IBM RT Series Hardware Reference Manual page 417

This instruction causes the data present on pins 1, 14, 16, 17, and
the IRQ bit to be read by the system unit's microprocessor. In
the absence of external drive applied to these pins, data read by
the system unit's microprocessor will match data last written to
hex 3BE in the same bit positions. Notice that data bits 0-2 are
not included. If external drivers are dotted to these pins, that
data will be ORed with data applied to the pins by the hex 3BE
latch.
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3
Bit 2 Bit 1 BitO
- - -
IRQ Pin 17 Pin 16 Pin 14 Pin 1
Enable
Par=O Par= 1 Par=O Par= 1 Par= 1
These pins assume the states shown after a reset from the system
unit's microprocessor.
Monochrome Adapter 15