From LQWiki

RS-232 is a standard usually found in computers on serial ports. It specifies how data is exchanged in serial binary form between Data terminal equipment and Data communication equipment.

Until the late 1990s RS-232 ports were commonly used on PC hardware for communicating with modems, mice, and PDA synchronization cradles, among other things. These ports were typically 9- or 25-pin male "D" connectors on the computer, with female connectors on the device end. Now, RS-232 ports are still reasonably common on PC hardware, but for the most part their roles have been taken over by USB.

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