The keyboard is interfaced to the system via the serial  shift register 
on one of the 8520 CIA chips.  The keyboard data line (KDAT) is connected
to the  SP pin , the keyboard clock (KCLK) is connected to the  CNT pin .
 Appendix G  contains a full description of the interface.

 How the Keyboard Data is Received 
 Type of Data Received 
 Limitations of the Keyboard