Appendix: RS 232

The EIA RS232 specification resembles the CCITT V24 specification, the basic difference being in the conventions in naming the connecting pins to be used. Much of the communication between the various units that make up a teletext system are linked with RS232 serial links which use the established signal levels and connections. These are ±5 to ±20 volts for data levels, and the connections for a standard 25-way D-type connectors are as follows:

Pin number	Signal

1		Protective ground
2		Transmitted data
3		Received data
4		Request to send
5		Clear to send
6		Data set ready
7		Signal ground/common return
8		Received line signal detector
9		+Voltage
10		-Voltage
11		Unassigned
12		Secondary received line signal detector
13		Secondary clear to send
14		Secondary transmitted data
15		DCE transmitter element timing
16		Secondary received data
17		Receiver element signal timing
18		Unassigned
19		Secondary request to send
20		Data terminal ready
21		Signal quality detector
22		Ring indicator
23		Data signal rate selector
24		DTE transmitter signal element timing
25		Unassigned

RS232 connections can cause problems as the specification does not call for all signals to be present. When one piece of equipment requires, say, the 'clear-to-send-signal' (CTS) to be used and the sending end does not support it, the two pieces of equipment will not function when linked together. The solution is to arrange for the missing signals to be substituted by linking the appropriate pin to the required level, either ground or +12 volts, to act as a substitute for the missing signal. This linking is often made between connector pins, and care must be taken that the linking is made at the appropriate end of the cable if all the leads are not present. Such a lead may cause problems when used with a different item of equipment which has a slightly different set of requirements and therefore modified leads should be marked.

The RS232 specification is designed for data over a telephone line, where a modem is usually needed, and the connections are suitably arranged. However, where no modem is used the necessary change of the interconnections is made with a device called a 'null modem' which ensures that the transmit pin of the transmitting device is connected to the receive pin of the receiving device.

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