samples/secondemploys a thermo-array recordingsystem, with 200 dots per inch (dpi)resolution on a 200-mm wide ther-mally sensitive chart. Referred to astherographic recording, this methoduses a linear array of fixed thermal-printing elements (stylii) across thechart width. A heated stylus affectsthe heat sensitive paper to producethe chart record. The chart grid sys-tem is generated by the printer.

A portable, benchtop model thathandles 10 or 20 channels uses a print-ing technique described as raster scan.With this printing method, therecorder can handle high-speed scan-ning of 20 points/second and high-speed recording of 50 points/second.

A highly-specialized “transient”

recorder with a 170-mm wide thermal-ly sensitive paper chart is offered torecord signals for which standardrecorders are too slow. It offers digitalsampling and storage up to 1 MHz. It

has a modular design that allowsexpansion to 30 channels. Its 16 Kmemory per channel can store up to16,000 samples and its standard analogoutput can directly interface to anoscilloscope or strip-chart recorder.

• Flatbed Recorders For testing and other laboratorywork, flatbed recorders are offeredwith a variety of switch-selectable,full-scale ranges to accommodatedifferent inputs. Designed to meetvarious application needs, they typi-cally offer a range of chart speedsfrom centimeters/minute to cen-timeters/hour. A typical slow speedis 1 cm/hr; a fast speed generally is60 cm/min.

Most flatbed recorders, includingX-Y types, record with ink, using

fiber-tipped, disposable cartridges.Chart width is typically 100 mm; larg-er models range from 200 to 250mm. Notable is the instrument’sspeed—typically 0.4 seconds full-

scale response for this class.Offerings in various flatbed mod-

els include: • Ability to accept a variety of inputs,

such as from thermocouples andRTDs, ac/dc voltage/current, andrelative humidity—with availableinputs depending on the model;

• Two and three pens with pen off-set compensation. (One model hasfour channels with a choice of dis-play modes: measured data, bargraph, or range data);

• Widely adjustable chart speeds.One portable, battery-operatedmodel for recording temperatureand relative humidity comes withchart paper pre-printed for chartcapacity of 1, 7, and 32 days;

• Built-in 3.5-in. digital (LCD) dis-play (on a one-channel model)



Many trend recorders include a digital display of the variable in question as well as a long term

paper record.

indicating true value of input;• Protocol printing on chart to per-

manently record measurementrange, chart speed, etc.;

• Built-in remote control of paperadvance, pen lift (automaticallylifts the pen after 30 seconds ifchart is not advancing), and eventmarker; and,

• One model type, called a “func-tion recorder,” is offered with upto three channels, each of whichcan have its own plug-in inputmodule to equip the recorder forspecific inputs types.More sophisticated units add

RS-232 serial or IEEE-488 parallelcomputer interface to handle dataoutput and remote configuration,3.5-in. disk drive for data logging, anddigital data print-out.

• XY RecordersXY recorders, though usually flatbedin design, are really a class by them-selves. Most employ a potentiomet-ric pen-positioning system, but somenewer models feature a digital servosystem. The instrument’s basic func-tion is to plot the relationship of twoinput variables with no regard fortime. Input signals to the recordercan be analog or digital, with suitableconditioning as required. Commonsizes for the chart paper are 8.5 by 11and 11 by 17 inches.

In addition to features as describedabove for flatbed recorders, optionsavailable in certain X-Y recordersinclude:• Ability to also plot either the X or

Y inputs versus time. Such a capa-bility is expressed as X-Y, Y-t and X-trecording. The mode to be used isselectable on the instrument’sfront adjustment panel.

• An additional servo-actuated mea-suring element and recording pen

yields an XX’Y or XYY’ recorderthat can plot the relationshipamong three variables.

• To distinguish different chartrecords plotted on the same chart,one design with a digital plottercan use eight different color pens(disposable felt tipped). Penchange is selected manually or viaRS-232C interface.

• Recorder responsiveness is express-ed in a “slewing” speed along eachaxis. The speed may be, say, 0.6

meter/second for the X-axis and0.8 m/s for the Y-axis.Some thermal-array recorders also

include an X-Y recording mode. Suchinstruments offer much higher real-time frequency response and theability to print many Y channelsagainst a single X channel. Some

thermal-array recorders include amemory X-Y mode, so that the usercan capture high-speed transients forlater X-Y replay.

• Circular Chart Recorders Basic characteristics of this type ofrecorder were covered with referenceto Figure 7-2. In this class, too, themicroprocessor has had its beneficialeffects. Recent developments includecircular chart recorders that use blankcircular chart paper, printing the

scales and traces for up to four chan-nels, as recording progresses.

One of the newer types uses heat-sensitive paper on which a single sty-lus printhead produces up to four ana-log traces and also prints alphanumer-ic chart data and time lines. The instru-ment is user configurable via English



Today’s circular chart recorders come equipped with microprocessors and offer an array of display,

recording, and control functionality.

language prompts. Another unit can beprogrammed to print the equivalent of10-inch, 11-inch, or 12-inch diametercharts. It is fully configurable from a PCor a keypad and prints straight timelines (not curved, as conventionalrecorders have used for many years).

Other capabilities that the micro-processor has brought to the circu-lar-chart recorder include: • Ability to handle a variety of

inputs such as all common typesof thermocouples, RTDs, dc volt-ages and currents;

• Control outputs (including PID)and adjustable alarms settings;

• Flow totalization, including datalogging of totals; and

• Retransmission options. One model, custom-designed for

measuring and recording tempera-ture and relative humidity, uses adouble-sided, 200-mm (8-inch) chart.Available chart capacities are 1, 7, or32 days. Here again, an embeddedmicroprocessor affords capabilitiessuch as digital display with user-selectable read-out of temperaturein °F or °C, relative humidity, andalarm setpoints.

Data Loggers Based upon the way data is recordedand stored, there are two types ofstandalone data loggers. One typestores the data in memory to beretrieved at a later time; the othertype automatically records the dataon paper for immediate viewing andanalysis. Many designs combinethese two functions, with one or theother of primary importance

The discussion here is confined tovarious types of standalone units. Inaddition, there is a wide variety ofsoftware packages that can beloaded into PCs, DCSs, or other com-puter-based devices to provide

excellent data-logging and digitaldata storage capabilities.

Note, too, that hybrid recorders,as mentioned earlier, serve many ofthe functions formerly reserved fora separate data logger. Conversely,

modern data loggers also can pro-vide visual displays and digital read-outs. They also can perform com-plex mathematical operations onincoming data.

Functional highlights of a typicalprogrammable, multi-functional datalogger include:• Automatic measurement, digital

display, recording (storage), andprinting of data from a variety ofinputs (e.g, thermocouples of dif-ferent types and voltage sources).

• Wall, panel, and bench-top config-urations.

• Programmable parameters includ-ing date and time, print interval,

contents and format of print-out,configuration of each channel, highand low alarms, and much more.

• Two-way communication via RS-232serial port—for programminginputs and data outputs via a

remote PC or other system. • Optional alarm boards with high

and low relays for each channel. • Built-in thermal printer with exter-

nal trigger control.• Data cache with buffer. • Graphical software for an associat-

ed PC. Not all standalone units, of

course, offer all such features. Someare smaller and more portable, usingbuilt-in memory to store data forlater review. These usually include anRS-232 serial port for data downloadand a digital display.

Some models are battery-operatedfor easy portability in field applica-



This portable data logger handles up to 16 channels and can download data to Windows-based

programs for later analysis.

tions. One variation is a self-con-tained unit that can, for example,profile temperatures as it passesthrough an in-line bake oven.

Oscillographic RecordersUsed primarily for applications in thetest and research fields, the capabili-ties of oscillographic recorders andthe newer digital oscilloscopes haveexpanded greatly over the past sev-eral years. An oscillograph is a devicefor determining waveforms by plot-ting instantaneous values of a quan-tity such as voltage as a function oftime. A decade ago, this impliedeither a recording galvanometer or aCRT recorder—analog instrumentsthat afforded the needed band-widths in excess of 20 kHz.

As in other recorder develop-ments, however, digital is the buzz-word today and digital storage oscil-loscopes (DSOs) or simply digitaloscilloscopes have proliferated.These may be defined as oscillo-scopes that digitize an input signalfor storage in memory for later dis-play or analysis. It is a logical and rel-atively simple step to use the storeddata to provide a chart record, andmany DSOs do just that, essentiallyacting as data loggers.

