Ack(Hexfa); Overrun (Hex 00); Diagnostic Failure (Hex Fd); Break Code Prefix (Hex Fo) - IBM AT 5170 Technical Reference

ACK(HexFA)
The keyboard issues an ACK response to any valid input other
than an Echo or Resend command.
If
the keyboard is interrupted
while sending ACK, it will discard ACK and accept and respond
to the new command. ,.,-.....,.
Overrun (Hex 00)
An overrun character is placed in position 17 of the keyboard
buffer, overlaying the last code if the buffer becomes full. The
code is sent to the system as an overrun when it reaches the top of
the buffer.
Diagnostic Failure (Hex FD)
The keyboard periodically tests the sense amplifier and sends a
diagnostic failure code if it detects any problems. If a failure
occurs during BAT, the keyboard stops scanning and waits for a
system command or power-down to restart. If a failure is
reported after scanning is enabled, scanning continues.
Break Code Prefix (Hex FO)
This code is sent as the first byte of a 2-byte sequence to indicate
the release of a key.
BAT Completion Code (Hex AA)
Following satisfactory completion of the BAT, the keyboard
sends hex AA. Hex FC (or any other code) means the keyboard
microprocessor check failed.
ECHO Response (Hex EE)
This is sent in response to an Echo command from the system.
4-10 Keyboard