View source for PS/2 port

{{Short description|6-pin mini-DIN connector for connecting keyboards and mice to a PC compatible computer}}
{{redirect2|PS/2 keyboard|PS/2 mouse|the keyboards normally supplied with IBM PS/2 computers|Model M keyboard|peripherals and game conversions for the video game console|PlayStation 2}}
{{Use dmy dates|date=January 2021}}
{{Infobox connector
| name = PS/2 port
| type = [[Computer keyboard|Keyboard]] and [[computer mouse]] data connector
| image = [[File:PS2 keyboard and mouse jacks.jpg|300px]]
| caption = The color-coded PS/2 connection ports (purple for keyboard and green for mouse)
| designer = [[IBM]]
| design_date = {{start date and age|1987}}
| superseded = [[DIN connector]], [[DE-9 connector]] and [[InPort|Mini-DIN-9 InPort]]
| superseded_by = [[USB]] ([[USB human interface device class]])
| hotplug = No
| external = Yes
| cable = 4&nbsp;wires plus shield
| num_pins = 6
| physical_connector = [[Mini-DIN|Mini-DIN-6]]
| electrical = 5&nbsp;V [[Direct current|DC]]
| earth = Dedicated pin and shield
| maximum_voltage = {{val|5.0|0.5|u=V}}
| maximum_current = 275&nbsp;mA
| data_signal = Serial data at 10.0–16.7&nbsp;kHz with 1&nbsp;start bit, 8&nbsp;data bits ([[Least significant bit|LSB]] first), 1&nbsp;parity bit (odd), 1&nbsp;stop bit, [1&nbsp;ack bit (if host-to-device)]
| data_bandwidth = 7–12&nbsp;[[Kilobit|kbit/s]]
| data_devices = 1 or 2{{efn|Keyboard and mouse ports may be combined into a single port which can be used to connect both by splitter cable.}}
| data_style = [[Serial communication|Serial]]
| pinout_image = [[Image:MiniDIN-6 Connector Pinout.svg|150px|center]]
| pinout_caption = Female connector from the front
| pin1 = Data
| pin1_name = +DATA
| pin2 = Not connected{{efn|Sometimes, mouse Data for splitter cable.}}
| pin2_name = 
| pin3 = Ground
| pin3_name = GND
| pin4 = +5&nbsp;V DC at 275&nbsp;mA
| pin4_name = [[IC power-supply pin|Vcc]]
| pin5 = [[Clock signal|Clock]]
| pin5_name = +CLK
| pin6 = Not connected{{efn|Sometimes, mouse Clock for splitter cable.}}
| pin6_name = 
| pinout_notes = {{noteslist}}
}}

The '''PS/2 port''' is a 6-pin [[mini-DIN connector]] used for connecting [[Computer keyboard|keyboards]] and [[computer mouse|mice]] to a [[PC compatible]] computer system. Its name comes from the [[IBM Personal System/2]] series of [[personal computer]]s, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older [[DE-9 connector|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 keyboard port is electrically and logically identical to the IBM AT keyboard port, differing only in the type of electrical connector used. The PS/2 platform introduced a second port with the same design as the keyboard port for use to connect a mouse; thus the PS/2-style 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 different sets of commands and the device drivers generally are hard-coded to communicate with each device at the address of the port that is conventionally assigned to that device. (That is, keyboard drivers are written to use the first port, and mouse drivers are written to use the second port.<ref>There is actually no technical reason that either port could not work with either type of device, if appropriate software was written to support that arrangement.</ref>)

