National Instruments PC-DIO-24/PnP User Manual

Page 75

Appendix C

C-15

PC-DIO-24/PnP User Manual

Port C status-word bit definitions for output (port A and port B):

Bit

Name

Description

OBFA*

Output Buffer for Port A—A low setting indicates that
the CPU has written data to port A.

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.

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.

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.

OBFB*

Output Buffer for Port B—A low setting indicates that
the CPU has written data to port B.

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.