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Appendix C Register-Level Programming
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National Instruments Corporation C-15 PC-DIO-24/PnP User Manual
Port C status-word bit definitions for output (port A and port B):
Bit Name Description
7 OBFA* Output Buffer for Port A—A low setting indicates that
the CPU has written data to port A.
6 INTEA Interrupt Enable Bit for Port A—Setting this bit
enables interrupts from port A of the 82C55A. This bit
is controlled by setting/resetting PC6.
5–4 I/O Input/Output—These bits can be used for general-
purpose I/O when port A is in mode 1 output. If these
bits are configured for output, the port C bit set/reset
function must be used to manipulate them.
3 INTRA Interrupt Request Status for Port A—When INTEA
and OBFA* are high, this bit is high, indicating that an
interrupt request is pending for port A.
2 INTEB Interrupt Enable Bit for Port B—Setting this bit
enables interrupts from port B of the 82C55A. This bit
is controlled by setting/resetting PC2.
1 OBFB* Output Buffer for Port B—A low setting indicates that
the CPU has written data to port B.
0 INTRB Interrupt Request Status for Port B—When INTEB
and OBFB* are high, this bit is high, indicating that an
interrupt request is pending for port B.
At the digital I/O connector, port C has the following pin assignments
when in mode 1 output. Notice that the status of ACKA* and the status
of ACKB* are not included when port C is read.
D7 D6 D5 D4 D3 D2 D1 D0
OBFA* INTEA I/O I/O INTRA INTEB OBFB* INTRB