PS/2 port
The PS/2-interface ( PS/2-connector, 2 port, originally auxiliary port ) is a widely used serial interface for input devices; usually keyboard and mouse, rarely other pointing devices such as a trackball or graphics tablet can be connected. It is for example used in personal computers, but there are increasingly displaced by the USB; some new motherboards no longer have PS/2-connectors, or only one for the keyboard (as of 2010 ).
History
First, the PS/2-interface was only on the RS/6000 computers and staff System/2-Reihe ( hence the name) from IBM to find and gained little importance. After the end of the personal System/2-Reihe the company but the IBM PC series continued again and used there now also PS2 ports for input devices. Thus, the PS/2-connector has established itself on the market of PC clones and appeared in the late 1990s with the introduction of the ATX standard for motherboards more on. Even Hewlett -Packard used it for the HP 9000 computers, as well as the Digital Equipment Corporation for some terminals.
Adapter
The many mice and some keyboards attached to USB adapter or are purely mechanical adapter to adapt the plug standard or shape; connected devices must be different based on the signal level, whether they are connected to a USB or PS2 format, and behave accordingly. A non -blown Web PS/2-connector USB works with these adapters as little as a continuous tape not to PS / 2 USB device.
Typical characteristic for a "real" PS/2-USB-Umwandler, can be connected to the pure PS/2-Geräte also, on the one hand is a housing for the electronics (mostly integrated in the cable), on the other hand offer almost all converter two PS/2-connectors, one for mouse and keyboard. Such adapters may be slightly more expensive than very cheap keyboards and mice, but are still a rewarding opportunity, quality PS/2-Tastaturen of computers to operate, have the USB, but no PS/2-connectors more.
Connection
The PS2 format uses a six pin mini - DIN connector, while the keyboard interface was provided in the previous AT- format with the larger five-pin DIN connector. Both connectors are identical on the electrical properties and ensure that it can be keyboards with DIN connector via a simple adapter cable to the PS2 format and vice versa. The only exception is the even older XT keyboard ( from the time of the IBM PC XT), although the same plug type as the AT used, but electric is compatible neither with the PS/2- even with the AT interface, some older keyboards have built-in switch. Mice were usually connected with the AT format via the RS- 232 interface.
For desktop PCs usually two PS/2-Buchsen are available, one for mouse (or pointing device), and keyboard. Although they have the same connectors and even the same pinout, come for mouse and keyboard different protocols used, so that the plug must not be interchanged. The connections are from the end of the 90 color-coded in the rule; the keyboard and the mouse has purple green plugs or sockets. The shades are set 99 System Design Guide Intel and Microsoft in the PC. In her PC 2001 System Design Guide and there is no longer that. These and other leading manufacturers recommend since the 1990s, the alternative USB.
On some notebooks and compact industrial PC motherboards, however, only a PS/2-socket is present, which is almost always suitable for mouse and keyboard, and - except for very old models - even for both at the same time. Mouse and keyboard differ in their behavior at power up and at reset, so that the notebook can distinguish between the two when they are directly connected. In order to connect the two at the same time, the two otherwise unused pins in the connector are used as additional clock and data lines. A Y- cable connects 5 V and GND of connector (notebook) with two clutches, and one pair of clock and data lines, each with a clutch. It will not be easy all contacts connected in parallel, otherwise the mouse and keyboard would block each other.
Some manufacturers have used the free pins for other purposes, such as a mouse for the combined operation of PS / 2 and RS- 232 or construed to turn with an extra key on the keyboard to the PC. This may occur when certain combinations of computers and mice or keyboards cause problems.
Some of the current PC systems, manufacturers have the PS2 format in favor of the hot -pluggable USB completely abandoned. For old operating systems, the BIOS emulates but still more or less the PS2 format, even if the mouse and keyboard are actually connected via USB.
Electrical interface
The physical interface of the PS2 format is identical to that of AT and MF2 keyboard. It has similar to the I2C data line and a clock line Data Clock. The levels on these lines are about 5 V. As can be seen in the diagrams or the wiring diagram, each participant can actively pull data or clock line low. The high level is passive, in each case by a pull-up resistor on clock and data (in this example 4.7 kOhm ) realized. To avoid data collision, each participant must consider whether perhaps during transfer another participant the same line disabled = high, on low = active attracts. The level low = active is so dominant over the level high = passive.
The interface is not for hot plugging, so plugging during operation, provided, neither the Protocol nor the connectors allow das. In many cases it does work ( more on keyboards than in mice ), but theoretically could be motherboard or keyboard or mouse is destroyed - at least but just not recognized. Gladly the operating system hung up earlier also when plugging 2 mouse.
Protocol
As the article or keyboard scan code already describe the protocol is different for a PC / XT keyboard as a historical predecessor of the other keyboards. Here, only the currently used protocol on an MF2 keyboard (AT keyboard) will be described.
Basically, the clock comes from the keyboard, both for sending data to the PC and to receive data from the PC. The frequency can vary considerably, since the clock is generated by software in the microcontroller of the keyboard. The communication between the keyboard and PC takes only two bidirectional lines ( data and clock). Thus, only four different initial states are possible.
Pull the PC at any time the clock line low, the keyboard must cancel a transmission in progress may immediately and begin again when the clock line is released. Unfortunately, many keyboards do not observe what often leads to problems with adapters and KVM switches.
In the graphs, the transitions marked as edges, to make the development easier to detect. If you take it exactly happens, the transition from high to low, almost abruptly ( through the low transistor). The transition from low to high, however, is rather slow and follows the shape of an exponential function ( RC element of transistor capacitances and pull-up resistor ).
The first diagram shows the course of the data shortly after turning on the computer. In this example from the keyboard BAT = $ AA is sent.
After this command Set LED keyboard with Ack would ' acknowledge byte-level command as understood. Next, the PC can then, for example, the parameter transfer $ 02 ( NumLock LED on). The command reference is under scan code.