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
of
their 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
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