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Interfacing Microcontroller with PC Keyboard

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 keyboard’s protocols in action. This interfacing example uses a 89s51 MCU to decode an IBM AT keyboard and output the ASCII equivalent of the key pressed at 9600 BPS.

Note that this page only deals with AT Keyboards. If you have any XT keyboards, you wish to interface, consider placing them in a museum. We will not deal with this type of keyboard in this document. XT Keyboards use a different protocol compared to the AT, thus code contained on this page will be incompatible.

PC Keyboard Theory

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 it’s serial line. If you are still holding it down, for longer than it’s typematic delay, another 1C will be sent. This keeps occurring until another key has been pressed, or if the ‘A’ key has been released. Read the rest of this entry »

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