PS/2 Tutorial

What is PS/2?

PS/2 refers to a connector interface for computer mice and keyboards. PS/2 has been a standard interface since it was first introduced by IBM on its Personal System/2 PC in 1987, from which the PS/2 connector derives its name. The PS/2 connector is becoming obsolete as an increasing number of mice and keyboards rely on USB and wireless connections--especially on laptops where space efficiency and versatility are prized--but it is still common among desktops and older computer models. The PS/2 connector itself is a 6-pin mini-DIN.

Applications and Usages

PS/2 is used exclusively for connecting keyboards and mice. The initial PC System Design Guide (PC 97) standardized the color code for the connectors and ports of borth devices. Purple corresponds to keyboards and green corresponds to mice. Some laptops may have only one PS/2 port for either device, but most computers have one port for each. When connecting a mouse or keyboard it is important to connect the device to its appropriate port. Although the PS/2 ports are practically identical, a keyboard will not work in a mouse port (and vice versa), because a keyboard requires bidirectional communication and utilizes open collector outputs on both sides of the interface.

PS/2 devices are not hot swap compatible. PS/2 connector pins directly link digital I/O lines in the motherboard's device controller and the PS/2 device itself. But artificial hot swapping is common and is often harmless because of high-strength I/O wiring. In terms of functionality, artificial hot swapping has mixed results. When two similar devices are swapped, the device driver may not detect a change of equipment and continue to function to the best of its ability. For example, if a new keyboard is hot swapped for an old keyboard, the new keyboard may be able to input normal letters and keys, but the web shortcuts and volume controls on the keyboard may not work until the computer is rebooted.

Practical Info

PS/2 extension cables for keyboards and mice are rarely greater than 50 feet long; 25 ft cords are more common