A recent survey lists some 30 differ-ent suppliers of DSOs, many of themPC-based. They cover a range of band-widths—some around 40 to 50 MHzwhile others go as high as 350 MHz.These are sophisticated electronicinstruments that have capabilities farbeyond traditional analog CRT-basedoscilloscopes, which have beenaround for many decades.

• Analog Oscilloscopes An early form of oscillograph, alsocalled a “light-beam recorder” usesa mirror connected to the rotating



Given the wide variety of instruments for recording or logging acquired data, the prospective user

needs to review the key factors involved in making a suitable selection. To help, here are some

key questions and basic points to consider:

1) How many inputs are to be recorded or logged? These are often called channels, points, or records.

2) What types of inputs are involved? Voltages, T/C or RTD, 4-20 mA.

3) Do different input types need to be handled by the same unit?

4) What type of recording or logging is required—continuous single-point or multi-point? And if

the latter, what minimum scan cycle is required?

5) What degree of accuracy is required? Accuracy may be defined as the closeness to the actual

signal that the measured value or trend position takes—stated in either a plus/minus per-

centage of full scale or percentage of reading.

6) Is a communication interface required to transmit measured data to a host computer? Set up

the instrument remotely? Connect to an external printer?

7) Is the instrument to be mounted bench-top or in a panel?

8) What type of instrument power is available?

9) Is log-type recording desired instead of or in addition to continuous trend recording?

10) Is color differentiation desirable for multiple trend lines?

11) If the instrument is to perform alarm functions, how many alarm setpoints are needed per

channel? What types of alarms are desired—e.g. threshold, rate, or delta? Are relays contacts

required to handle external alarm outputs? If so, how many?

12) What, if any, signal processing is required? These may include linear scaling (conversion to

engineering units), thermocouple characterization, difference calculation, square root calcu-

lation (as for an orifice flowmeter).

13) Are any higher-level math functions required? These may include absolute value, logarithm,

exponential, maximum, minimum, time average, group average, summation, standard devia-

tion, and integration functions.

14) For conventional, paper-based recorders, what writing method is preferable? Capillary ink?

Disposable felt-tip ink cartridge? Dot printing, using an ink ribbon cassette or pressure sensi-

tive paper? Thermal moving head or stationary linear array? Rotating ink wheel?

15) If a strip-chart recorder is used, is a Z-fold or roll type of chart desired? Z-fold has become the

primary choice for process applications because of the ease with which an operator can

review past traces without disrupting active recording.

16) Should the chart speed be fixed or programmable?

17) What kinds of annotations on the chart are desired? Tags, lists, engineering units, messages,

scales, channel identification (numeric or alphanumeric), date and time, chart speed, and

instantaneous, “snapshot” values. Continuous writing instruments provide the annotation by

a separate printhead.

18) For a strip-chart recorder, what chart width is desired—100 mm, 180 mm, or 250 mm, other?

19) What secondary visual indications are desired? Analog bar graph in percentage of full scale?

Analog scale indication in percentage of full scale? Digital channel (point) number and mea-

sured value? Alarm status? Engineering units?

20) What operating modes will be required? Options include normal monitoring, which involves mon-

itoring at a set scan interval and trend recording at a set chart speed, or logging at set intervals;

print on alarm, which involves monitoring at a set scan interval but not trend recording or logging

until an alarm condition occurs; and change on alarm, which allows tending or logging at a base

chart speed or log interval when no alarm condition exists and an automatic switch to an alter-

nate (usually faster) chart speed or log interval when an alarm condition exists.

20 QUESTIONS IN RECORDER SELECTION

armature. An input voltage causes adegree of armature rotation which,in turn, moves the mirror a propor-tionate amount. A light source, usu-ally ultraviolet, is reflected off themirror onto a light-sensitive paperwhich moves at a fixed chart speedlike a strip-chart recorder. Theresult is a series of light spots (datapoints) on the recording chart.Frequency response of this device issome 25,000 Hz.

A step in the advancement ofrecording analog oscilloscopes isthe CRT-based instrument. Theseinstruments can display not onlyinstantaneous values of voltage vs.time, but using suitable input trans-ducers, can convert other vari-ables—such as current, strain, accel-eration, and pressure—into voltagefor display on a CRT.

The CRT display is a graphical rep-resentation of electrical signals pro-duced by varying the position of afocused spot where an electron beamstrikes the fluorescent (phosphor)coating on the inside surface of theCRT face. This type of oscilloscopeover the years has benefited fromadvances in solid-state, semiconduc-tor electronics, as well as improve-ments in phosphors which are alsoused today for CRTs in other applica-tions. In fact, some CRT-based termi-nals and workstations are, in essence,highly-sophisticated oscilloscopes.

One typical fiber-optic DSO can beadapted to record over 30 channels.Records that are relatively insensitiveto normal room light are produced byultraviolet light generated by the CRT.The UV light is directed through a spe-cial CRT fiber-optic faceplate ontophotosensitive direct print paper.

The CRT faceplate consists ofmillions of tiny glass fibers, fusedtogether within a 5- by 200-mm

(0.2 - by 8-inch) area. These fibers arethe means for transfer of ultravioletlight from the phosphor inside thefaceplate to the photosensitiverecording paper—enhancing writingspeed and trace resolution.

Note that the recorder has fouraxes: an X axis (time base or horizon-tal), Y axis (amplitude or vertical), Zaxis (spot intensity), and Y axis(movement of recording medium).The four axes provide an instrumentthat is very versatile. For example,video pictures from television or fac-simile can be recorded by use of theZ-X axes, together with either the Yor Y axis. The quality of the pictureswill depend on such factors as sensi-tivity of the recording medium, band-width of the Z-axis amplifier, and thespot size that can be recorded.

• Oscilloscope SelectionThere are many technical details inoscillographic recorder or digital

oscilloscope designs that can only behighlighted here. Some of the termsused in evaluating and selecting onetype versus another are: • Bandwidth: In data communica-tions, this term means the differencebetween the highest and lowest fre-quencies of a transmission channel.With oscilloscopes, it is expressed inMHz. One supplier defines band-width as the maximum frequencythat a scope can capture withoutreducing the amplitude by more that3 dB. Generally, the greater the band-width, the more waveform informa-tion can be sent through in a giventime. With such increases, however,can come an increase in cost andcomplexity. DSOs can have a band-width as high as 500 MHz at a rela-tively low cost, whereas similar per-formance in an analog unit is notreadily available.• Trigger Capability: Triggering thedevice means initiating the measuring



Extremely compact, battery-powered data loggers can even ride along through a process for accurate

temperature and humidity profiling.

function at appropriate times versushaving the instrument running all thetime. Flexibility of the device in trig-gering extends its ability to capture,for example, any aberrations thatoccur on a signal or group of signals.• Mode of Operation: An analogoscilloscope is best for displayingsignals with high information con-tent, such as video. This is due to itshigh real-time screen update charac-teristic. On the other hand, a storage

oscilloscope may be better suited fortransients, alphanumeric display,curve traces, etc. • Analog and Digital Storage: Severalmanufacturers offer oscilloscopesthat combine both analog and digitalstorage in one unit. The display flexi-bility this affords is well suited to gen-eral purpose applications, from singleuse to complex signal handling. • Sampling Rate: In a DSO, this isthe maximum speed at which the ana-log-to-digital converter samples theincoming waveform. It is usually spec-ified in millions of samples per second(MS/s). The Nyquist theorem statesthat a signal must be sampled at least

twice as fast as the highest frequencycontent of the signal, therefore theuser must always be conscious of hav-ing enough sampling points to recon-struct the waveform accurately. • Memory Length: This is the num-ber of samples that a DSO takes eachtime a waveform is captured. • Vertical Resolution: In a DSO thisis the number of digitizing levelsfrom the A/D converter; 8-bit res-olution gives 256 steps and 12-bit

resolution gives 4,096 steps. • Interface for Remote Control:Most DSOs have interfaces (IEEE-488or RS-232C) that allow them to beremotely configured and controlled.

