RS-232 has been around as a standard for decades is an electrical interface between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) such as modems or DSUs. It appears under different incarnations such as RS-232C, RS-232D, V.24, V.28 or V.10 but essentially all these interfaces are interoperable. RS-232 is used for asynchronous data transfer as well as synchronous links such as SDLC, HDLC, Frame Relay and X.25. In its simplest form, the RS-232-C interface consist of only two wires-one to carry data, plus a circuit common. The circuit common is the absolute voltage reference for all the interface circuitry, the point in the circuit from which all voltages are measured.
A typical DTE device is an ordinary video terminal with a keyboard and a video display. Data on pin 2 of the DTE is transmitted, while the same data on pin 2 of a DCE (modem) is received data.