The color-coded PS/2 connection ports (purple for keyboard and green for mouse)
|Type||Keyboard and computer mouse data connector|
|Superseded||DIN connector, DE-9 connector and Mini-DIN-9 InPort|
|Cable||4 wires plus shield|
|Signal||5 V DC|
|Max. voltage||±0.5 V5.0|
|Max. current||275 mA|
|Data signal||Serial data at 10.0–16.7 kHz with 1 start bit, 8 data bits (LSB first), 1 parity bit (odd), 1 stop bit, [1 ack bit (if host-to-device)]|
|Max. devices||1 or 2|
|Female connector from the front|
|Pin 2||Not connected|
|Pin 4||Vcc||+5 V DC at 275 mA|
|Pin 6||Not connected|
The PS/2 port is a 6-pin mini-DIN connector used for connecting 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, unlike the otherwise similar Apple Desktop Bus connector used by Apple, a given system's keyboard and mouse port may not be interchangeable since the two devices use a different set of commands.
Communication is serial, synchronous and bidirectional. The attached device generates the clock signal. The host controls communication using the clock line; when the host pulls the clock low, communication from the device is inhibited.
Older laptops and most contemporary motherboards 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. This configuration is common on IBM/Lenovo Thinkpad notebooks among many others.
The PS/2 keyboard interface was electrically the same as for the 5-pin DIN connector on former AT keyboards, and keyboards designed for one can be connected to the other with a simple wiring adapter. In contrast to this, 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 passive wiring adapter, where the mice would detect the presence of the adapter due to its wiring and then switch protocols accordingly.
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. Various Macintosh clone computers from the late 1990s featured PS/2 mouse and keyboard ports, including the Motorola StarMax and the Power Computing PowerBase.
Legacy port status and USB
However, PS/2 ports continue to be included on many computer motherboards, and are favoured by some users, for various reasons including:
- PS/2 ports may be favored for security reasons in a corporate environment as they allow USB ports to be totally disabled, preventing the connection of any USB removable disks and malicious USB devices.
- The PS/2 interface provides no restriction on key rollover. The USB HID keyboard interface requires that it explicitly handle key rollover, with the full HID keyboard class supporting n-key rollover. However, the USB boot keyboard class (designed to allow the BIOS to easily provide a keyboard in the absence of OS USB HID support) only allows 6-key rollover. Some keyboard peripherals support only the latter class, and some OSes may fail to switch to using the full HID keyboard class with a device after boot. Short rollover hinders fast typists (including use in games), as well as users who depend on unusual hotkeys to be recognized by special software (for example, for system development purposes) or seek to use alternative input methods like Braille or chorded input.
- Some USB keyboards may not be able to operate the BIOS on certain motherboards due to driver issues or lack of support. The PS/2 interface has near-universal compatibility with BIOS.
- They cause fewer problems when KVM switching with non-Wintel systems.
USB mice have lower latencies than PS/2 mice because standard USB mice are polled at a default rate of 125 hertz while standard PS/2 mice send interrupts at a default rate of 100 hertz when they have data to send to the computer. Also, USB mice do not cause the USB controller to interrupt the system when they have no status change to report according to the USB HID specification's default profile for mice. Both PS/2 and USB allow the sample rate to be overridden, with PS/2 supporting a sampling rate of up to 200 hertz and USB supporting a polling rate up to 1 kilohertz as long as the mouse runs at full-speed USB speeds or higher.
Conversion between PS/2 and USB
Many keyboards and mice were specifically designed to support both the USB and the PS/2 interfaces and protocols, selecting the appropriate connection type at power-on. Such devices are generally equipped with a USB connector and ship with a passive wiring adapter to allow connection to a PS/2 port. Such passive adapters are not standardized and may therefore be specific to the device they came with. They cannot be used to adapt other devices to PS/2 ports. While combi-devices supporting USB and PS/2 are still available, most USB keyboards and mice in the 2010s no longer come with or even support the PS/2 protocol. Connecting them to a PS/2 port would require an adapter, actively translating between the protocols. Such adapters only support certain classes of USB devices such as keyboards and mice, but are not model- or vendor-specific.
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.) 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.
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.
PS/2 connectors are not designed to be plugged in and out very often, which can lead to bent or broken pins. PS/2 connectors insert in only 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 damaged 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 a PS/2 connector plugged into the PS/2-to-USB adapter at all times and not risk damaging the pins this way. A USB-to-PS/2 adapter does not have this problem.
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 and vice versa).
- "PS/2 Keyboard (IBM Thinkpad) Y adapter". RU: Pinouts. Retrieved 2011-06-14.
- Lenerz, Gerhard (7 November 2006). "Common Input Devices". Hardware. SGIstuff. Archived from the original on 2007-06-26. Retrieved 2007-03-14.
- "Power Computing PowerBase". Low end Mac. Retrieved 2011-04-04.
- "Massive, undetectable security flaw found in USB: It's time to get your PS/2 keyboard out of the cupboard". ExtremeTech. Retrieved 26 October 2015.
- "N-key Rollover via PS/2 and USB". Geek hack. Archived from the original on 2010-12-25.
- "Device Class Definition for HID 1.11" (PDF). Archived from the original (PDF) on 11 August 2014.
- "The pros and cons of PS-2 to USB adapters and converters".
- Adam Chapweske (2003-09-05). "The PS/2 Mouse/Keyboard Protocol". Retrieved 2016-11-26.
|Wikimedia Commons has media related to PS/2 connector.|
- "Keyboard and Auxiliary Device Controller" (PDF). Hardware Interface Technical Reference -Common Technical-. IBM. October 1990. Retrieved 2016-11-26.
- PS/2 keyboard and mouse mini-DIN 6 connector pinouts, Burton sys .
- PS/2 In-depth information, Computer engineering .
- Technical information on Interfacing with the AT keyboard, Beyond logic .