The RS232 protocol

The RS232 standard was drawn up by the CCITT in Europe and the Electronics Industries Association (EIA) in the United States in order to ensure that there was a worldwide common format for serial communication between computers and peripherals. This standard outlines the characteristics of the connectors to be used, (size, number of pins, shape, and so on), the voltage levels to be supported, and the control functions assigned to each pin. It also identifies the simple handshaking protocol which determines when the terminal equipment is ready to send and to receive data.

The voltage levels for the RS232 interface are defined to lie between –3 V and  15 V for logic 1, and +3 V and +15 V for logic 0. Typically they are set to –12 V and +12 V. Note that the RS232 interface uses so-called 'negative logic'.

The interface is normally limited to speeds of 20 kbps over distances in the order of 15 metres due to pulse rise times over these lengths of cables. Higher speeds are possible over much shorter distances, however.

 A similar standard to the RS232, termed RS449, can deliver much higher interconnection speeds by specifying the use of 'balanced signalling' and much tighter specification of interconnection cable parameters. The RS449 interface can signal at speeds in excess of 2 Mbps.

A table of pin designations for the RS232 interface is given below.

Table of RS232 Pin Designations
1 Ground
2 Transmitted data (TD)
3 Received data (RD)
4 Request to send (RTS)
5 Clear to send (CTS)
6 Data set ready (DSR)
7 Ground
8 Data carrier detect (DCD)
9 Not connected
10 Not connected
11 Not connected
12 Secondary data carrier detect
13 Secondary clear to send
14 Secondary transmitted data
15 Secondary transmitted clock
16 Secondary received data
17 Receiver clock (RC)
18 Divided clock receiver (DCR)
19 Secondary request to send
20 Data terminal ready (DTR)
21 Signal quality detect (SQ)
22 Ring indicator (RI)
23 Data rate selector
24 External transmitter clock
25 Busy