Programming Considerations - IBM 5150 Hardware Reference Manual

Programming Considerations
The Printer Adapter responds to 5 I/O instructions - 2 output and 3
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 CPU to read back the
contents of the two latches. The third allows the CPU to -read the real
time status of a group of pins on the connector.
A description of each instruction follows.
IBM Monochrome Display
&
Printer Adapter
Parallel Printer Adapter
Output to address 3BCH Output to address 378H
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2
Bit 1 Bit 0
Pin 9 Pin 8 Pin 7 Pin 6 Pin 5
Pin 4 Pin 3 Pin 2
This instruction captures data from the data bus and is present on the
respective pins. These pins are each capable of sourcing 2.6 rna and
sinking 24 rna.
It is essential that the external device not try to pull these lines to
~
ground.
IBM Monochrome Display
&
Printer Adapter
Parallel Printer Adapter
Output to address 3BEH Output to address 37AH
Bit 4 Bit 3 Bit 2
Bit 1 Bit 0
IRQ
Pin 17
Pin 16 Pin 14 Pin 1
Enable
This instruction causes this latch to capture the five least significant
bits of data bus. The four least significant bits present their outputs, or
inverted versions oftheir outputs to the respective pins shown above. If
bit 4 is written I, the card will interrupt the CPU on the condition that
Pin 10 transitions high to low.
These pins are driven by open collector drivers pulled to
+
5V through
4.7K OHM resistors. They can each sink approximately 7 rna and
maintain 0.8 volts down level.
~
Note: For pin references, see Parallel Interface Connector
Specifications, page 2-69.
2-67