8/12/2019 C2-RS232-RS422-RS485-phan 1

1/40

1

Serial CommunicationEIA RS232, RS422 and RS485

Recommended Standard 232The purpose of this course note is to attempt to describe the main elements in SerialCommunication. This application note attempts to cover enough technical details ofRS232, RS422 and RS485.

EI A - Electroonic Industries Association

8/12/2019 C2-RS232-RS422-RS485-phan 1

2/40

Must be a Sender and Receiver

A protocol is a set of rules which governs the transfer of databetween computers. Protocols allow communication between

computers and networks.

Handshaking is used to establish which protocols to use.Handshaking controls the flow of data between computers

Protocols will determine the speed of transmission, error checkingmethod, size of bytes, and whether synchronous or asynchronous

Examples of protocols are: RS-232, TCP/IP, ....

Characteristics of CommunicationSystems

8/12/2019 C2-RS232-RS422-RS485-phan 1

3/40

5 Basic ComponentsEvery communication system has 5 basic requirements Data Source (where the data originates) Transmitter (device used to transmit data) Transmission Medium (cables or non cable) Receiver (device used to receive data) Destination (where the data will be placed)

8/12/2019 C2-RS232-RS422-RS485-phan 1

4/40

5 Basic Components

8/12/2019 C2-RS232-RS422-RS485-phan 1

5/40

- simplex: One direction only

Transmission Direction

8/12/2019 C2-RS232-RS422-RS485-phan 1

6/40

Half Duplex Transmission

half duplex: Bothdirections but

only onedirection at atime

8/12/2019 C2-RS232-RS422-RS485-phan 1

7/40

Full Duplex Transmission

full duplex:send and

receive bothdirections atonce

RS232

8/12/2019 C2-RS232-RS422-RS485-phan 1

8/40

Analog ADC Line coding Digitaltransmission

SamplingQuantizationCoding

RZ, NRZ,AMI

ASK,FSK,PSK

Block diagram for digital transmission system

Before any digital signal can be transmitted (viatransmission medium or wireless) it mustundergo line coding. What is line coding??

8/12/2019 C2-RS232-RS422-RS485-phan 1

9/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

10/40

PCM Procedures

30 August 201310

Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the messageinformation is encoded in the amplitude of a series of signal pulses.

Sampled signal (discretesignal): discrete time,continuous values.

Quantized signal: continuoustime, discrete values

Digital signal (sampled,quantized): discrete time,

discrete values.

Quantization is the procedure of constraining something from a relatively large or continuousset of values (such as the real numbers) to a relatively small discrete set (such as the integers).

8/12/2019 C2-RS232-RS422-RS485-phan 1

11/40

Mch ly mu v gi

- ly mu, cng tc c ng li trong mt khong thi gian t np n gi

tr V in. Khi cng tc c nh ra, gi tr in th c gi nguyn. - Thi gian cng tc ng li (t ) c xc nh nh sau (trong khong thi gian ny, tc th np n 99% gi tr V in)

8/12/2019 C2-RS232-RS422-RS485-phan 1

12/40

nh l ly mu (tiu chun Nyquist)

Tt c thng tin ca tn hiu gc c th c phc hi nu n c ly mu vi tn s(f s) ln gp t nht 2 ln tn s cao nht (f h) trong tn hiu gc.

Thi gian ly mu:

8/12/2019 C2-RS232-RS422-RS485-phan 1

13/40

informationPCM

Linecoder

channel

Reasons for linecoding:

1. Synchronization2. Error detection

3. Error correction

The purpose of a line code is tomatch the output signal

to the channel for basebandtransmission.

8/12/2019 C2-RS232-RS422-RS485-phan 1

14/40

drawbacks are that it is not self-clocking(Receiver Setting the clock matching the senders )

Line Coding Unipolar encoding (NRZ)

RZ encoding

8/12/2019 C2-RS232-RS422-RS485-phan 1

15/40

Digital Data Analog Signals

Encoding is by modulation of a continuoussinusoidal carrier signal. This involvesalteration of some characteristics of thecarrier signal - amplitude, frequency, or phase.

Various encoding techniques, ASK, FSK, PSK,...

8/12/2019 C2-RS232-RS422-RS485-phan 1

16/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

17/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

18/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

19/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

20/40

Time Division Multiplexing In time division, the communications link is subdivided in terms of time. Each sub-circuit is given the channel for a limited amount of time, before it

is switched over to the next user, and so on In the picture bellow it can be seen that each sub-channel occupies the

entire bandwidth of the channel, but only for a portion of the time

8/12/2019 C2-RS232-RS422-RS485-phan 1

21/40

Frequency Division Multiplexing In frequency division multiplexing, each sub-channel is

separated by frequency (each sub-channel is allocated partof the bandwidth of the main channel)

The speed or bandwidth of the main link is the sum of theindividual sub-channel speeds or bandwidth.

8/12/2019 C2-RS232-RS422-RS485-phan 1

22/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

23/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

24/40

Pulse Rate versus Bit Rate Pulse rate number of pulses per second Bit rate number of bits per second

If pulse defines only one bit, Pulse rate and Bit rate are the same.

If pulse defines more than one bit, Bit rate is greater than Pulse rate . Bit Rate = Pulse Rate x log 2 L

L is the number of data levels of the signal.

