Aku No Onna Kanbu [2/2]

http://www.computer-engineering.org/ps2keyboard/

A History Lesson:
The most popular keyboards in use today include:
• USB keyboard - Latest keyboard supported by all new computers (Macintosh and IBM/compatible).  These are relatively complicated to interface and are not covered in this article.
• IBM/Compatible keyboards - Also known as "AT keyboards" or "PS/2 keyboards", all modernPCs support this device.  They're the easiest to interface, and are the subject of this article.
• ADB keyboards - Connect to the Apple Desktop Bus of older Macintosh systems.  These are not covered in this article 
IBM introduced a new keyboard with each of its major desktop computer models.  The original IBM PC, and later the IBM XT, used what we call the "XT keyboard."  These areobsolete and differ significantly from modern keyboards; the XT keyboard is not covered in this article.  Next came the IBM AT system and later the IBM PS/2.  They introduced the keyboards we use today, and are the topic of this article.  AT keyboards and PS/2 keyboards were very similar devices, but the PS/2 device used a smaller connector and supported a few additional features.  Nonetheless, itremained backward compatible with AT systems and few of the additional features ever caught on (since software also wanted to remain backward compatible.)  Below is a summary of IBM's three major keyboards.
IBM PC/XT Keyboard (1981):
• 83 keys
• 5-pin DIN connector
• Simple uni-directional serial protocol
• Uses what we now refer to as scan code set 1
• No host-to-keyboardcommands
IBM AT Keyboard (1984) - Not backward compatible with XT systems(1).
• 84 -101 keys
• 5-pin DIN connector
• Bi-directional serial protocol
• Uses what we now refer to as scan code set 2
• Eight host-to-keyboard commands
IBM PS/2 Keyboard (1987) - Compatible with AT systems, not compatible with XT systems(1).
• 84 - 101 keys
• 6-pin mini-DIN connector• Bi-direction serial protocol
• Offers optional scan code set 3
• 17 host-to-keyboard commands 
The PS/2 keyboard was originally an extension of the AT device.  It supported a few additional host-to-keyboard commands and featured a smaller connector.  These were the only differences between the two devices.  However, computer hardware has never been about standards as much ascompatibility.  For this reason, any keyboard you buy today will be compatible with PS/2 and AT systems, but it may not fully support all the features of the original devices.
Today, "AT keyboard" and "PS/2 keyboard" refers only to their connector size.  Which settings/commands any given keyboard does or does not support is anyone's guess.  For example, the keyboard I'm using right now has a PS/2-styleconnector but only fully supports seven commands, partially supports two, and merely "acknowledges" the rest.  In contrast, my "Test" keyboard has an AT-style connector but supports every feature/command of the original PS/2 device (plus a few extra.)  It's important you treat modern keyboards as compatible, not standard.  If your design a keyboard-related device that relies on non-general features,it may work on some systems, but not on others...
Modern PS/2 (AT) compatible keyboards
• Any number of keys (usually 101 or 104)
• 5-pin or 6-pin connector; adaptor usually included
• Bi-directional serial protocol
• Only scan code set 2 guaranteed.
• Acknowledges all commands; may not act on all of them.

Footnote 1) XT keyboards use a completely different protocolthan that used by AT and PS/2 systems, making it incompatible with the newer PCs.  However, there was a transition period where some keyboard controllers supported both XT and AT (PS/2) keyboards (through a switch, jumper, or auto-sense.)  Also, some keyboards were made to work on both types of systems (again, through the use of a switch or auto-sensing.)  If you've owned such a PC or keyboard,...