== Communication protocol ==
Each port implements a [[Duplex (telecommunications)|bidirectional]] [[Synchronization#Communication|synchronous]] [[Serial communication|serial]] channel.<ref name="computer-engineering.org">{{cite web|url=http://www.computer-engineering.org/ps2mouse/ |title=The PS/2 Mouse Interface |date=1 April 2003|archive-url=https://web.archive.org/web/20080916065845/http://computer-engineering.org/ps2mouse/ |archive-date=16 September 2008}}</ref> The channel is slightly asymmetrical: it favors transmission from the input device to the computer, which is the majority case. The bidirectional IBM AT and PS/2 keyboard interface is a development of the unidirectional IBM PC keyboard interface, using the same signal lines but adding capability to send data back to the keyboard from the computer; this explains the asymmetry.<ref>Compare the logic diagrams in the IBM Personal Computer Technical Reference manual with those in the IBM Personal Computer AT Technical Reference manual.</ref>

The interface has two main signal lines, Data and Clock. These are single-ended signals driven by open-collector drivers at each end. Normally, the transmission is from the device to the host. To transmit a byte, the device simply outputs a serial frame of data (including 8 bits of data and a parity bit) on the Data line serially as it toggles the Clock line once for each bit. The host controls the direction of communication using the Clock line; when the host pulls it low, communication from the attached device is inhibited. The host can interrupt the device by pulling Clock low while the device is transmitting; the device can detect this by Clock staying low when the device releases it to go high as the device-generated clock signal toggles. When the host pulls Clock low, the device must immediately stop transmitting and release Clock and Data to both float high. (So far, all of this is the same as the unidirectional communication protocol of the IBM PC keyboard port, though the serial frame formats differ.) The host can use this state of the interface simply to inhibit the device from transmitting when the host is not ready to receive. (For the IBM PC keyboard port, this was the only normal use of signalling from the computer to the keyboard. The keyboard could not be commanded to retransmit a keyboard scan code after it had been sent, since there was no reverse data channel to carry commands to the keyboard, so the only way to avoid losing scan codes when the computer was too busy to receive them was to inhibit the keyboard from sending them until the computer was ready. This mode of operation is still an option on the IBM AT and PS/2 keyboard port.)<ref>IBM Personal Computer Technical Reference, IBM Personal Computer AT Technical Reference</ref>

To send a byte of data back to the device, the host pulls Clock low, waits briefly, pulls Data low and releases the Clock line again. The device then generates a Clock signal while the host outputs a frame of bits on the Data line, one bit per Clock pulse, similar to what the attached device would do to transmit in the other direction. However, while device-to-host transmission reads bits on falling Clock edges, transmission in the other direction reads bits on rising edges. After the data byte, the host releases the Data line, and the device will pull the Data line low for one clock period to indicate successful reception. A keyboard normally interprets the received byte as a command or a parameter for a preceding command. The device will not attempt to transmit to the host until both Clock and Data have been high for a minimum period of time.<ref>IBM Personal Computer AT Technical Reference</ref>

Transmission from the device to the host is favored because from the normal idle state, the device does not have to seize the channel before it can transmit—the device just begins transmitting immediately. In contrast, the host must seize the channel by pulling first the Clock line and then the Data line low and waiting for the device to have time to release the channel and prepare to receive; only then can the host begin to transmit data.

== Port availability ==
{{multiple image
| total_width       = 300
| image1            = PS2_dual_connector.JPG
| image2            = PS-2_Y_cable.png
| image3            = PS-2 Y-cable pinout female.gif
| footer            = PS/2 dualport, corresponding splitter (Y-cable) and pinout (female)
}}
Older [[laptop]]s and some 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.<ref>{{cite web | title = PS/2 Keyboard (IBM Thinkpad) Y adapter | url = http://pinouts.ru/InputCables/Ps2KeyboardYThinkpad.shtml |  publisher = Pinouts | location = RU | access-date = 2011-06-14 }}</ref> This configuration is common on [[IBM]]/[[Lenovo]] [[Thinkpad]] notebooks among many others.

The PS/2 keyboard interface is electrically the same as the 5-pin DIN connector on earlier [[IBM PC keyboard#Connection|AT keyboard]]s, and keyboards designed for one can be connected to the other with a simple wiring adapter. Such wiring adapters and adapter cables were once commonly available for sale. Note that IBM PC and PC XT keyboards use a different unidirectional protocol with the same DIN connector as AT keyboards, so though a PC or XT keyboard can be connected to PS/2 port using a wiring adapter intended for an AT keyboard, the earlier keyboard will not work with the PS/2 port. (At least, it cannot work with normal PS/2 keyboard driver software, including the system BIOS keyboard driver.)

