PS/2 interface
The PS/2 interface is a serial keyboard and mouse interface developed by IBM for their PS/2 line of computers. The physical connector is a 6-pin mini-DIN, with four of the pins connected.
Computers generally came with two PS/2 ports, one for the keyboard, and the other for the mouse. The ports were separately marked, as computers required the correct device to be inserted into each socket. 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. Contrast this with with Apple Desktop Bus, which was a daisy chain bus that accepted devices in any order. Modern PCs however more often tend to feature only a single PS/2 port that can be used by either type of device.
To avoid confusion in identifying the ports, the first version of the PC System Design Guide in 1997 specified that mouse and keyboard ports would be coloured purple and green (specifically, mauve and green) for keyboard and mouse respectively.
PS/2 ports are not hot-swappable. Connecting a keyboard or mouse to a running machine can cause damage to some motherboards and/or some older operating systems to hang.
Contents
Keyboard connection
The PS/2 keyboard connection uses the same keyboard protocol as the AT keyboard interface.
On the IBM RS/6000, the two pins not used by the PS/2 protocol connect to a speaker inside the type of IBM Model M Enhanced Keyboard that came with the system. Full-size and space-saving Model M keyboards for the IBM PC also have speaker grilles on the bottom of their cases, even though no speaker is fitted.
Mouse connection
For mice, IBM introduced a new protocol very similar to the AT interface. This replaced serial mice and older mouse connections.
Combined mouse/keyboard ports
Many contemporary desktop PC motherboards, and some older laptops have only a single PS/2 port.
There are unfortunately different types of combined ports:
- Primarily a keyboard port, but where the two pins otherwise unused by the PS/2 interface are used for mouse signals. A mouse can be connected only through a Y-splitter cable or a dedicated mouse port on a connected keyboard.
- A port that can connect either a mouse or keyboard directly (through auto-detection), or both using a Y-splitter cable.
- A port that can connect either a mouse or keyboard directly through auto-detection, but which do not support Y-splitter cables.
Y-cables should have coloured and/or labelled sockets to show which device connects to which socket. Use of it should be safe with either type of combo port except that the mouse might just not be detected at all.
Y-cables without any label or colour should be used only with computers that you are sure support both mouse and keyboard simultaneously and through auto-detection.
Keyboards with PS/2 mouse out are quite rare, typically bundled only with desktop PCs they were intended for.
Non IBM platforms
The PS/2 port is typically associated with the IBM PS/2 and PC clones, but it has been adopted on some other platforms.
Macintosh clones
Many Macintosh clones from the late 1990s also featured PS/2 mouse and keyboard ports. One advantage of this arrangement was that you could connect a two-button PC mouse to the computer and use a program such as TheMouse2B to map the second button to control+click, thereby achieving contextual menus from the right button as with a Windows computer. Shortly after the demise of the Macintosh clone industry, Apple introduced the iMac, which featured USB ports for the mouse and keyboard; this was followed three years later with Mac OS X, with native support for multiple mouse buttons and right-click contextual menu activation.
Silicon Graphics
Silicon Graphics used PS/2 mice and keyboards for their Unix workstations, starting with the Indigo² (and Challenge M). Other workstation models with the port include Indy, O2 and Octane.
Digital Equipment
DEC used PS/2 mice and keyboards on their AlphaStation line of Unix workstations.
Adaptors
There are passive adaptors between PS/2 and AT keyboard interfaces.
Passive adaptors between PS/2 and USB ports work only with keyboards and mice that support both protocols but have only one cable. Passive adaptors are often spotted by being small and coloured green or mauve. Both green and the mauve adaptors are wired identically.
The same applies to adaptors between PS/2 mouse and 9-pin serial mouse interface: the mouse or trackball must support both protocols.
Converters
Protocol converters between PS/2 and USB tend to be on a wire. Many have a boxy shape or are split into two, supporting both a PS/2 mouse and keyboard to one USB port.
