PS/2

PS/2 is a six-pin mini-DIN connector standard that carries serial data and power for computer input devices, most commonly keyboards and mice. The standard emerged from IBM's Personal System/2 line in 1987, where it replaced the older five-pin DIN keyboard socket with a smaller, more reliable interface. Each PS/2 port is electrically independent: a keyboard connects to one port, a mouse to another, though both use identical connectors and pinouts.

The physical connector is cylindrical with six pins arranged in a 120-degree offset pattern, measuring about 8 millimeters in diameter. Pin 1 carries data, pin 3 is ground, pin 4 provides +5V DC power at up to 100 milliamps, pin 5 carries the clock signal, and pins 2 and 6 remain unused. The offset design prevents accidental insertion of incompatible devices, though users could force a PS/2 connector into a standard DIN socket and damage both cable and port. Communication is synchronous and bidirectional at roughly 10 to 16 kilobits per second, allowing the host to send commands to the device as well as receive scancodes or motion data.

Two variants emerged early: the original keyboard version and the mouse version, which differ only in firmware and protocol interpretation. A PS/2 keyboard transmits 8-bit scancodes; a PS/2 mouse transmits relative motion and button state in three-byte packets. Many systems included a PS/2 splitter, a passive Y-adapter with one male connector and two female ports, allowing a single motherboard port to accommodate both keyboard and mouse simultaneously, though electrical loading could degrade signal integrity on longer cables.

Decline and persistence

USB began displacing PS/2 in the early 2000s, offering higher bandwidth, hot-swapping, and better power delivery. Most consumer systems dropped the port by 2010. However, industrial and legacy equipment still relies on PS/2: mechanical keyboards in manufacturing facilities, point-of-sale systems, kiosks, and cnc control panels often ship with PS/2 interfaces because they are simple, robust, and require no driver. The low power consumption and electrically quiet operation also made PS/2 valuable in noise-sensitive measurement and control applications.

Common failure modes include pin corrosion, intermittent contact from cable flexing at the connector strain relief, and accidental shorting of pins 1 and 5 (data and clock), which can lock up both the peripheral and the host. Many industrial environments still stock PS/2 peripherals and adapter cables, and motherboard manufacturers continue to include PS/2 headers on industrial and embedded boards even as consumer boards abandoned them entirely.