PC Hardware in a Nutshell, 3rd Edition

Serial Port Standards

Because they transfer only one bit at a time, using serial communications interfaces to establish a unidirectional connection between two devices in theory requires only one data circuit. This circuit comprises one data (or signal) wire and a second wire (called the ground, return, or common) that completes the electrical circuit. In practice, most PC serial communications devices use additional wires to enable bidirectional communication, to provide control circuits between the devices, and so on.

Serial port interface and signaling specifications are defined by the Electronic Industries Association (EIA) standard Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange, Revision D. This standard is properly abbreviated EIA-232D, but is usually called RS-232C, for the earlier and nearly identical revision C. The ITU (formerly CCITT) defines essentially identical standards as V.24 (interface) and V.28 (signaling).