Receiving Data From The Keyboard; Scan Code Translation - IBM AT 5170 Technical Reference

Receiving Data from the Keyboard
The keyboard sends data in a serial format using an 11-bit frame.
The first bit is a start bit, and is followed by eight data bits, an
odd parity bit, and a stop bit. Data sent is synchronized by a
clock supplied by the keyboard. At the end of a transmission, the
keyboard controller disables the interface until the system accepts
the byte. If the byte of data is received with a parity error, a
Resend command is automatically sent to the keyboard. If the
keyboard controller is unable to receive the data correctly after a
set number of retries, a hex FF is placed in its output buffer, and
the parity bit in the status register is set to 1, indicating a receive
parity error. The keyboard controller will also time a byte of data
from the keyboard. If a keyboard transmission does not end
within two milliseconds, a hex FF is placed in the keyboard
controller's output buffer, and the receive time-out bit in the
status register is set. No retries will be attempted on a receive
time-out error.
Note: When a receive error occurs in the default mode (bits
5,6, and 7 of the command byte set to 0), hex 00 is placed in
the output buffer instead of hex FF. See "Commands (I/O
Address Hex 64)" on page 1-51 for a detailed description of
the command byte.
Scan Code Translation
Scan codes received from the keyboard are converted by the
keyboard controller before being placed into the controller's
output buffer. The following figure shows the keyboard layout.
Each key position is numbered for reference.
System Board 1-43