PS/2 connector

"PS/2 keyboard" redirects here. For the physical keyboards normally supplied with IBM PS/2 computers (among other hardware), see Model M keyboard.

PS/2 connector

The color-coded PS/2 connection ports (purple for keyboards and green for mice)
Type	Keyboard and computer mouse data connector
Production history
Designer	IBM
Designed	1987
Superseded	DIN connector and DE-9 connector
Superseded by	Universal Serial Bus
General specifications
Pins	6
Connector	Mini-DIN
Data
Data signal	Serial data at 10 to 16 kHz with 1 stop bit, 1 start bit, 1 parity bit (odd)
Pin out
Female connector from the front
Pin 1	+DATA	Data
Pin 2	Not connected	Not connected*
Pin 3	GND	Ground
Pin 4	Vcc	+5 V DC at 275 mA
Pin 5	+CLK	Clock
Pin 6	Not connected	Not connected**
* On some computers mouse data for splitter cable. ** On some computers mouse clock for splitter cable.

The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older DE-9 RS-232 "serial mouse" connector, while the PS/2 keyboard connector replaced the larger 5-pin/180° DIN connector used in the IBM PC/AT design. The PS/2 designs on keyboard and mouse interfaces are electrically similar and employ the same communication protocol. However, a given system's keyboard and mouse port may not be interchangeable since the two devices use a different set of commands.

Port availability

Old laptops generally have a single port that supports either a keyboard or a mouse. Sometimes the port also allows one of the devices to be connected to the two normally unused pins in the connector to allow both to be connected at once through a special splitter cable.^[1] This configuration is common on IBM/Lenovo Thinkpad notebooks among many others.

The PS/2 keyboard interface was electrically the same as the 5-pin AT system, and keyboards designed for one can be connected to the other with a simple wiring adapter. The PS/2 mouse interface is substantially different from RS-232 (which was generally used for mice on PCs without PS/2 ports), but nonetheless many mice were made that could operate on both with a simple wiring adapter.

PS/2 mouse and keyboard connectors have also been used in non-IBM PC-compatible computer systems, such as the DEC AlphaStation line, early IBM RS/6000 CHRP machines and SGI Indy, Indigo 2, and newer (Octane etc.) computers.^[2] Various Macintosh clone computers from the late 90s featured PS/2 mouse and keyboard ports, including the Motorola StarMax and the Power Computing PowerBase^[3]

Legacy port status and USB

PS/2 was considered a legacy port by the Intel/Microsoft PC 2001 specification of 2000; USB ports were preferred for connecting keyboards and mice. Despite this, PS/2 ports are included on most new motherboards.^[4] These PS/2 ports cause fewer problems when KVM switching with non-Wintel systems.^{[citation needed]} PS/2 ports may also be favored for security reasons in a corporate environment. Use of PS/2 ports for keyboard and mouse connectivity would allow USB ports to be totally disabled, preventing the connection of any USB removable disks. Also, with high-end keyboards, the only way to provide full n-key rollover is by using PS/2, as USB has a cap of 6 keys plus modifiers at the same time.^[5]

Many keyboards and mice can connect via either USB or PS/2, selecting the appropriate protocol at power-on. Such devices are generally equipped with a USB connector, and ship with a simple wiring adapter to allow connection to a PS/2 port. Older PS/2-only peripherals can be connected to a USB port via an active adapter, which generally provides a pair of PS/2 ports at the cost of one USB port.

Color code

Original PS/2 connectors were black or had the same color as the connecting cable (mainly white). Later the PC 97 standard introduced a color code: the keyboard port, and the plugs on compliant keyboards, were purple; mouse ports and plugs were green. Some vendors initially used a different color code: Logitech used the color orange for the keyboard connector for a short period, but soon switched to purple.^{[citation needed]} Today this code is still used on most PCs. The pinouts of the connectors are the same, but most computers will not recognize devices connected to the wrong port.

Color		Description
	Purple	Keyboard
	Green	Mouse

Wiring inside keyboard cable varies widely. Here are some more common color codes, but the reader is cautioned that the only reliable method of determining color assignment is to confirm by measuring continuity to the connector.

