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.