RS 232 Serial Communication Standards

8.4 RS 232 Serial Communication Standards

In serial I/O, data can be transmitted as either current or voltage. When data is transmitted as voltage, the commonly used standard is known as RS-232C. This standard was developed by Electronic Industries Association (EIA), USA and adopted by IEEE. RS-232 standard proposes a maximum of 25 signals for the bus used for serial data transfer.

8.4.1 RS-232 Pin Names and Signal Descriptions

Pin Number

Common Name

RS-232 Name

Description

1

AA

Protective Ground

2

TxD

BA

Transmitted Data

3

RxD

BB

Received Data

4

RTS

CA

Request to Send

5

CTS

CB

Clear to Send

6

DSR

CC

Data Set Ready

7

GND

AB

Signal Ground

8

CD

CF

Received Line signal detector

9

Reserved for Data set testing

10

Reserved for Data set testing

12

SCF

Secondary Receiver Line Signal Detector

13

SCB

Secondary Clear to Send

14

SBA

Secondary Transmitted Data

15

DB

Transmission signal element timing

16

SBF

Secondary Received Data

17

DD

Receiver Signal element timing

18

Unassigned

19

SCA

Secondary request to send

20

DTR

CD

Data terminal ready

21

CG

Signal quality detector

22

CE

Ring indicator

23

CH / CI

Data signal rate selector

24

DA

Transmitted Signal element timing

25

Unassigned

In practice, the first 9-signals are sufficient for most of the serial data transmission scheme. Hence, the RS-232C bus signals are transmitted on a D-type 9-pin connector. When all the 25 signals are used, then RS-232C serial bus is terminated on a 25-pin connector.

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Fig. 8.16 Connections used for terminating RS-232C bus

The voltage levels used for all RS-232C signals are :

Logic Low = -3V to -15V under load (-25V on no load)

Logic High = +3V to +15V under load (+25V on no load)

Commonly used voltage levels are,

+12V (Logic high) and -12V (Logic low)

• The RS-232C signal levels are not compatible with TTL logic levels.

• For interfacing TTL devices, level converters or RS-232C line drivers are employed.

• The popularly used level converters are :

1. MC 1488 – TTL to RS-232C level converter.

2. MC 1489 – RS-232C to TTL level converter.

image

MAX 232 is a bidirectional level converter. It is equivalent to a combination of MC 1488 and MC 1489 in single IC

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