From: siim@waterloo.hp.com (Brad Siim)
Subject: Re: Help with IBM-PC keyboard interfacing

[...] The PS2 keyboard interface is not a very friendly or easy to understand 
interface.  The best place to get information is in IBM's technical reference 
manual. All of the documentation that I have seen on this interface is the 
same/similar (ie. Compac's Technical Reference spec) and almost seems to be a 
direct copy of IBM's reference manual.

The core of ALL/most PS2 keyboard interfaces is the Intel 8042 microcontroller
or some derivative of it.  You will need to get a copy of this datasheet as
well as IBM's keyboard spec.

Basic Description:  

1. The 8042 side of the interface is two bidirectional open collector
   lines called CLOCK and DATA.
2. The KEYBOARD side of the interface is identical.
3. Both sides can drive the lines and often do so at the same time.   Each
   side has to figure out what is going on if the other side has decided to
   interrupt.
4. When the system CPU wants to send something to a KEYBOARD device it
   writes a command to the 8042 which tells the 8042 that the next command
   written to the 8042 should be passed on to the KEYBOARD device across the
   serial DATA line.
5. Assume that when idle both the DATA and CLOCK lines are high.
6. The 8042 pulls the clock line low to inhibit any transmission from the
   KEYBOARD.
7. The 8042 pulls the data line low to get the KEYBOARD's attention to the
   fact that it wants to transmit.
8. The 8042 release the CLOCK line and waits for the KEYBOARD to pull the
   clock line low.  When the CLOCK line has been pulled low the 8042 places
   its first bit of data on the DATA line.
9. The keyboard toggles the clock line and clocks data across on the data line.
   The 8042 controller will place a new bit on the data line each time the 
   clock is pulled LOW by the KEYBOARD. 

The following picture will explain better:

CK HHllllHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHllll
DA HHHllllll000000111111222222333333444444555555666666777777ppppppssssssLLLLHHH
        ^                                                                ^
    start bit                                                   important bit

where:  l = 8042 driving the line 
        L = KEYBOARD driving the line
        1..7 data
        p = Parity 8042 driving
        s = stop   8042 driving
        P = PARITY KEYBOARD driving the line
        S = STOP   KEYBOARD driving the line
       
11. The last bit in the last clock cycle is a data bit that is returned by the
    KEYBOARD. The keyboard may also be required to return a whole byte/bytes to
    the controller depending upon the data that was sent by the controller to
    the KEYBOARD.  After the controller has seen the last bit it usually
    will inhibit the keyboard by yanking the clock line low. Please note that
    if the keyboard has to return data to the 8042 it will do so under the 
    following protocal after the 8042 has released the clock line.
12. During the above transmission the contoller may abort the operation by
    holding the CLOCK line low.  The keyboard has to montitor the CLOCK line
    and stop clocking if it sees that the CLOCK line is not going high.
13. If the KEYBOARD wants to send data:  Assume both lines are high. If they
    are not high the keyboard can't send. If the clock line is being held low
    by the controller, the controller does not want data.  If the data line
    is being held low by the controller the controller wants to send data and
    the keyboard must start clocking it. 
14. So the lines are high:  The keyboard pulls the data line low and then pulls 
    the clock line low and then back to high. The data line is changed while
    the clock is HIGH.  The KEYBOARD pulls the clock line low and high and
    repeats the operation as follows:

CK HHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLLHHHLLlllll
DA HLLLLLL000000111111222222333333444444555555666666777777PPPPPPssssssLLLLLHHHH
      ^
    start bit

I have explained this in detail because the documentation is VERY hard to 
decripher.  I hope this is accurate and helps you. I have not discussed timing,
implementation or voltage levels as I'm sure you will not have trouble with
these aspects.