A basic characteristic of an analogscope is the variable intensity of thedisplayed trace. Changes in tracebrightness and line width result fromthe fact that light emission from theCRT phosphor coating is proportion-al to the speed of the electron beamexciting it. Operators have learned tointerpret and rely upon intensityvariations to the extent that somemanufacturers of combined analogand digital oscilloscopes recommend

that the user switch to analog modeif there is any doubt about a wave-form’s true characteristics.

There are other reasons for usinganalog oscilloscopes, but DSOsoffer three capabilities which leadto their preference in many applica-tions. These include the ability tocapture and display very slow sig-nals, the conversion of signals intodigital form for further analysis, andthe ability to more easily handlehigher bandwidths.

In short, today’s digital oscillo-scopes are marvels of modern, state-of-the-art technology—offering testand research technicians and engi-neers extremely versatile tools.

Videographic RecordersEvidence of the many benefits ofvideographic, or paperless, recordersare the large number of suppliersthat now offer these types of instru-ments. Some are relative newcomersto the recorder field, utilizing theirexpertise in electronic technology tomeet user needs. One example is therecent offering of a circular chartvideographic recorder with a 10-inchscreen display—equivalent in size tothe common paper chart versions.

Display technology has been get-ting better and better in sharpness ofdetail, with a wide range of brightcolors, similar to the proven qualityof a newer PC. Some models offerthe flexibility of plotting recordshorizontally or vertically. A commonscreen size on such recorders issmall—on the order of 76 mm wideby 38 mm high (3 in. x 1.5 in.).

Some suppliers offer a 5-inch widedisplay area or larger. Most aredesigned for panel mounting andcome in a size as small as 1/4 DIN. Inaddition to displaying trend recordsor bar graphs, the instruments can be



Videographic, or paperless recorders provide trend indication without the maintenance requirements

of pen-and-paper systems.

configured at will to provide largedigital displays of the measurementsin engineering units or small digitalreadings along with the trend curves.

These recorders have some formof storage media—either a 3-1/2 inchfloppy disk or a PCMCIA card—whichcontinuously stores all the measure-ment inputs as well as the recorderconfiguration. From such stored data,an operator can use a touchscreen ormembrane keypad to provide such

display functions as: a split screen(two trend windows), zooming, face-plate displays, alarm summaries, andselected trends (such as six out of upto 24 inputs).

The recorder’s disk or card can beremoved and used to download allits accumulated data to a PC that hasthe necessary software. An engineercan bring up any of the stored datafor review and analysis, zooming inon a desired time, say, of a processupset (as well as just before and justafter the event).

Videographic recorders may includea serial port that permits data to besent, for example, over an RS-422/485communication network directly to a

PC or DCS with suitable software. Thiscan be two-way communication. Thus,an operator at a PC in a control room

could monitor the recorder readings inreal-time, call for specific data, andeven reconfigure the recorder. T



References and Further Reading• The Data Acquisition Systems Handbook, Omega Press LLC, 1997.• New Horizons in Data Acquisition and Computer Interfaces, Omega Press


Omega Press LLC, 1997.• “Boards, Recorders, and Software Ease Data Acquisition, Temperature

Recording Tasks,” Peter Cleaveland, I&CS, August, 1997. • Dictionary of Measurement and Control: Guidelines for Quality, Safety,

and Productivity, 3rd ed., ISA, 1995.• “DSO—Digital Storage Oscilloscopes,” T. Lecklider, et al, Measurements &

Control, October, 1996.• Encyclopedia of Instrumentation and Control, Douglas M.Considine,

McGraw-Hill, 1971.• Instrument Engineers’ Handbook, Third Edition, Bela G. Liptak, Chilton, 1995.• Instrumentation Catalogue, Measurement and Automation, National

Instruments, 1998.• “Melding Markets Via Evolution,” M.P. Minneman and W. H. Manning,

InTech, February, 1997. • “Monitoring and Data Collection Improved by Videographic Recorder,”

Water Engineering & Management, November, 1995.• “New Breed of Industrial Process Recorder,” Collen Baker, Measurements

& Control, September, 1996.• “Oscilloscopes—DSO Versus Analog,” Richard Parish, Measurements &

Control, September, 1997.• “Paperless Recorders Meet Expanding Information Needs,” Greg Kleinert,

I&CS, March 1995.• “Paperless: The Trend in Recording,” John Ham, InTech, June 1996.• “Portable Multifunction Paperless Data Loggers,” Ed Palko, Plant

Engineering, September, 1995.• “Power Analysis with a DSO,” Kevin J. Cassidy, Measurements & Control,

October, 1997.• Process/Industrial Instruments and Controls Handbook, Fourth ed.,

Douglas M. Considine, McGraw-Hill, 1993. • “Process Recorders Keep Getting More Flexible and Capable,” Control

Engineering, September, 1996.• “Recent DSO Developments,” Charles Holtom. Measurements & Control,

October, 1996.• “Regulators Embrace Paperless Recorders,” Phil Shook, Control, April 1995.• “Smarter, Faster, Data Logging Systems,” Steve Lekas, Sensors, October, 1997.• “Videographic Recorders,” Brian Mattox and Patrick Bridge,

Measurements & Control, September, 1997.• “X-Y Recorders,” Measurements & Control, October, 1997.

Magnetic storage media, on PCMCIA card or

floppy disk, allow videographic recorders to

download data for archival storage.

American Institute of Chemical Engineers (AIChE) 345 East 47 Street, New York, NY 10017-2395 212/705-7338 www.aiche.org

American National Standards Institute11 West 42nd Street, New York, NY 10036 212/642-4900 web.ansi.org

American Society for QualityP.O. Box 3005, Milwaukee, WI 53201-3005 414/272-8875 www.asqc.org

American Society of Mechanical Engineers (ASME)345 East 47th Street, New York, NY 10017 212/705-7722 www.asme.org

American Statistical Association1429 Duke Street, Alexandria, VA 22314-3415 703/684-1221 www.amstat.org

Electric Power Research Institute (EPRI)3412 Hillview Avenue, Palo Alto, CA 94303 415/855-2000 www.epri.com

Electronic Industries Association 2500 Wilson Boulevard, Arlington, VA 22201-3834 703/907-7500 www.eia.org

Federal Communications Commission1919 M Street N.W., Washington DC 20554 888/225-5322 www.fcc.gov

Fieldbus Foundation1998 Fieldbus Foundation9390 Research Blvd., Ste. II-250, Austin, TX 78759 512/794-8890 www.fieldbus.org

Institute of Electrical & Electronics Engineers445 Hoes Lane, Piscataway, NJ 08855-1331 732/981-0060 www.ieee.org

International Organization for Standardization (ISO)1, rue de Varembé, Case postale 56CH-1211 Genève 20, Switzerland + 41 22 749 01 11 www.iso.ch

ISA—The International Society for Measurement & Control67 Alexander Drive, Research Triangle Park, NC 27709 919/549-8411 www.isa.org

National Institute of Standards & TechnologyGaithersburg, MD 20899-0001 301/975-3058 www.nist.gov

OPC FoundationP.O. Box 140524, Austin, TX 76714-0524 512/834-7200 www.opcfoundation.org

Profibus Trade Organization5010 East Shea Blvd., Suite C-226, Scottsdale, AZ 85254-4683 602/483-2456 www.profibus.org

Information Resources

Information ResourcesORGANIZATIONSNAME/ADDRESS PHONE WEB ADDRESS


For the LatestInformation onData AcquisitionProducts:

Omega Engineering, Inc.One Omega Drive, P.O. Box 4047, Stamford, CT 06907-0047Phone: 800-82-66342 (800-TC-OMEGASM)Email: [email protected]: www.omega.com



The Data Acquisition Systems Handbook, Omega Press LLC, 1997.

New Horizons in Data Acquisition and Computer Interfaces, Omega Press LLC, 1997.

Omega® Universal Guide to Data Acquisition and Computer Interfaces, Omega Press LLC, 1997.

Book of Books®: Scientific & Technical Books, Software & Videos, Omega Press 1998.

Activex Programming Unleashed, Weiyung Chen (editor), Sams, 1996 Weiyung Chen, Sams, 1996.

Analog and Digital Communication Systems, Martin S. Roden, 1995.

Analog and Digital Signal Processing, H. Baher, John Wiley & Sons, 1990.

