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PS/2 Keyboard Basics

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The origins of the PS/2 keyboard are back in the 1980s with the proliferation of the IBM Personal Computer (PC). This had the generic name of Personal System; hence, PS and the second version of this was therefore called the PS/2. The interface technology has remained under that name ever since. The keyboard interface evolved from the XT (83 key, 5pin DIN), through the AT (84-101 key, 5pin DIN) and eventually settled on the PS/2 (84-101 key, 6pin miniDIN). 

The PS/2 interface is essentially a custom serial interface with one device supported per connector (unlike the modern USB, Universal Serial Bus, which can handle numerous devices on a single port). The data rate is relatively slow (40 kbps) and the device is powered off a 5 V dc supply. 

Unlike the USB approach where devices are generally “hot swappable,” that is, they can be plugged in or unplugged at will, the PS/2 device cannot be removed without a system crash or freeze resulting.

The PS/2 keyboard supports communication from the keyboard to the host and vice versa, and the supply is provided from the host to the keyboard in the form of a 5 V line. 

Unlike the mouse, the keyboard has an on-board processor that checks the matrix of keys for any key presses and sends the appropriate code down the PS/2 data line.

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