Keyboards: There are two basic types of keyboards used with the PC. The older keyboard used with the IBM PC and XT read out keyboard scan codes that were interpreted by the PC's BIOS. The second type of keyboard contains a microcontroller that communicates with a microcontroller in the PC using a modified serial interface. The second type of Keyboard was introduced with the IBM PC AT. Curiously, both keyboards use the same 5 pin DIN connector even though they are not interchangeable. A few keyboards built in the late 1980s and early 1990s incorporated a switch that allowed the keyboard to be switched between XT and AT modes. The PS/2 introduced a 6 pin mini-DIN keyboard connector which -- some "experts" notwithstanding -- is neither better nor worse than the older connector. It's just different.

Some variation in electrical interfaces and connectors exist, but they are rare.

For the most part, keyboards can be "hot plugged" without damaging either the keyboard or the computer.

Keyboards come in a wide variety of keyboard layouts including non-English language keyboards. The initial common layout was "88" keys used with the IBM PC and XT. The AT introduced a 102 key keyboard that added two additional function keys (to the 10 already present); and word processing keys separate from the keypad. In 1995 a couple of Windows 95 specific keys were added to the keyboard. The Windows keys generally generate codes like Ctrl-Esc that have special meaning to Windows. Most keyboards use the QWERTY layout with few variations in the positions of most non-typewriter keys. The position of a few keys -- Alt, Ctrl, |, Esc changes from keyboard to keyboard.

Keyboard characters are read out at something like 500 to 1000 characters per second -- which is much faster than anyone can type.

Return To Index Copyright 1994-2002 by Donald Kenney.