Analog and Digital Signal Transmission, Lawrence M. Thompson, ISA, 1985.

Analog I/O Design: Acquisition, Conversion, Recovery, Patrick Garrett, Reston Publishing Co., 1981.

Analog Signal Processing and Instrumentation, Arie F. Arbel, Cambridge University Press, 1980.

Analog-Digital Conversion Handbook, Analog Devices, Inc., Prentice Hall, 1985.

Analog-To-Digital and Digital-To-Analog Conversion Techniques, David F. Hoeschele, John Wiley & Sons, 1994.

Analog-To-Digital Conversion: A Practical Approach, Kevin M. Daughtery, McGraw Hill, 1995.

Automation Systems for Control and Data Acquisition, Lawrence T. Amy, ISA, 1992.

Beginning Java,Ivor Horton, Wrox Press, Inc., 1997.

Business Data Communications, Fourth Edition, D. A. Stamper, Benjamin/Cummings, 1994.

Computer Interfacing: A Practical Approach to Data Acquisition and Control, William H. Rigby and Terry Dalby, Prentice Hall, 1994.

Computer Networks, Second Edition, A. S. Tanenbaum, Prentice Hall, 1988.

Concise Encyclopedia of Software Engineering, Derrick Morris and Boris Tamm (editors) Pergamon Press, 1993.

Data Acquisition and Control, Microcomputer Applications for Scientists and Engineers,Joseph J. Carr, Tab Books Inc., 1988.

OMEGA PRESS REFERENCES

OTHER REFERENCE BOOKS



Data Acquisition and Process Control Using Personal Computers, Tarik Ozkul, Marcel Dekker, 1996.

Data Acquisition Techniques Using Personal Computers, Howard Austerlitz, Academic Press, 1991.

Data Communications—Video and Workbook, Industrial Training Corp. for ISA, 1993.

Data Communications, A Beginner’s Guide to Concepts and Technology, Scott A. Helmers, Prentice-Hall, Inc., 1989.

Data Communications, Computer Networks and Open Systems, Fourth Edition, F. Halsall, Addison-Wesley, 1996.

Dictionary of Measurement and Control: Guidelines for Quality, Safety, and Productivity, 3rd ed., ISA, 1995.

Digital Control Systems: Theory, Hardware, Software, Constantine Houpis and Gary Lamont, McGraw, 1991.

Distributed Operating Systems: Concepts and Design, Pradeep K. Sinha, IEEE, 1996.

Distributed Systems—Concepts and Design, Second Edition, G. Coulouris, J. Dollimore, & T. Kindberg, Addison-Wesley, 1994.

Encyclopedia of Instrumentation and Control,Douglas M.Considine, McGraw-Hill, 1971.

Essential Visual Basic 5.0 Fast, John R. Cowell, Springer Verlag, 1997.

The Essential Windows NT Book, Richard Cravens, Prima Publishing, 1996.

GUI Design for Dummies, Laura Arlov, IDG Books, 1997.

Guide to Networking and Internetworking Terms, P. Simoneau, American Research Group, 1994.

How Networks Work, F. J. Derfler, Jr. and L Freed, Ziff-Davis Press, 1993.

Instrument Engineers’ Handbook, Third Edition, B. Liptak (editor), Chilton Book Co. (CRC Press), 1995.

Introduction to Analog and Digital Communications, Simon Haykin, John Wiley & Sons, 1989.

The Java Programming Language, Ken Arnold and James Gosling, Addison-Wesley, 1997.

The McGraw-Hill Encyclopedia of Science and Technology, 8th ed., Vol. 9, McGraw-Hill, 1997.

Microcomputer Interfacing and Applications, A. Mustafa, Butterworth-Heinemann, 1994.

Microsoft Windows NT Workstation 4.0: At a Glance, Jerry Joyce and Marianne Moon, Microsoft Press, 1997.



Microsoft Windows NT Workstation 4.0 Starts Here, Microsoft Press Interactive, 1997.

Modern Digital and Analog Communication Systems, B.P. Lathi, Holt, Rinehart, & Winston, 1983.

Modern Operating Systems, Andrew S. Tannenbaum, Prentice Hall, 1992.

Object-Oriented Software for Manufacturing, S. Adiga, Chapman & Hall, 1992.

PC Bus Performance Brief: PC Bus Overview, Intel Corp., 1997.

PC Interfacing for Data Acquisition and Process Control, Sanjay Gupta and J.P. Gupta, ISA, 1995.

Process/Industrial Instruments & Controls Handbook, Fourth Edition, Douglas M. Considine, McGraw-Hill Inc., 1993.

Signal Conditioning & PC-Based Data Acquisition Handbook: A Reference on Analog & Digital Signal Conditioning for PC-Based Data Acquisition, Steve Lekas, IOTech, Inc., 1997.

Simplified Design of Data Converter, John D. Lenk, Butterworth-Heinemann, 1997.

Teach Yourself C++ in 21 Days, Jesse Liberty, Sams Publishing, 1997.

Technical Overview: Foundation Fieldbus, Fieldbus Foundation, 1996.

What Is PC/104?, PC/104 Consortium, 1994.


Accuracy: Closeness of a reading or indication of a mea-surement device to the actual value of the quantitybeing measured.ActiveX Control (OLE Control): Custom control appletbased on Microsoft’s OLE architecture. Can run in anyOLE-enabled container application or web browser. Address: Label or number identifying the memory loca-tion of a unit of information.Alias: A false lower frequency component that appearsin sampled data acquired at too low a sampling rate. Alphanumeric: A character set containing both lettersand digits.Alternating current (ac): An electric current that revers-es its direction at regularly recurring intervals.Ambient compensation: The design of an instrumentsuch that changes in ambient temperature do not affectthe readings of the instrument. Ambient temperature: The average or mean tempera-ture of the surrounding air in contact with equipment orinstruments under test.American National Standards Institute (ANSI): TheUnited States standards body responsible for designatingstandards developed by other organizations as nationalstandards.American Standard Code for Information Interchange(ASCII): Standard seven or eight-bit code used to repre-sent alphanumeric characters for communication amongcomputers.Ampere (amp): A unit used to define the rate of flow ofelectricity (current) in an electrical circuit; units are onecoulomb (6.25x1018 electrons) per second. Symbolized by A.Amplifier: A device that magnifies an input or outputsignal using power drawn from a source other than thesignal itself.Amplitude flatness: A measure of gain consistency of acircuit over a range of frequencies. Amplitude: A measurement of the distance from highestto lowest excursion of a variable or physical motion.Often used with reference to waveforms.Analog trigger: A trigger that occurs at a user-selectedpoint on an incoming analog signal. Can be set to occurat a specific level on either an increasing or a decreasingsignal (positive or negative slope).

Analog-to-digital (A/D): Generic term referring to theconversion of analog information to the digital languageof computers.Analog-to-digital converter (ADC): An electronic devicethat converts analog signals to an equivalent digital form.Application program: Computer program designed toaccomplish a specific task, such as word processing. Application-specific integrated circuit (ASIC): Customsemiconductor component designed and manufacturedto perform a set of specific functions, typically for asingle application.ARCnet: Token-passing network technology in whichnodes “take turns” talking according to which node hasthe token.Arithmetic logic unit (ALU): The part of a digital pro-cessing system where binary data is operated uponmathematically. Asynchronous: An event that occurs at an arbitrary time,without synchronization to a reference clock. May referto network messaging, in which a node sends a messagewithout waiting for the receiver to signal that it is readyto receive.

Backbone: High capacity network designed to provide highbandwidth for corporate communication requirements.Backplane: Multi-conductor assembly into which com-puter-based boards are inserted. Typically supplies powerand allows boards to communicate at high speeds.Base address: A memory address that serves as the start-ing address for programmable registers. Baud rate: Serial communications data transmission rateexpressed in bits per second (bps).Bipolar: A signal range that includes both positive andnegative values (i.e., -10 to +10 V).Bit (b): A single binary digit, valued as either 0 or 1. Block: A high-speed data transfer in which the address ofthe data is sent followed by a specified number of back-to-back data words. Breakdown voltage: Threshold voltage at which circuitcomponents begin to be damaged. See also “workingvoltage.” Burst mode: High-speed data acquisition mode in whichdata points are measured as quickly as possible then heldfor subsequent conversion. Helps reduce skew.