Description	Common		Alternate		Alternate		Alternate		Alternate		Alternate		Alternate
+CLK		Green		Blue		White		Yellow		White		Blue		Yellow
Data		White		Yellow		Green		Red		Green		Yellow		Gray
GND		Yellow		Black		Orange		Gray		Black		White		Black
Vcc		Red		Red		Blue		Brown		Red		Orange		Red

Software issues

As of 2010, version 8.0 of Microsoft's keyboard and mice drivers no longer supports PS/2 (even with USB adapters) in its supported keyboards and supported mice.^{[citation needed]}

Hardware issues

Hotplugging

PS/2 ports are designed to connect the digital I/O lines of the microcontroller in the external device directly to the digital lines of the microcontroller on the motherboard. They are not designed to be hot swappable. Hot swapping PS/2 devices usually does not cause damage because more modern microcontrollers tend to have more robust I/O lines built into them which are harder to damage than those of older controllers; however, hot swapping can still potentially cause damage on older machines, or machines with less robust port implementations.

If they are hot swapped, the devices must be similar enough that the driver running on the host system recognizes, and can be used with the new device. Otherwise, the new device will not function properly. While this is seldom an issue with standard keyboard devices, the host system rarely recognizes the new device attached to the PS/2 mouse port. In practice most keyboards can be hot swapped but this should be avoided.

Durability

PS/2 connectors are not designed to be plugged in and out very often, which can easily lead to bent or broken pins. PS/2 connectors only insert in one direction and must be rotated correctly before attempting connection. Most but not all connectors include an arrow or flat section which is usually aligned to the right or top of the jack before being plugged in. The exact direction may vary on older or non-ATX computers and care should be taken to avoid damage or bent pins when connecting devices. This issue is slightly alleviated in modern times with the advent of the PS/2 to USB adapter. Users can just leave the PS/2 plugged into the USB at all times and not risk damaging the pins this way.

Fault isolation

As noted, in a standard implementation both PS/2 ports are usually controlled by a single microcontroller on the motherboard. This makes design and manufacturing extremely simple and cheap. However, a rare side effect of this design is that a malfunctioning device can cause the controller to become confused, resulting in "both" devices acting erratically. The resulting problems can be difficult to troubleshoot (e.g. a bad mouse can cause problems that appear to be the fault of the keyboard).

A way to isolate the problem is to use a USB keyboard or mouse to determine which of the devices is at fault. A USB to PS/2 adapter can also be used to connect either of the devices to a USB port but the result can be undetermined because PS/2 and USB ports can use different voltages. Quality USB to PS/2 adapters have an integrated circuit that compensates the PS/2 - USB port voltage differences. Due to this difference it is recommend to avoid cheap adapters or try using a USB Keyboard or Mouse when troubleshooting PS/2 related errors.

See also

• Bus mouse
• DIN connector
• Din or AT Keyboard
• DE-9 connector
• Universal Serial Bus

References

1. ^ "PS/2 Keyboard (IBM Thinkpad) Y adapter". RU: Pinouts. http://pinouts.ru/InputCables/Ps2KeyboardYThinkpad.shtml. Retrieved 2011-06-14. 
2. ^ Lenerz, Gerhard (7 November 2006). "Common Input Devices". Hardware. SGIstuff. http://sgistuff.g-lenerz.de/hardware/input.php. Retrieved 2007-03-14. 
3. ^ "Power Computing PowerBase". Low end Mac. http://lowendmac.com/power/base.shtml. Retrieved 2011-04-04. 
4. ^ "Crosshair V Formula – Specifications". ASUS. http://www.asus.com/Motherboards/AMD_AM3Plus/Crosshair_V_Formula/#specifications. 
5. ^ "Keyboards With Full N-key Rollover". http://www.techpowerup.com/forums/showthread.php?t=115181. 

External links

• PS/2 keyboard and mouse mini-DIN 6 connector pinouts
• PS/2 In-depth information on PS/2
• Technical information on Interfacing with the AT keyboard