IBM 5170 Technical Reference page 63

Personal Computer. This includes converting a
two-byte break sequence to the one-byte IBM
Personal Computer format.
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 to a O.
Bit 0 Enable Output-Huffer-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
keyboard clock and data lines. The test result is placed in
the output buffer as follows:
00
No error detected.
01 The
I
keyboard clock
I
line is stuck low.
02 The
I
keyboard clock
I
line is stuck high.
03 The
I
keyboard data
I
line is stuck low.
System Board 1-41