GlossaryA

B

Glossary

Glossary


Bus master: A type of a plug-in board or controller with theability to read and write to devices on the computer bus.Bus: Group of conductors that interconnect individualcircuitry in a computer. Examples of PC buses includeISA, EISA, and PCI. Byte (B): Eight related bits of data, an eight-bit binarynumber. Also used to denote the amount of memoryrequired to store one byte of data.

Cache: High-speed processor memory that buffers com-monly used instructions or data to increase processingthroughput. Calibration: The process of adjusting an instrument orcompiling a deviation chart so that its reading can becorrelated to the actual value being measured.Central processing unit (CPU): Microprocessor thatfunctions as the primary computational “brain” of a PC orother computer-based device.Code width: Smallest voltage an A/D converter candetect; a function of resolution, gain, and range.Cold junction compensation (CJC): The referencing ofthermocouple voltage outputs to ambient temperaturein a thermocouple measurement circuit. Carrier sense multiple access with collision detection(CSMA/CD): Non-deterministic networking scheme inwhich any node can send messages at any time.Collisions are detected and messages resent as needed.Common-mode range: Input range over which a circuitcan handle a common-mode signal. Common-mode rejection ratio (CMRR): Measure of aninstrument’s ability to reject interference from a com-mon-mode signal, usually expressed in decibels (dB). Common-mode signal: Mathematical average voltage,relative to a computer’s ground, of the signals from a dif-ferential input. Compiler: Software utility that converts code from ahigh-level programming language, such as C or VisualBasic, into a machine language. Compiled programs run10 to 1,000 times faster than interpreted programs. Seealso Interpreter. Component Object Model (COM): UnderlyingMicrosoft software architecture that allows applicationsto be readily integrated. Component software: Object-oriented softwareapproach in which standalone objects, or components,can readily interact in a container environment withoutcustom program development.

Controller Area Network (CAN): A serial bus developedby Bosch for use in automotive in-vehicle communica-tions. With added communication layers, is the founda-tion of DeviceNet industrial automation protocol.Conversion time: Time required to convert an analog ordigital signal into its converse.Counter/timer: Circuit that counts external or clockpulses.Crosstalk: Unwanted signal on one channel due to inter-ference from another channel. Current drive capability: Amount of current a digital oranalog output channel is capable of sourcing or sinkingwhile still operating within voltage range specifications. Current sink capability: Ability of a data acquisitionboard to dissipate current for analog or digital outputsignals. Current source capability: Ability of a data acquisitionboard to supply current for analog or digital output signals.Current: The rate of flow of electricity. The unit is theampere (A), which equals one coulomb per second.

Data acquisition (DAQ): The activity of measuring,transmitting, and recording electrical signals from sen-sors, switches, and transducers. Often implies the con-version of these signals into computer-compatible digitalinformation.Data communications equipment (DCE): Generictelecommunications term referring to the equipmentused to actually communicate messages among data ter-minal equipment (DTE).Data terminal equipment (DTE): Generic telecommuni-cations term referring to a computer-based device thatneeds to communicate, also referred to as a generator orreceiver. See also Data communications equipment.Decibel (dB): Unit for expressing a logarithmic measureof the ratio of two signal levels. Degree: An incremental value in a temperature scale.Derivative control: Control action in which output cor-rection is proportional to the rate of change of the errorsignal. Derivative control anticipates the magnitude dif-ference between the process variable and the setpoint. DeviceNet: Non-proprietary device-level networkingprotocol for industrial automation. Developed byAllen-Bradley based on Bosch’s CAN chips, now theprovince of the non-profit Open DeviceNet VendorsAssociation. (ODVA).Differential input: Analog input consisting of the volt-

C

D

Glossary


age difference between two terminals, both of which aredifferent from computer ground. Digital input/output (DIO): Input or output pointsallowed only two discrete states, typically on or off, 1 or 0. Digital signal processing (DSP): The manipulation of sig-nal information while it exists in digital rather than ana-log form.Digital-to-analog converter (D/A): Electronic device,often an integrated circuit, that converts a digital numberinto a corresponding analog voltage or current.Digital-to-analog: Generic term referring to the conver-sion of digital data to a corresponding analog voltage orcurrent.DIN: German agency responsible for setting engineeringand dimensional standards.Direct memory access (DMA): Method by which datacan be rapidly transferred to or from computer memorywithout the processor’s intervention.Distributed control system (DCS): Typically a large-scale process control system characterized by a distrib-uted network of processors and I/O subsystems thatencompass the functions of control, user interface, datacollection, and system management. DCSs are common-ly used in large industrial facilities, such as chemicalplants, petroleum refineries, and paper mills. Distributed Component Object Model (DCOM):Distributed version of Microsoft’s COM architecture, allow-ing applications to be readily integrated across networks.Drift: A change in an instrument’s reading or setpointvalue over extended periods due to factors such as time,line voltage, or ambient temperature effects.Driver: Software that controls a specific hardware devicesuch as a PC board or programmable controller. Dynamic Data Exchange (DDE): Microsoft inter-process communication protocol for transferring dataamong Microsoft Windows-based applications.Predecessor to OLE.Dynamic Link Library (DLL): Executable code and datathat can be called or used by Microsoft Windows-basedapplications or other DLLs. Functions and data in a DLLare loaded and linked when referenced .Dynamic range: Ratio of the largest to smallest signallevel a circuit can handle, normally expressed in dB.

EEPROM: Electrically erasable programmable ROM; read-only memory that can be erased and reprogrammed. Electromagnetic interference (EMI): Electrical noise

induced upon signal wires with the possible effect ofobscuring the instrument signal. Electromotive force (EMF): A measure of voltage in anelectrical circuit.Encoder: Device that converts linear or rotary displace-ment into digital or pulse signals. EPROM: Erasable programmable ROM; read only memo-ry that can be erased (usually by ultraviolet light expo-sure) and reprogrammed. Error: The difference between the correct or desiredvalue and the actual reading or value taken.Ethernet: Popular local area networking technology usedfor office applications and, increasingly, control networkand data acquisition applications.External trigger: Voltage pulse from an external sourcethat triggers an event such as A/D conversion.

Fiber Distributed Data Interface (FDDI): Standarddefining a high capacity, dual-ring fiber optic communi-cation protocol.Fieldbus: All-digital communication network used toconnect process instrumentation and control systems.Designed to replace existing 4-20 mA analog signalswith bidirectional, multivariable data communicationscapability.Fieldbus Foundation: Austin, Texas-based nonprofit con-sortium of instrumentation suppliers that is developing astandard digital communication network (fieldbus) forprocess control applications. The network developed bythe Foundation is referred to as the Foundation Fieldbus. First-in first-out (FIFO): Memory buffer in which thefirst data stored is the first data sent.Flash ADC: High-speed ADC whose output code is deter-mined in a single step by a bank of comparators andencoding logic. Frequency: The number of cycles over a specified timeperiod over which an event occurs. Normally expressedin cycles per second (hertz, Hz).Function: A set of software instructions executed by asingle line of code that may have input and/or outputparameters and returns a value when executed.

Gain accuracy: Measure of deviation of the gain of anamplifier from the ideal gain. Gain: The amount of amplification used in an electricalcircuit, sometime expressed in decibels, dB.General Purpose Interface Bus (GPIB): Synonymous

E

F

G

Glossary


with HP-IB (for Hewlett-Packard), the standard bus usedfor controlling electronic instruments with a computer.Also called IEEE 488 in reference to defining ANSI/IEEEstandards. Graphical user interface (GUI): An intuitive, graphicalmeans of communicating information between comput-er-based devices and human users. GUIs can resemblethe front panels of instruments or other objects associ-ated with a computer program. Ground: The electrical neutral line having the samepotential as the surrounding earth; the negative side of adirect current power system; the reference point for anelectrical system.

Hardware: Typically refers to the physical componentsof a data acquisition system, such as PCs, boards, dataconversion devices, etc.Hertz (Hz): Unit of frequency, defined as one cycle persecond.Hierarchical: Method of organizing computer programswith a series of levels, each with further subdivisions. Human-machine interface (HMI): The means by whicha human operator interacts with an industrial machineor other piece of equipment. See also graphical userinterface.

