IBM AT 5170 Technical Reference page 162

The keyboard checks the state of the
I
clock
I
line at intervals of
no less than 60 milliseconds. If a request-to-send is detected, the
keyboard counts 11 bits. After the tenth bit, the keyboard forces
the
I
data
I
line low and counts one more (the stop bit). This
action signals the system that the keyboard has received its data.
Upon receipt of this signal, the system returns to a ready state, in
~
which it can accept'keyboard output, or goes to the inhibited state
until it is ready.
Each system command or data transmission to the keyboard
requires a response from the keyboard before the system can send
its next output. The keyboard will respond within 20 milliseconds
unless the system prevents keyboard output. If the keyboard
response is invalid or has a parity error, the system sends the
command or data again. A Resend command should not be sent
in this case.
4-14 Keyboard