IBM XT 5160 Technical Reference page 99

The keyboard interface is defined so that system software has
maximum flexibility in defining certain keyboard operations. This
is accomplished by having the keyboard return scan codes rather
than American Standard Code for Information Interchange
(ASCII) codes. In addition, all keys are typematic (if held down,
they will repeat) and generate both a make and a break scan code.
For example, key 1 produces scan code hex 01 on make and code
hex 81 on break. Break codes are formed by adding hex 80 to
make codes. The keyboard 110 driver can define keyboard keys
as shift keys or typematic, as required by the application.
The keyboard microprocessor (Intel 8048) performs several
functions, including a power-on self test when requested by the
system unit. This test checks the keyboard's ROM, tests memory,
and checks for stuck keys. Additional functions are keyboard
scanning, buffering of up to 16 key scan codes, maintaining
bidirectional serial communications with the system unit, and
executing the handshake protocol required by each scan-code
transfer.
Several different keyboard arrangements are available. These are
illustrated on the following pages. For information about the
keyboard routines required to implement non-US keyboards, refer
to the Guide to Operations and DOS manuals.
4-4 83-Key Keyboard