IEEE 488: See General Purpose Interface Bus. Impedance: The total opposition to electrical flow in anelectrical circuit.Input bias current: Current that flows into circuit inputs. Input impedance: The measured resistance and capaci-tance between the input terminals of a circuit.Input offset current: Difference in the input bias cur-rents of the two inputs of an instrumentation amplifier. Input/output (I/O): The transfer of data to and from acomputer system involving communications channels,operator interface devices, and/or data acquisition andcontrol interfaces. Institute of Electrical and Electronic Engineers (IEEE):Professional and standards-writing organization of elec-trical and computer-related professionals.Integral control: Control action that eliminates theoffset inherent in proportional control. Offset fromsetpoint is “integrated” over time resulting in an out-put correction.Integral nonlinearity (INL): A measure in LSB of theworst-case deviation from the ideal A/D or D/A transfer

characteristic of analog I/O circuitry. Integrating ADC: ADC that works by integrating anunknown voltage over time. Time required is comparedto the time required to integrate a known referencevoltage.Interchangeability error: A measurement error that canoccur if two or more sensors are used to make the samemeasurement. Caused by slight variations from sensor tosensor.Interpreter: Software utility that executes source codefrom a high-level language such as Visual Basic or C byreading one line at a time and executing the specifiedoperation. See also Compiler. Interrupt: Computer signal indicating that the CPU shouldsuspend its current task to service a designated activity. Intrinsically safe: An instrument in which electricalenergy is limited such that it will not spark or otherwiseignite a flammable mixture. ISA: Formerly the Instrument Society of America, nowreferred to as the International Society for Measurement& Control. Isolation voltage: Voltage an isolated circuit can nor-mally withstand, usually specified from input to inputand/or from any input to the amplifier output, or to thecomputer bus.

Joint time-frequency analysis (JTFA): Technique forspectral analysis of rapidly changing waveforms.

Linearity: The deviation of an instrument’s responsefrom a straight line.Local area network (LAN): Network of computer-baseddevices, typically PCs and associated peripherals in ageographically limited location.Logical address: Digital number that uniquely identifieseach device in a system. Loop resistance: The total resistance of a completeelectrical circuit.Least significant bit (LSB): Refers to the smallest incre-ment of resolution in an A/D or D/A conversion.

Mainframe: Chassis that mechanically contains boardsor modules inserted into a backplane. Provides environ-ment conditioning and vibration and shock-resistantconnections. Man-machine interface (MMI): See Human-machineinterface.

J

H

I

L

M

Glossary


Media access control (MAC): Networking layer thatdetermines which node can access the physical media.Million floating-point operations per second (MFLOPS):Unit for expressing the computational power of aprocessor. Milliamp (mA): One thousandth of an ampere.Million instructions per second (MIPS): Unit forexpressing the speed of processor execution of machinecode instructions. Millivolt (mV): One thousandth of a volt.Module: A board assembly and its associated mechanicalparts. A module contains everything required to occupyone or more slots in a mainframe chassis.Multiplexer (Mux): A switching device that sequentiallyconnects multiple inputs or outputs in order to processseveral signal channels with a single A/D or D/A converter. Multitasking: Property of an operating system in whichseveral processes can be run simultaneously.

Noise: Any undesirable electrical signal, from externalsources such as ac power lines, motors, electrical storms,and radio transmitters, as well as internal sources such aselectrical components.Normal-mode rejection ratio: The ability of an instru-ment to reject electrical interference across its input ter-minals, normally of line frequency (50-60 Hz).Nyquist theorem: Law of sampling theory stating thatdata sampling frequency should be at least twice that ofthe highest frequency variations in the signal of interest.Must be observed to preserve patterns in data and to notintroduce artificial, lower frequency patterns.

Object Linking and Embedding (OLE): Set of Microsoftsystem services that provides a means for applicationintegration. Based on COM technology, OLE allows appli-cations to dynamically identify and use the data and ser-vices of other applications.Objects: Self-contained software modes that encapsu-late both data and processing logic.Ohmeter: A device used to measure electrical resistance.OLE Control: See ActiveX Control. Open Systems Interconnect (OSI): A seven-layer modelfor generically describing modern networking technologies.Operating system: Base-level software that controls acomputer, runs programs, interacts with users, and com-municates with installed hardware or peripheral devices. Optical isolation: Two networks or circuits in which an

LED transmitter and receiver are used to maintain elec-trical discontinuity between the circuits.Output settling time: Time required for the analog out-put voltage to reach its final value within specified limits. Output slew rate: Maximum rate of change of analogoutput voltage from one level to another. Overhead: Amount of computer processing resourcesrequired to accomplish a task.

Personal Computer Memory Card InternationalAssociation (PCMCIA): An expansion card form factorpopular in laptop and notebook computers. Peripheral Component Interconnect (PCI): High-per-formance expansion bus architecture common in mod-ern desktop PCs. Phase: A time-based relationship between a periodicfunction and a reference. Plug and Play ISA: Specification that allows PC ISAboards to be fully configured in software, withoutjumpers or switches on the boards. Polarity: In electricity, the quality of having two chargedpoles, one positive and one negative.Port: A communications connection on a computer-based device.Postriggering: The acquisition of a programmed numberof samples by a data acquisition board after trigger con-ditions are met. Power supply: A separate unit or part of a circuit thatprovides power to the rest of a circuit.Pretriggering: A data acquisition board that maintains acontinuous buffer of data, so that when trigger condi-tions are met, data generated before the conditionoccurred can be acquired.Profibus: Family of fieldbus, device-level, and cell con-troller protocols for industrial automation.Programmable gain amplifier (PGA): Signal amplifierthat can be programmed to apply a different signal gaindepending on the input voltage. Effectively increasesdynamic range and sensitivity of A/D converter.Programmable logic controller (PLC): Special-purposecomputer used in industrial monitoring and controlapplications. Propagation delay: Amount of time required for a signalto pass through a circuit. Proportional control: Control action in which outputcorrections are directly proportional to the process vari-able’s deviation from setpoint.

O

P

N

Glossary


Proportional-integral-derivative (PID) control: Three-term control algorithm combining proportional, integral,and derivative control actions. Protocol: Defined sequence of bits, characters, andcontrol codes used to transfer data among computer-based devices.

Quantization error: Inherent uncertainty in an A/D con-version due to the finite resolution of the conversionprocess.

Radio frequency interference (RFI): Noise inducedupon signal wires by ambient radio-frequency electro-magnetic radiation with the effect of obscuring theinstrument signal. Range: An area between two limits within which a quanti-ty is measured, stated in terms of a lower and upper limit.Relative accuracy: Measure in LSB of the accuracy of anADC. It includes all non-linearity and quantization errors. Remote terminal unit: Industrial control and data col-lection device similar to a PLC, but designed for remotecommunication via wire-based or radio telemetry.Repeatability: The ability of an instrument to give thesame output or reading under repeated, identical con-ditions.Resistance temperature detector (RTD): A metallicprobe that measures temperature based upon its coeffi-cient of resistivity.Resistance: The resistance to the flow of electric cur-rent, measured in ohms. Resolution: The smallest signal increment that can bedetected by a measurement system. Resolution can beexpressed in bits, in proportions, or in percent of fullscale. For example, a system has 12-bit resolution, onepart in 4,096 resolution, and 0.0244 percent of full scale.Ribbon cable: Flat cable in which the conductors areside by side. Root-mean-square (RMS): Averaging algorithm thatresults from the square root of the arithmetical mean ofthe squares.

Sample-and-hold (S/H): Circuit that acquires andstores an analog voltage on a capacitor for subsequentconversion. Self-calibrating: Data acquisition board that calibratesits own A/D and D/A circuits with reference to a stableonboard reference.