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 based on 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]] [[Common Hardware Reference Platform|CHRP]] machines and [[SGI Indy]], [[SGI Indigo2|Indigo 2]], and newer ([[SGI Octane|Octane]], etc.) computers.<ref>{{cite web | title = Common Input Devices | url = http://sgistuff.g-lenerz.de/hardware/input.php | work = Hardware |  publisher = SGIstuff | first = Gerhard | last = Lenerz | date = 7 November 2006 | access-date = 2007-03-14 | archive-url = https://web.archive.org/web/20070626100840/http://sgistuff.g-lenerz.de/hardware/input.php | archive-date = 2007-06-26 }}</ref> Macintosh clone computers based on the "LPX-40" logic board design featured PS/2 mouse and keyboard ports, including the [[Motorola StarMax]] and the Power Computing PowerBase.<ref>{{cite web | publisher = Low end Mac | title = Power Computing PowerBase| url = http://lowendmac.com/1996/power-computing-powerbase/ | access-date = 2011-04-04}}</ref>

=== Legacy port status and USB ===
PS/2 is now considered a [[legacy port]], with [[USB]] ports now normally preferred for connecting keyboards and mice. This dates back at least as far as the [[Intel Corporation|Intel]]/[[Microsoft]] [[PC 2001]] specification of 2000.

However, as of 2023, although PS/2 ports are rarely included in off the shelf computer systems, they continue to be included on some computer motherboards and are favored by some users for various reasons including the following:
* 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.<ref>{{cite web|url=http://www.extremetech.com/computing/187279-undetectable-indefensible-security-flaw-found-in-usb-its-time-to-get-your-ps2-keyboard-out-of-the-cupboard|first1=Sebastian|last1=Anthony|date=31 July 2014|title=Massive, undetectable security flaw found in USB: It's time to get your PS/2 keyboard out of the cupboard|website=ExtremeTech|publisher=Ziff Davis, LLC|access-date=26 October 2015}}</ref>
* The PS/2 interface provides no restriction on [[key rollover]], although USB keyboards have no such restriction either, unless [[USB human interface device class#Keyboards|operated in BOOT]] mode, which is the exception.
* To free USB ports for other uses like removable USB devices.
* 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.
* The simplicity of PS/2 makes it useful for embedded systems and retro computers that may lack USB host peripheral. This also takes advantage of the wide availability of USB keyboard and mouses with PS/2 compatibility mode. <ref>{{cite web |last1=Eater |first1=Ben |title=So how does a PS/2 keyboard interface work? |url=https://www.youtube.com/watch?v=7aXbh9VUB3U |website=www.youtube.com |publisher=YouTube |access-date=14 December 2024}}</ref>

==== Latency of mice ====
USB mice send data more quickly than PS/2 mice because standard USB mice are polled at a default rate of 125&nbsp;Hz while standard PS/2 mice send interrupts at a default rate of 100&nbsp;Hz when they have data to send to the computer.<ref name="cybergamer">{{cite web|url=http://eu.cybergamer.com/forums/thread/260663/Mouse-Optimization-Guide:-Acceleration-Fix-and-Polling-Rate/ |title=Mouse Optimization Guide: Acceleration Fix and Polling Rate |publisher=eu.cybergamer.com |date=2011-07-26 |access-date=2024-10-16}}</ref><ref>{{cite web|url=http://web.fe.up.pt/~pfs/aulas/lcom2012/labs/lab5/lab5.html |title=LCOM 2012/2013: Lab5 |publisher=web.fe.up.pt |access-date=2024-10-16}}</ref>

