AT Keyboard Source Code Interfacer

Why would you want to interface the Keyboard? The IBM keyboard can be a cheap alternative to a keyboard on a Microprocessor development system. Or maybe you want a remote terminal, just couple it with a LCD Module.
Maybe you have a RS-232 Barcode Scanner or other input devices, which you want to use with existing software which only allows you to key in numbers or letters. You could design yourself a little box to convert RS-232 into a Keyboard Transmission, making it transparent to the software.
An interfacing example is given showing the keyboards protocols in action. This interfacing example uses a 68HC705J1A MCU to decode an IBM AT keyboard and output the ASCII equivalent of the key pressed at 9600 BPS.
The IBM keyboard you most probably have sitting in front of you, sends scan codes to your computer. The scan codes tell your Keyboard Bios, what keys you have pressed or released. Take for example the A Key. The A key has a scan code of 1C (hex). When you press the A key, your keyboard will send 1C down its serial line. If you are still holding it down, for longer than its typematic delay, another 1C will be sent. This keeps occurring until another key has been pressed, or if the A key has been released.
However your keyboard will also send another code when the key has been released. Take the example of the A key again, when released, the keyboard will send F0 (hex) to tell you that the key with the proceeding scan code has been released. It will then send 1C, so you know which key has been released.
Your keyboard only has one code for each key. It doesnt care it the shift key has been pressed. It will still send you the same code. Its up to your keyboard BIOS to determine this and take the appropriate action. Your keyboard doesnt even process the Num Lock, Caps Lock and Scroll Lock. When you press the Caps Lock for example, the keyboard will send the scan code for the cap locks. It is then up to your keyboard BIOS to send a code to the keyboard to turn on the Caps lock LED.
Now theres 101 keys and 8 bits make 256 different combinations, so you only need to send one byte per key, right? Unfortunately a handful of the keys found on your keyboard are extended keys, and thus require two scan code. These keys are preceded by a E0 (hex). But it doesnt stop at two scan codes either. How about E1,14,77,E1,F0,14,F0,77! Now that cant be a valid scan code? Wrong again. Its happens to be sent when you press the Pause/break key. Dont ask me why they have to make it so long! Maybe they were having a bad day or something?
When an extended key has been released, it would be expect that F0 would be sent to tell you that a key has been released. Then you would expect E0, telling you it was an extended key followed by the scan code for the key pressed. However this is not the case. E0 is sent first, followed by F0, when an extended key has been released.
Tthe PCs keyboard implements a bi-directional protocol. The keyboard can send data to the Host and the Host can send data to the Keyboard. The Host has the ultimate priority over direction. It can at anytime (although the not recommended) send a command to the keyboard.
The keyboard is free to send data to the host when both the KBD Data and KBD Clock lines are high (Idle). The KBD Clock line can be used as a Clear to Send line. If the host takes the KBD Clock line low, the keyboard will buffer any data until the KBD Clock is released, ie goes high. Should the Host take the KBD Data line low, then the keyboard will prepare to accept a command from the host.