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Appendix B
Register-Level Programming
DAQCard-DIO-24 User Manual
B-16
©
National Instruments Corporation
The following are the port C status-word bit definitions for output
(port A and port B).
Bit
Name
Description
7
OBFA*
Output Buffer Full for Port A—Low
indicates that the CPU has written data to
port A.
6
INTEA
Interrupt Enable Bit for Port A—If this
bit is high, interrupts are enabled from
the 82C55A for port A. Controlled by bit
set/reset of PC6.
5–4
I/O
Input/Output—Extra I/O status lines
when port A is in mode 1 output.
3
INTRA
Interrupt Request Status for
Port A—When INTEA is high and
OBFA* is high, this bit is high,
indicating an asserted interrupt request.
2
INTEB
Interrupt Enable Bit for Port B—If this
bit is high, interrupts are enabled from
the 82C55A for port B. Controlled by bit
set/reset of PC2.
1
OBFB*
Output Buffer Full for Port B—Low
indicates that the CPU has written data to
port B.
0
INTRB
Interrupt Request Status for Port B—
When INTEB is high and OBFB* is high,
this bit is high, indicating an asserted
interrupt request.
At the digital I/O connector, port C has the following pin assignments
when in mode 1 output. Notice in Figure B-3 that the status of ACKA*
and ACKB* is not included when port C is read.
7
6
5
4
3
2
1
0
OBFA*
INTEA
I/O
I/O
INTRA
INTEB
OBFB*
INTRB