Signals within RS232C interface
As well as the RS232C circuit descriptors for each circuit within the standard, there are alternative abbreviation names for each circuit. They are both shown below.
The first list shows the main circuits, which are specified for both the 25 pin D-type connector and the 9 pin D-type connector.
RS232C
circuit |
Abbrev
-iation |
25pin
D-type
pin no |
9 pin
D-type
pin no |
|
Circuit function |
| AA |
PG |
1 |
|
|
Protective ground - used to connect chassis together for protection against electric shock under fault conditions |
| AB |
SG |
7 |
5 |
|
Signal ground - provides a reference for all other signals |
| BA |
TXD |
2 |
3 |
|
Transmitted data - from computer to modem |
| BB |
RX |
3 |
2 |
|
Received data - from modem to computer |
| CA |
RTS |
4 |
7 |
|
Request to send - sent by computer to tell modem to set up for a data transfer |
| CB |
CTS |
5 |
8 |
|
Clear to send - sent by the modem to tell the computer that data sent by the computer on BA will be onward transmitted. Usually used in response to RTS, and also in association with DTR and DSR |
| CC |
DSR |
6 |
6 |
|
Data set ready - sent by the modem to the computer to indicate that it is ready to transmit data out to the network. Does not indicate that the remote modem is ready to receive. Usually used in response to DTR, and in association with RTS and CTS |
| CD |
DTR |
20 |
4 |
|
Data terminal ready - sent by the computer to tell the modem that the computer is ready to send data |
| CE |
RI |
22 |
9 |
|
Ring indicator - sent by the modem to the computer to indicate that ringing tone is being received |
| CF |
DCD |
8 |
1 |
|
Data carrier detect or Received line signal detector - sent by the modem to the computer when the modem receives an acceptable signal via the network. Used when the modem is acting as a receiver of data, not as a transmitter |
In the RS 232 standard, the data signals on circuits TXD and RXD use the voltage levels as follows :-
logic 0 = +3v to +15 v logic 1 = - 3v to -15v
The control signals use the voltage levels as follows :-
Other signals which exist within the RS232C specification, but are only specified for use with the 25 pin D-type connector are :-
RS232C
circuit |
25pin
D-type
pin no |
|
Circuit function |
| CG |
21 |
|
Signal quality detector - sent by the modem to the computer to indicate that the modem believes the received signal quality was adequate |
| CH |
23 |
|
Data signalling rate selector - sent by the computer to the modem to indicate data transfer rate ( on dual speed machines ) |
| CI |
23 |
|
Data signalling rate indicator - sent by the modem to the computer to indicate data transfer rate ( on dual speed machines ) |
| DA |
24 |
|
Transmitter signal element timing - sent by the computer to the modem to indicate clock rate of data - used for synchronous data transfers |
| DB |
15 |
|
Transmitter signal element timing - as above, but modem provides signal |
| DD |
17 |
|
Receiver signal element timing - as above, but sent by modem to computer to indicate clock rate of incoming data |
| SBA |
14 |
|
Secondary transmitted data - as BA |
| SBB |
16 |
|
Secondary received data - as BB |
| SCA |
19 |
|
Secondary request to send - as CA |
| SCB |
13 |
|
Secondary clear to send - as CB |
| SCF |
12 |
|
Secondary received line signal detector - as CF |
© 1998 Ron Turner
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