IBM AT 5170 Technical Reference page 68

Bit 5 IBM Personal Computer Mode-Writing a 1 to
this bit programs the keyboard to support the
IBM Personal Computer keyboard interface. In
this mode the controller does not check parity or
convert scan codes.
Bit 4 Disable Keyboard-Writing a 1 to this bit
disables the keyboard interface by driving the
I
clock
I
line low. Data is not sent or received.
Bit 3 Inhibit Override-Writing a 1 to this bit disables
the keyboard inhibit function.
Bit 2 System Flag-The value written to this bit is
placed in the system flag bit of the controller's
status register.
Bit
1
Reserved-Should be written as a
O.
Bit 0 Enable Output-Buffer-Full Interrupt-Writing a
1 to this bit causes the controller to generate an
interrupt when it places data into its output
buffer.
.~
AA
Self-Test-This commands the controller to perform
internal diagnostic tests. A hex 55 is placed in the output
buffer if no errors are detected.
AB Interface Test-This commands the controller to test the
I
keyboard clock
I
and
I
keyboard data
I
lines. The test
result is placed in the output buffer as follows:
00
01
02
03
04
No error detected.
The
I
keyboard clock
I
line is stuck low.
The
I
keyboard clock
I
line is stuck high.
The
I
keyboard data
I
line is stuck low.
The
I
keyboard data
I
line is stuck high.
1-52
System Board