IBM 5170 Technical Reference page 66

The following figures show bit definitions for the input, output,
and test-input ports.
Bit 0 Undefined
Bit 1 Undefined
Bit 2 Undefined
Bit 3 Undefined
Bit 4 RAM on the system board
o
=
Disable 2nd 256Kb of system board RAM
1
=
Enable 2nd 256Kb of system board RAM
Bit 5 Manufacturing jumper
o
=
Manufacturing jumper installed
1
=
Jumper not installed"
Bit 6 Display type switch
0= Primary display attached to Color/Graphics adapter
1
=
Primary display attached to Monochrome adapter
Bit 7 Keyboard inhibit switch
o
=
Keyboard inhibited
1
=
Kevboard not inhibited
Input-Port Definitions
Bit 0 System reset
Bit 1 Gate A20
Bit 2 Undefined
Bit 3 Undefined
Bit 4 Output buffer full
Bit 5 Input buffer empty
Bit 6 Keyboard clock (output)
Bit 7 Kevboard data (out out)
Output-Port Bit Definitions
Test-Input Port Bit Definitions
1-44 System Board