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.<ref name="hidspec">{{cite web|title=Device Class Definition for HID 1.11|url=http://www.usb.org/developers/devclass_docs/HID1_11.pdf|url-status=dead|archive-url=https://web.archive.org/web/20140811112340/http://www.usb.org/developers/devclass_docs/HID1_11.pdf|archive-date=11 August 2014}}</ref> Both PS/2 and USB allow the sample rate to be overridden, with PS/2 supporting a sampling rate of up to 200&nbsp;Hz<ref name="computer-engineering.org"/> and USB supporting a polling rate up to 1&nbsp;kHz<ref name="cybergamer"/> as long as the mouse runs at full-speed USB speeds or higher, while USB 2.0 capable devices can support up to 8 kHz polling rates.

==== USB key rollover limitations ====
The [[USB human interface device class#Keyboards|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.<ref>{{cite web | url = http://geekhack.org/showwiki.php?title=USB+versus+PS+2#Full+NKRO | publisher = Geek hack | title = N-key Rollover via PS/2 and USB | archive-url = https://web.archive.org/web/20101225222038/http://geekhack.org/showwiki.php?title=USB+versus+PS+2 | archive-date = 2010-12-25 }}</ref>

=== Conversion between PS/2 and USB ===
[[File:USB to PS2 keyboard and mouse adapter.jpg|thumb|Passive PS/2  to USB adapters]]
[[File:Usb to ps 2 adapter IMGP1414.jpg|thumb|Active USB to PS/2 converter]]
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 ''[[Passivity (engineering)|passive]]'' wiring adapter to allow connection to a PS/2 port. Such passive adapters may be specific to the devices they came with; however, the most common configuration involves sharing the GND and Vcc (+5 V) pins of both protocols, connecting the [[USB hardware#Pinouts|USB D+ pin]] to the PS/2 +CLK pin, and connecting the USB D- pin to the PS/2 +DATA pin. Using such adapters requires a dual-mode controller on the keyboard or mouse side.<ref name=":0">{{Cite web |title=USB to PS/2 mouse or keyboard adapter pinout signals @ PinoutGuide.com |url=https://pinoutguide.com/InputCables/usb_ps2_mouse_pinout.shtml |access-date=2024-10-28 |website=pinoutguide.com}}</ref><ref>{{Cite book |url=https://media.digikey.com/pdf/Data%20Sheets/Cypress%20PDFs/CY7C63722,23,43.pdf#page=3 |title=enCoRe™ USB Combination Low-Speed USB and PS/2 Peripheral Controller. CY7C63722, CY7C63723, CY7C63743 |date=2004-09-27 |publisher=[[Cypress Semiconductor]] |pages=3}}</ref><ref>{{Cite book |url=https://www.nxp.com/docs/en/reference-manual/DRM014.pdf#page=8 |title=USB and PS/2 Multimedia Keyboard Interface. Designer Reference Manual. M68HC08 Microcontrollers |date=2006 |publisher=[[Freescale Semiconductor]] |pages=8}}</ref>
{| class="wikitable"
|+USB to PS/2 passive adapter pinout<ref name=":0" />
!USB pin Number
!USB pin Name
!PS/2 pin Number
!PS/2 pin Name
|-
|1
|V<sub>BUS</sub>
|4
|Vcc
|-
|2
|D−
|1
| +DATA
|-
|3
|D+
|5
|<nowiki>+CLK</nowiki>
|-
|4
|GND
|3
|GND
|}
Older PS/2-only peripherals can be connected to a USB port via an active converter, which generally provides a pair of PS/2 ports (which may be designated as one keyboard and one mouse, even though both ports may support both protocols) at the cost of one USB port on the host computer.<ref name="PS/2Adaptors">{{cite web |title=The pros and cons of PS-2 to USB adapters and converters |url=http://searchenterprisedesktop.techtarget.com/tip/The-pros-and-cons-of-PS-2-to-USB-adapters-and-converters |url-access=registration |website=TechTarget}}</ref>

