General Information About RS-232 (Serial) Connectors

There are two basic types of RS-232 connectors in general use among PC equipment today. They are DCE and DTE. The DCE connector is usually on the back of the PC and the DTE connector is usually found on a device (such as a modem) which needs to talk to a DCE. The acronyms stand for "Data Communications Equipment" and "Data Terminal Equipment" respectively. A DCE can talk to a DTE through a direct serial cable in which each pin on one device is wired directly to the same pin number on the other device. All that is needed for a DCE to talk to another DCE (or DTE to DTE for that matter) is for the appropriate pins on each connector to be wired in reverse to each other. This is what's commonly refered to as a "NULL MODEM" adaptor. The important pins that must be swapped in order to create such an adaptor are TD/RD (Pins 2 and 3), RTS/CTS (Pins 4 and 5) and DSR/DTR (Pins 6 and 20). The rest can be straight wired, but the only other pin usually really useful is pin 7 (ground). (The pin examples given are for the 25 pin version of the RS-232 connector.) See the RS-232 Connectors for more information.

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