Programming Considerations - IBM RT Series Hardware Reference Manual

Programming Considerations
The printer adapter portion of the IBM Monochrome Display and
Printer Adapter responds to five I/O instructions: two output
and three input. The output instructions transfer data into 2
latches whose outputs are presented on pins of a 25-pin D-shell
connector.
Two of the three input instructions allow the system unit's
microprocessor to read back the contents of the two latches. The
third allows the system unit's microprocessor to read the real-time
status from a group of pins on the connector.
A description of each instruction follows.
IBM Monochrome Display &
Printer Adapter
Output to address hex 3BC
Bit 7 Bit 6 Bit 5 Bit4
Pin 9 Pin 8 Pin 7 Pin 6
The instruction captures data from the data bus and is present on
the respective pins. Each of these pins is capable of sourcing 2.6
rnA and sinking 24 rnA.
It is essential that the external device does not try to pull these
lines to ground.
IBM Monochrome Display &
Printer Adpater
Output to address hex 3BE
Bit4
IRQ
Enable
This instruction causes the latch to capture the five least
significant bits of the data bus. The four least significant bits
present their outputs, or inverted versions of their outputs, to the
Monochrome Adapter 13