== Color code ==
{{Main|PC System Design Guide#Color-coding scheme for connectors and ports}}
{| class="wikitable floatright"
|-
! colspan="2" | Color / [[Pantone]] !! Function !! Connector on PC
|-
! style="background-color: #00c389;" | &nbsp;
! [[Green]] / 3395C
| PS/2 [[Mouse (computing)|mouse]] / [[pointing device]]
| rowspan="2" | 6-pin [[Mini-DIN connector|mini-DIN]] female
|-
! style="background-color: #8b84d7;" | &nbsp;
! [[Purple]] / 2715C
| PS/2 [[Keyboard (computing)|keyboard]]
|}
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.
{{clear}}

== Hardware issues ==
{{more citations needed |section: Hardware issues|date=March 2011}}
{{Missing information|section|i8042 issues as described in [https://github.com/GlasgowEmbedded/glasgow/blob/832f6ff362261ebe629e480f365e3e761b951603/software/glasgow/applet/interface/ps2_host/__init__.py#L17-L50 Glasgow]|date=December 2020}}

=== 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 swapping|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;<ref>{{cite web |author=Adam Chapweske |url=http://www.computer-engineering.org/ps2protocol/ |title=The PS/2 Mouse/Keyboard Protocol |date=2003-09-05 |access-date=2016-11-26 |archive-url=https://web.archive.org/web/20161116003545/http://computer-engineering.org/ps2protocol/|archive-date=2016-11-16 |url-status=dead}}</ref> 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 lead to bent or broken pins. Additionally, PS/2 connectors only insert in one direction and must be rotated correctly before attempting connection. (If a user attempts to insert the connector in the wrong orientation and then tries to rotate it to the correct orientation without first pulling it out, then bent pins could result.)

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.

=== Fault isolation ===
In a standard implementation both PS/2 ports are usually controlled by a [[Super I/O|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. (A well designed and programmed controller will not behave in this way.) 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).

== See also ==
* [[BIOS interrupt call]]
* [[IBM PC keyboard#Connection|DIN connector]] on [[IBM PC keyboard]]s
* [[Bus mouse]]
* [[Computer mouse#Connectivity and communication protocols|Connections]] on [[Computer mouse|mice]]
* [[DE-9 connector]]
* [[USB]]

== References ==
{{reflist |32em}}

== External links ==
{{commons |PS/2 connector}}
* {{cite book|url=http://www.mcamafia.de/pdf/pdfref.htm |title=Hardware Interface Technical Reference -Common Technical- |chapter-url=http://www.mcamafia.de/pdf/ibm_hitrc07.pdf |chapter=Keyboard and Auxiliary Device Controller |publisher=IBM |date=October 1990}}
* {{Citation | url = http://www.burtonsys.com/PS2_keyboard_and_mouse_mini-DIN-6_connector_pinouts.html | title = PS/2 keyboard and mouse mini-DIN 6 connector pinouts | publisher = Burton sys}}.
* {{Citation | url = http://www.computer-engineering.org/ps2protocol | title = PS/2 In-depth information | publisher = Computer engineering | access-date = 11 September 2006 | archive-url = https://web.archive.org/web/20060901070855/http://www.computer-engineering.org/ps2protocol/ | archive-date = 1 September 2006 | url-status = dead }}.
* {{Citation | url = http://retired.beyondlogic.org/keyboard/keybrd.htm | title = Technical information on Interfacing with the AT keyboard | publisher = Beyond logic | access-date = 25 March 2012 | archive-url = https://web.archive.org/web/20180830070412/http://retired.beyondlogic.org/keyboard/keybrd.htm | archive-date = 30 August 2018 | url-status = dead }}.

{{Basic computer components}}

{{DEFAULTSORT:PS 2 connector}}
[[Category:Computer connectors]]
[[Category:Deutsches Institut für Normung]]
[[Category:Computer keyboards]]
[[Category:Pointing devices]]
[[Category:Computer-related introductions in 1987]]
[[Category:Computer hardware standards]]