Sensitivity: The minimum change in a physical variableto which an instrument can respond.Sequenced Packet Exchange/Internetwork PacketExchange (SPE/IPE): Novell implementation of networkaddressing scheme.Shielded twisted pair (STP): Cable construction thatincludes an external grounded shield, as well as twistingon a regular basis to help minimize noise interferences.Signal-to-noise ratio (SNR): The ratio of the overall rmssignal level to the rms noise level, expressed in dB. Simultaneous sampling (SS): System in which eachinput or output channel is digitized or updated at thesame time.Single-ended (SE): An analog input that is measured withrespect to a common ground. Software trigger: A programmed event that triggers anevent such as data acquisition. Span: The difference between the upper and lower lim-its of a range, expressed in the same units as the range.Stability: The ability of an instrument or sensor to main-tain a consistent output when a constant input isapplied.Statistical process control (SPC): Analysis methodologyin which characteristics of a process are measured orcounted and tracked. Statistical rules are used to deter-mined whether variations are random or need correction.Strain gauge: Sensor whose resistance varies withapplied force. Structured Query Language (SQL): Computer languageused to interact with databases. Subroutine: Set of software instructions executed by asingle line of code that may have input and/or outputparameters.Successive-approximation ADC: ADC that sequentiallycompares a series of binary-weighted values with an ana-log input to produce an output digital word in n steps,where n is the bit resolution of the ADC. Supervisory control and data acquisition (SCADA):Automation and control architecture in which directcontrol and data acquisition are performed by PLCsand/or RTUs and supervised by another computer-baseddevice, often a PC.Synchronous: An event or action that is synchronized toa reference clock. System noise: Measure of the amount of noise seen byan analog circuit or an ADC when the analog inputs aregrounded.

Q

R

S

Glossary


Thermistor: A semiconductor sensor that exhibits arepeatable change in electrical resistance as a function oftemperature. Most thermistors exhibit a negative tem-perature coefficient. Thermocouple: The junction of two dissimilar metalsthrough which a measurable current flows depending onthe temperature difference between the two junctions.Throughput rate: Speed of execution, in samples or bitsper second, for a given continuous operation, calculatedto include software overhead. Transfer rate: Rate at which data is moved from sourceto destination.Transistor-to-transistor logic (TTL): Voltage-levelchanges readily communicated and interpreted by micro-processors, typically in the 0-5 V range.Transmission Control Protocol/Internet Protocol(TCP/IP): A set of standard protocols for communicatingacross a single network or interconnected set of networks.

Unipolar: A signal range that is always positive (for

example, 1 to 5 V). Unshielded twisted pair (UTP): Cable constructionconsisting of pairs of wires twisted at regular intervals(called the pitch) in order to reduce electrical noiseinterferences.

Volt (V): The electrical potential difference between twopoints in a circuit. One volt is the potential needed tomove one coulomb of charge between two points whileusing one joule of energy.

Wide area network (WAN): Computer network typical-ly designed to include and embrace multiple local areanetworks across multple geographic locations.Word: Standard number of bits that a processor or mem-ory manipulates at one time. Microprocessors typicallyuse 8, 16, or 32-bit words.

Zero offset: The non-zero output of an instrument,expressed in units of measure, under conditions of truezero.

T

U

V

W

Z


Index

A

Index

B

C

D

E

F

G

Acronyms (table) 3ActiveX Controls 51Alarm modules 44Aliasing 10Amplifiers,

function 14programmable gain (PGA) 14

Analog-to-digital conversion (ADC), accuracy 12acquisition time 12conversion time 12definition 10selection factors 11throughput 12transfer time 12types 12-13

Application-specific integrated circuits (ASICs) 37ARCnet 28Auto dialers 47

Backbones 32Backplanes 43Burst mode sampling 14Bus topologies 28

C++ 50Cabling,

(also see Wiring)analog options 22coaxial 23digital network 27

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 29

Code width 39Cold junction compensation (CJC) 44Compact PCI 41Comparators 13Component Object Model (COM) 51Contact closure 18Current-to-voltage conversion 14

Data acquisition,definition 10

Data acquisition systems,boards/cards 38hardware 36high accuracy 39high speed 38rack-mount 42selection criteria 36software 48specialty boards 40

Data communication equipment (DCE) 25Data terminal equipment (DTE) 25Data Link Layer (DLL) 35Data loggers 63Decimal prefixes 2Differential inputs 17Digital input/output

(see Input/output)Digital-to-analog (D/A) conversion 10, 15DIN rail 43Distributed Component Object Model (DCOM) 51Dynamic Data Exchange (DDE) 33

EIA-485 26Electrical,

relationships 19symbols 2units 19

Electrical Industries Association 26Ethernet 28Excitation 14

Fiber Distributed Data Interface (FDDI) 32Fiber optics,

multi-mode 27single mode 27

Fieldbus networks 34Filtering 11, 14Flash/parallel ADCs 12Foundation fieldbus 34Fourier, Jean Baptiste Joseph 19

Gain 14Gateways 31General Purpose Interface Bus (GPIB) 26

Index


O

H

I

J

M

N

Ground loops 22Grounding 20

Hypertext Markup Language (HTML) 33

IEEE-488 27Industry Standard Architecture (ISA) 41Input/output,

analog outputs 15differential analog 15digital inputs 17digital outputs 18general 10hardware 36pulse 18remote 45selection criteria 37single-ended analog 15

Integrating ADCs 12Isolation 15

Java 33Jitter 28

Media Access Control (MAC) 25Metropolitan area networks (MANs) 32Micro Channel Architecture (MCA) 41Modems,

fiber optic 46high-speed 46isolated 45limited distance 45

Multiplexing 14

Networking/networks,concepts 24device level 32fiber optic 27fieldbus 32media options 25, 29models 24parallel 27routing 32selection criteria 24serial 26shielding 27topologies 28wireless 30, 46

Noise,frequencies 20sources 20

Nyquist theorem 11

Object Linking & Embedding (OLE) 51Object orientation 51Ohm, George Simon 19Omega Engineering,

about 8contact information 68

Open Systems Interconnect (OSI) 24Operating systems 48Oscilloscopes,

analog 64digital storage (DSO) 64features 65recorders 64selection criteria 65

PC bus architecturescapacities 42descriptions 41speed comparisons 42

PC/104 41Peripheral Component Interconnect (PCI) 41Personal Computer Memory Card

International Association (PCMCIA) 41Profibus 33Programmable gain amplifier (PGA) 14Protocol converters 46

Quantization error 10

Ramping/counter ADCs 12Recorders,

circular chart 62classifications 59flatbed 61hybrid 60oscillographic 64selection criteria 64strip-chart 60trend 60videographic 66XY 62

Relay, driving 18Resistor,

P

Q

R

Index


U

color codes 2networks 16standard values 2

Resolution 10, 37Ring topology 28RS-232 25RS-422 26RS-485 25

Sample-and-hold (SH) circuitry,concept 14functionality 41

Sample rate 10, 37Satellite applications 32Sensitivity 11Sensor,

input boards 40types 20, 40

Sequenced Packet Exchange/Internetwork Packet Exchange (SPX/IPX) 31Settling time 16Signal,

scaling 14types 19typical ranges 10, 37

Signal conditioning,functions 11, 36hardware 44

Signal-to-noise ratio 21Simultaneous sampling 14Single-ended inputs 15Star topology 28Software,

components 50curve-fitting 55data interface 57development 48process monitoring 54

mathematical manipulation 55operating systems 50selection procedure 53statistical analysis 56

Statistical process/quality control (SPC/SQC) 56Successive approximation ADCs 13

Temperature input boards 40ThinNet 29Token networks 28Transistor-to-transistor logic (TTL) 10Transmission Control Protocol/Internet

Protocol (TCP/IP) 31Transmitters 44Triggering 18

Universal Serial Bus (USB) 41

Video Electronics Standards Association (VESA) 41VMEbus 43VXIbus 43

Waveform 19Wide area networks (WANs) 32Wireless networks,

cellular/trunked 46Ethernet 30licensed radio frequency 46microwave 46spread spectrum 46unlicensed 46

Wiring,(also see Cabling)grades 27plain pair 22unshielded twisted pair (UTP) 23, 27shielded twisted pair (STP) 23, 27shielding 27

