IBM RT Series Hardware Reference Manual page 416

respective pins as shown in the previous figure. If bit 4 is written
as aI, the card will interrupt the system unit's microprocessor on
the condition that pin 10 changes from high to low.
These pins are driven by open-collector drivers pulled to +5 Vdc
through 4.7 kQ resistors. They can each sink approximately 7 mA
and maintain 0.8 volts down-level.
IBM Monochrome Display &
Printer Adapter
Input from address hex 3BC
This instruction presents the system unit's Inicroprocessor with
data present on the pins associated with the output to hex 3BC.
This should normally reflect the exact value that was last written
to hex 3BC. If an external device should be driving data on these
pins at the time of an input (in violation of usage ground rules),
this data will be ORed with the latch contents.
IBM Monochrome Display &
Printer Adapter
Input from address hex 3BD
This instruction presents the real-time status to the system unit's
microprocessor from the pins as follows.
Bit 7
Bit 6 Bit 5 Bit4
Pin 11 Pin 10 Pin 12 Pin 13
IBM Monochrome Display &
Printer Adapter
Input from address hex 3BE
14 Monochrome Adapter
Bit 3 Bit 2 Bit 1 BitO
Pin 15
- - -