8/12/2019 C2-RS232-RS422-RS485-phan 1

25/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

26/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

27/40

Parity

The parity bit included at the end of the character to check for any error during

transmission Even Parity : Total no. of bits including parity bit is Even Odd Parity : Total no. of bits including parity is Odd Only data bits and parity bit are considered for finding parity bit value. Example for Even parity : 0101 0101 0, total 4 ones Example for Odd parity : 0101 0101 1, total 5 ones

Even Parity

8/12/2019 C2-RS232-RS422-RS485-phan 1

28/40

Graham Betts

Any transmission May be: Analog or Digital Serial or Parallel

Communication Concepts

8/12/2019 C2-RS232-RS422-RS485-phan 1

29/40

-Each bit has its own piece of wire along which it travels - Often used to send data to a printer

Parallel Transmission

All bits are sent simultaneously

R e c e i v e r r e c e i v e

d

S e n d e r t r a n s m i t t e d

10011

001

8/12/2019 C2-RS232-RS422-RS485-phan 1

30/40

Sender transmitted

Data is transmitted, on a single channel, one bit at atime one after another

Receiver received

Serial Transmission

101 0 0 1 1 0

8/12/2019 C2-RS232-RS422-RS485-phan 1

31/40

Serial Transmission

Shift Register

8/12/2019 C2-RS232-RS422-RS485-phan 1

32/40

Serial Transmission

0 1 0 1 0 0 0 1 1 1

Start bit

Stop bit

Asynchronous Communication

Synchronous Communication

1 0 1 0 0 0 1 1

Clock

Data

8/12/2019 C2-RS232-RS422-RS485-phan 1

33/40

Serial CommunicationName Sync

/Async

Type Duplex Max

devices

Max

speed(Kbps)

Max

distance(feet)

Pin

count(not including ground)

RS-232 async peer full 2 115.2 30 2 (or 4 with HW handshake)

RS-422 async multi-drop half 10 10000 4,000 1 (unidirectional only,additional pins for each

bidirectional comm.)

RS-485 async multi-point half 32 10000 4,000 2

I2C sync multi-master

half Limitation basedon buscapacitance andbit rate

3400 1000 625

8/12/2019 C2-RS232-RS422-RS485-phan 1

34/40

Serial Vs Parallel Communication System Comparison Data Transfer Speed

Complexity and Cost of Serial and Parallel Interface

Data Transfer Quality & Crosstalk Noise Interference

High Frequency Performance

Flexibility to Upgrade the Data CommunicationSoftware/Hardware What is the latest Trend? Serial or Parallel How to Choose the Best Data Transfer Interface from Serial

or Parallel?

Creating a parallel port relatively simple, requiring only a latch to copy data onto a data bus. In contrast, most serial communication must first be convertedback into parallel form by a universal asynchronous receiver/transmitter (UART) before they may be directly connected to a data bus. But in terms oftransmission channel costs such as data bus cable length, data bus buffers, interface connectors, area occupied on printed circuit board (PCB), the parallel datacommunication is somewhat costlier.

Crosstalk creates interference between the parallel lines, and the effect worsens with the length of the communication link.

Parallel buses are hard to run at high frequencies :1. It is hard to route many signals across a board without introducing timing variation (clock skew) betweenthem the more variation the lower maximum frequency is.2. Many wires switching simultaneously produce lots of EMI and interfere with each other. More EMI( ElectroMagnetic Interference ) leads to less maximum frequency.

Speed of the parallel data transfer is extremely high compared to serial data transfer.

Parallel data transfer mechanism more rely on hardwareresources and hence difficult and costlier.

Parallel- short distance. Serial longer distance.Parallel- Low speed/frequency data transfer. Serial High speed/frequency data transfer.

8/12/2019 C2-RS232-RS422-RS485-phan 1

35/40

Data Transmission Signals

8/12/2019 C2-RS232-RS422-RS485-phan 1

36/40

RS232 Standard Interface between Data Terminal Equipment (DTE) and Data

Communication Equipment (DCE) employing serial binary data

interchanges. Logic 0 : +3V to 15 VDC Logic 1 : -3V to -15 VDC Maximum Distance : 15m or 50ft

8/12/2019 C2-RS232-RS422-RS485-phan 1

37/40

8/12/2019 C2-RS232-RS422-RS485-phan 1

38/40

RS422 Standards The RS-422 standard defines a balanced (or differential) data communications

interface using two separate wires for each signal. Due to the high noise immunity of the RS-422 standard, high data speeds and long

distances can be achieved. The RS-422 specification allows reliable serial data communications for:

Distances of up to 1200 meters Data rates of up to 10 Mbps 0 to +5V nominal signal swing

RS-422 uses two separate twisted pairs and can be half or full duplex.

8/12/2019 C2-RS232-RS422-RS485-phan 1

39/40

RS485 Standards RS-485 interface has tri-state capability, so it can be used in multi-drop

systems. RS-485 can be half or full duplex and has a 2- or 4-wire version. The 2-wire version uses a single twisted pair; the 4-wire version uses two

twisted pairs. Each Slave device has a unique address and will only respond to requests

from the Master. The Slave never initiates communication.

8/12/2019 C2-RS232-RS422-RS485-phan 1

40/40

RS-485 can have multiple Commanding Devices and multipleListening Devices. RS-422 can have only one Commander andmultiple Listeners.