S

T

V

W

Index


1-1. Functional Diagram for Data Acquisition 10

1-2. The Analog-to-Digital Interface 10

1-3. A/D Conversion Compromises 11

1-4. Aliasing Due to Slow Sample Rate 11

1-5. Analog Input Flow Diagram 11

1-6. Alternative A/D Converter Designs 12

1-7. A/D Conversion by

Successive Approximation 12

1-8. A/D Conversion by Flash/Parallel Technique 13

1-9. A/D Conversion by Counting/Ramp Technique 13

1-10. Alternative Methods for Eliminating Time

Skew Among Multiplexed Channels 14

1-11. Conversion of 4-20 mA to 1-5 V 14

1-12. Single-Ended & Differential

Analog Input Configurations 15

1-13. Weighted Value & Single Value Resistor

Networks for D/A Conversion 15

2-1. Signal Processing Requirements

for Digital and Analog Signals 17

2-2. Digital Input Applied to a Contact Closure 17

2-3. Digital Output Applied to a Relay 18

3-1. A Basic Electric Current 19

3-2. Digital and Analog Signal Representations 19

3-3. Signal, Noise, and Filtering Frequencies 20

3-4. A Ground Conductor 20

3-5. Incorrect Grounding of Signal Circuit 21

3-6. Correct Grounding of Signal Circuit 21

3-7. Coaxial Cable Construction 22

4-1. The Seven-Layer OSI Network Model 24

4-2. Serial Data Transmission 25

4-3. Cable Length vs. RS 422 Data Rate 26

4-4. Shielded Twisted Pair (STP) Construction 27

4-5. Ring Network Topology 28

4-6. Bus Network Topology 29

4-7. Star Network Topology 29

4-8. A Remote File Request According to OSI 30

4-9. Dual-Ring Redundant FDDI Network 30

4-10. Profibus Applications 31

4-11. Protocol Architecture of Profibus 32

4-12. Foundation Fieldbus Protocol 33

4-13. Typical Enterprise Network 34

5-1. Computer Bus Architecture 42

6-1. Cost of Writing vs. Buying Software 49

6-2. Advantages of Component Software 50

6-3. Semi-Custom Development vs.

Off-the-Shelf Packages 52

6-4. Primary Data Analysis Techniques 55

7-1. Typical Strip-Chart Recorder Trace 58

7-2. Typical Circular Chart Recorder Trace 60

List of FiguresSection 1Analog I/O Functionality

Section 4Digital Signal Transmissions

Section 5Data Acquisition Hardware

Section 6Presentation & Analysis

Section 7Recording, Printing & Storage

Section 2Digital I/O Functionality

Section 3Analog Signal Transmissions

PLA

CE

STA

MP

HE

RE

PLA

CE

STA

MP

HE

RE

Com

plet

e Re

vers

e Si

de to

Rece

ive

Your

Ow

n

Dilb

ert C

ard

Deck

sO

ME

GA

EN

GIN

EE

RIN

G,I

NC

.O

NE

OM

EG

AD

RIV

EP.

O.B

OX

404

7S

TAM

FO

RD

CT

069

07-0

047

Com

plet

e Re

vers

e Si

de to

Rece

ive

Your

Ow

n

Hard

cove

r Han

dboo

ksO

ME

GA

EN

GIN

EE

RIN

G,I

NC

.O

NE

OM

EG

AD

RIV

EP.

O.B

OX

404

7S

TAM

FO

RD

CT

069

07-0

047 PLA

CE

STA

MP

HE

RE

PLA

CE

STA

MP

HE

RE

Com

plet

e Re

vers

e Si

de to

Rece

ive

Your

Ow

n

Gian

t Pos

ter B

ulle

tins!

OM

EG

AE

NG

INE

ER

ING

,IN

C.

ON

EO

ME

GA

DR

IVE

P.O

.BO

X 4

047

STA

MF

OR

D C

T 0

6907

-004

7

Com

plet

e Re

vers

e Si

de to

Rece

ive

Your

Ow

n

Tran

sact

ions

of

Mea

sure

men

t &Co

ntro

lO

ME

GA

EN

GIN

EE

RIN

G,I

NC

.O

NE

OM

EG

AD

RIV

EP.

O.B

OX

404

7S

TAM

FO

RD

CT

069

07-0

047

Fill out card completely. Please print inform

ation clearly:

Name

____________________________________________________Corporate e-m

ail:____________________________________________Personal e-m

ail:____________________________________________

Title______________________________________________________

Company

__________________________________________________Com

pany Address____________________________________________City__________________State______________Zip

________________Phone #

______________________Fax #

________________________If em

ployer does not permit handbook delivery at w

ork, give company address above

and home address below

. Catalogs cannot be delivered to P.O. Boxes.

Street Address________________________________________City

______________State__________Zip

______________

1. What is the principal activity at this location?

M

anufacturing

Service

R&D

2. What is the PRIM

ARY end product manufactured or service perform

edat your business location? (Please be specific):__________________________________________________________________________

3. What is the approxim

ate number of em

ployees at this location?

1-19 20-49

50-99 100-499

500-999 1000+

Already sent for your OMEGA

®Handbooks? Pass this card on. Your colleague w

ill thank you.©

COPYRIGHT 1998 OMEGA ENGINEERING, INC. ALL RIGHTS RESERVED.

YES!Send M

e the Following

BRAND NEW!, FREE!

OMEGA

®Handbook(s).

MP

urchasing Ag

ent Edition

MTem

perature and Hum

idityM

Pressure

,Strain,

and Force

MF

low

and Level

MpH

and Conductivity

MD

ata Acquisition H

andbookM

Universal G

uide to D

ata Acquisition

ME

lectric Heater

sM

The G

reen Book

®

M21

STC

enturyTM

Previe

w E

ditionM

Book of B

ooks®

Update

MTransactions

From

our affiliate companies

MO

ME

GA

DY

NE

TM P

ressureForce Load Torque D

atabook M

NE

WP

OR

T®Volum

e V D

atabook


ation clearly:

Name______________________________________________________

Corporate e-mail:____________________________________________

Personal e-mail:

____________________________________________

Title______________________________________________________

Company

__________________________________________________

Company Address____________________________________________

City__________________State______________Zip________________

Phone #______________________

Fax #________________________

If employer does not perm

it handbook delivery at work, give com

pany address aboveand hom

e address below. Catalogs cannot be delivered to P.O. Boxes.

Street Address________________________________________

City______________State

__________Zip______________


M

anufacturing

Service

R&D

2. What is the PRIM




ate number of em


1-19 20-49

50-99 100-499

500-999 1000+

YES!Send M

e the BRAND NEW!,

FREE! Dilbert Deck Cards!M

Dilbert D

eck Cards

YES!Send M

e the Following

BRAND NEW!, FREE!

OMEGA

®Transactions of

Measurem

ent andControl:

MInfrared

Temperature

Measurem

ent andC

ontrol (1Q98)

MD

ata Acqusition in

Process C

ontrol (2Q

98)

MP

ressure,F

orce,and

Load Measurem

ent and C

ontrol

(3Q98)

MM

easurement and

Control of E

nvironm

entalP

arameters (4Q

98)


ation clearly:

Name______________________________________________________

Corporate e-mail:____________________________________________

Personal e-mail:

____________________________________________

Title______________________________________________________

Company

__________________________________________________

Company Address____________________________________________

City__________________State______________Zip________________

Phone #______________________

Fax #________________________





Street Address________________________________________

City______________State

__________Zip______________


M

anufacturing

Service

R&D

2. What is the PRIM




ate number of em


1-19 20-49

50-99 100-499

500-999 1000+

YES!Send M

e the Following

BRAND NEW!, FREE!

OMEGA

®Giant Posters:

MS

pace Shuttle

Patc

hes

MR

ockets of the W

orld

MC

hart ofD

imensionless

Num

bers

MR

adio Frequencies

MD

ilbert Deck

Dilbert ©

United F

eature Syndicate, Inc.


ation clearly:

Name

____________________________________________________

Corporate e-mail:____________________________________________

Personal e-mail:

____________________________________________

Title______________________________________________________

Company

__________________________________________________

Company Address____________________________________________

City__________________State______________Zip________________

Phone #______________________

Fax #________________________





Street Address________________________________________City

______________State__________Zip

______________


M

anufacturing

Service

R&D

2. What is the PRIM




ate number of em


1-19 20-49

50-99 100-499

500-999 1000+

Already sent for your Dilbert DeckCards? Pass this card on. Your colleague w

ill thank you.©

COPYRIGHT 1998 OMEGA ENGINEERING, INC.

ALL RIGHTS RESERVED.


®Giant

Poster Bulletins? Pass this card on.Your colleague w

ill thank you.©




®Transactions

of Measurem

ent and Control? Pass thiscard on. Your colleague w

ill thank you.©



Transactions vol ii

Engineering

Transcript of Transactions vol ii