RS232C/D/E Connection Information

Glossary of Abbreviations etc.

Clear To Send [DCE --> DTE]
Data Carrier Detected (Tone from a modem) [DCE --> DTE]
Data Communications Equipment eg. modem
Data Set Ready [DCE --> DTE]
Data Signal Rate Selector [DCE --> DTE] (Not commonly used)
Data Terminal Equipment eg. computer, printer
Data Terminal Ready [DTE --> DCE]
Frame Ground (screen or chassis)
No Connection
Receiver (external) ClocK input
Ring Indicator (ringing tone detected)
Ready To Send [DTE --> DCE]
Received Data [DCE --> DTE]
Signal Ground
Secondary Clear To Send [DCE --> DTE]
Secondary Data Carrier Detected (Tone from a modem) [DCE --> DTE]
Secondary Ready To Send [DTE --> DCE]
SRxD (RxD2)
Secondary Received Data [DCE --> DTE]
STxD (TxD2)
Secondary Transmitted Data [DTE --> DCE]
Transmitted Data [DTE --> DCE]

Electrical Signal Levels and Data Format:

Typical RS232 pinouts and junction boxes:

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:

V.D.U. (DTE) <---> Modem (DCE) <-- (phone line) --> Modem (DCE) <---> Mainframe Computer (DTE)

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:

Pin No. Signal Description Abbr. DTE DCE
1 DCE Ready, Ring Indicator DSR/RI
2 Received Line Signal Detector DCD
3 DTE Ready DTR
4 Signal Ground SG
5 Received Data RxD
6 Transmitted Data TxD
7 Clear To Send CTS
8 Request To Send RTS

Master Cable

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