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PS/2接口

PS/2键盘及鼠标连接端口，分别以紫色及绿色标示。
类别	键盘及鼠标数据接口
产品历史
设计者	IBM
设计时间	1987年​（1987）
前身	DIN connector, DE-9 connector and Mini-DIN-9 InPort
取代	USB (USB human interface device class)
一般规格
热插拔	No
外接	Yes
缆线	4 wires plus shield
引脚	6
连接器	Mini-DIN-6
电力
信号	5 V DC
接地	Dedicated pin and shield
最大电压	7000500000000000000♠5.0±0.5 V
最大电流	275 mA
数据
数据信号	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)]
比特率	7–12 kbit/s
最多设备数	1 or 2
协议	Serial
引脚输出
PS/2接脚示意图
引脚1	+DATA	资料
引脚2		保留
引脚3	GND	地线
引脚4	Vcc	+5伏特直流电，可供应275mA的电流
引脚5	+CLK	时钟频率
引脚6		保留

PS/2端口是一种PC兼容型电脑系统上的接口，用来链接键盘及鼠标。其名称来源于1987年IBM推出的个人电脑：PS/2系列电脑。PS/2鼠标连接通常取代旧式的序列鼠标接口（DB-9 RS-232），而PS/2键盘连接则取代为IBM PC/AT设计的大型5针/180° DIN接口。PS/2的键盘及鼠标接口在电气特性上非常类似，采用了相同的通信协议，但键盘接口需要双向沟通。

PS/2平台引入了一个与键盘端口设计相同的第二个端口来连接鼠标，然而某些系统的键盘和鼠标端口可能无法互换，因为两种设备使用不同的指令集，且设备驱动程序通常被硬编码来分别在键盘与鼠标的端口地址上与这些设备通信。在早期，如果对调键盘和鼠标的插槽，大部分的台式机主板无法正确识别。现在已经出现共享接口，能够随意插入键盘或鼠标并正确识别处理。

目前，PS/2接口已逐渐被USB取代，仅少部分台式机仍提供完整的PS/2键盘及鼠标接口，部分机器甚至已无PS/2接口，大多数机器仅提供一组键盘及鼠标共享的PS/2接口或仅供键盘使用。有些鼠标及键盘可以使用转换器将USB接口转为PS/2，或者利用转接线从USB分接成键盘和鼠标的PS/2接口。不过，由于USB接口在未经特殊调整下仅能支持6键无冲突，而PS/2键盘接口则可支持所有按键同时无冲突。因此大多数主板上的PS/2键盘接口仍被保留，但仅保留一组键盘及鼠标共享的PS/2接口的主板日益普遍，键盘及鼠标分别拥有单独PS/2接口的主板已相当少见。

需要注意的是，PS/2接口在设计上不支持热插拔，尽管大多数PS/2键盘在实践中可以热插拔使用，但仍应避免这种行为。即使设备支持混用键盘和鼠标接口，跨接口类型的热插拔通常无法实现，需要重启设备后才能正常工作，这是由控制器和操作系统的限制共同导致的。

Communication protocol

Each port implements a bidirectional synchronous serial channel. 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.

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.)

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.

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

PS/2 dualport, corresponding splitter (Y-cable) and pinout (female)

Older laptops 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. 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 AT keyboards, 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 CHRP machines and SGI Indy, Indigo 2, and newer (Octane, etc.) computers. 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.

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/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.
• The PS/2 interface provides no restriction on key rollover, although USB keyboards have no such restriction either, unless 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.

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 Hz while standard PS/2 mice send interrupts at a default rate of 100 Hz 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 Hz and USB supporting a polling rate up to 1 kHz 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 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.

Conversion between PS/2 and USB

Passive PS/2 to USB adapters

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 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 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.

USB to PS/2 passive adapter pinout

USB pin Number	USB pin Name	PS/2 pin Number	PS/2 pin Name
1	VBUS	4	Vcc
2	D−	1	+DATA
3	D+	5	+CLK
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.

Color code

Color / Pantone		Function	Connector on PC
	Green / 3395C	PS/2 mouse / pointing device	6-pin mini-DIN female
	Purple / 2715C	PS/2 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.

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 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 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).

External links

• Keyboard and Auxiliary Device Controller (PDF). Hardware Interface Technical Reference -Common Technical-. IBM. October 1990. 
• PS/2 keyboard and mouse mini-DIN 6 connector pinouts, Burton sys .
• PS/2 In-depth information, Computer engineering, [11 September 2006], （原始内容存档于1 September 2006） .
• Technical information on Interfacing with the AT keyboard, Beyond logic, [25 March 2012], （原始内容存档于30 August 2018） .