The RS232 standard was originally defined for the inter-connection of Data Terminal Equipment (DTE) and Data Communication Equipment (DCE). A typical example would be:
Hence, when the full standard was applied, there would be a one-to-one connection between all pins of the 25-way connectors between the DTE and DCE devices. Should two DTE's need inter-connection (without the modems) a Null-Modem adaptor would be required which swapped wires between pins in such a way as each DTE device would appear to be a DCE device to the other. Where the full standard was not applied (again no modems), and not all of the possible signal lines used, variations of the null-modem principle were applied in which unused handshake output(s), (usually RTS), would be looped back to the appropriate input(s), (CTS, DSR, DCD), of the controlling DTE device (computer). Examples of various RS232 or RS232-like "pinouts" or junction boxes can be viewed via the links below:
A variant of RS232 is the three-wire, daisy-chainable, A.R.C. (Addressable RS232C?) interface illustrated below. Only software (Xon/Xoff) handshaking is supported and secondary TxD and RxD lines are used to form the daisy-chain. Each instrument (if not addressed?) echoes its input to the "opposite" output, thereby passing commands/replies down/up the chain. The PC and all instruments must be set to the same baud rate, parity etc.
RS232D uses RJ45 type connectors (similar to telephone connectors) wired as shown below:
|DCE Ready, Ring Indicator
|Received Line Signal Detector
|Clear To Send
|Request To Send
The circuit diagram of a typical, four-line, master cable is shown below. Such a cable can be used to link two computers together (with the switches set for the null-modem configuration, or to connect a computer to a printer, plotter or a laboratory instrument that uses a similar, limited rs232-like interface.
Prepared by F.T. Gowen
Last modified: 12th. January, 2000