PS/2 interface

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Colour-coded keyboard and mouse sockets on a Belkin converter

The PS/2 interface is a serial keyboard and mouse interface developed by IBM for their PS/2 line of computers. The PS/2 keyboard connector uses the same keyboard protocol as the AT keyboard interface, but uses a Mini-DIN 6 connector with four pins connected. Like the XT and AT interfaces before it, the PS/2 interface is not hot-swappable, and connecting a keyboard to a running computer can cause older operating systems to hang.

For many years, keyboard ports have been coloured mauve, and mouse sockets, green. Besides the modern colour-coding, PS/2 mouse and keyboard connections are visually identical, and unlike the contemporary Apple Desktop Bus, the two sockets are not compatible. Connecting a keyboard into the mouse port, or vice versa, will traditionally not work; an IBM PS/2 computer will hang on boot with a cryptic number displayed on the screen if the keyboard is mistakenly connected to the mouse port. Some more modern laptops however feature a single PS/2 port that can be used by either type of device.

For desktop keyboards, the PS/2 interface has a number of advantages over the USB interface:

• PS/2 keyboards send commands directly to the host computer, unlike USB keyboards which must be polled for keypress events.
• Providing that the keyboard circuitry supports it, there is no limit to the number of keys which can be registered simultaneously over PS/2. The USB protocol is limited to 6 simultaneous keypresses, while PS/2 supports true N-key rollover.