Serial interface

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A serial interface is a computer interface that delivers data down a single wire or single pair of wires. Serial interfaces are a concept, for which there are numerous implementations, and numerous connectors.

The most widespread is the RS-232 standard, using DB-25 and later DE-9 connectors[footnote 1], both male on the host/device and female on the cable (both ends).

Older Macintosh computers had often two female mini-DIN-8 sockets, using RS-422 signals to Apple-specific peripherals such as printers, and RS-232 signals (with adapter) to third-party peripherals such as modems.

RS-232 and RS-422 serial interfaces are now mostly superseded by USB; laptops in particular typically omit the serial port now.


On PCs the serial port is also called the COM port.

Desktop PCs may still have a 9-pin RS-232 port present, if not on the back I/O panel more often as a IDC-10 socket on the motherboard for connecting to an optional I/O bracket with the DE-9 port. Note that there exist two types of wiring to IDC-10 socket: AT/Everex ("Straight Through": same numbered pin) and DTK/Intel ("Cross over")[1].

PC mouse

Among its many uses, the PC's "COM" port was used for mice, especially before the PS/2 interface was widely adopted. Mouse Systems and Microsoft had different protocols over RS-232 and other mouse manufacturers supported either (or both with a switch on the mouse). Microsoft's protocol was extended for a third mouse-button and later for a scroll wheel.[2]

Be aware that some bus mice did also use DE-9 connectors but are incompatible with serial mice.





  1. DE-9 is very often mislabelled as "DB9". 'E' and 'B' stand for the size of the D-subminiature port. DE-9 is smaller than DB-25, whereas an actual DB-9 port (if one existed) would be the same size as a DB-25 but have fewer pins


  1. Bodenzord—RS-232 PC Serial Port Pinout Conventions. Dated 2013-10-03. Retrieved 2018-12-04
  2.—Mouse protocol. Archived from Dated 2003-01-12. Retrieved 2018-12-04