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