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Register-Level Programming Appendix E
Lab-PC+ User Manual E-30 © National Instruments Corporation
Port C status-word bit definitions for output (Port A and Port B):
76543210
OBFA* INTEA I/O I/O INTRA INTEB OBFB* INTRB
Bit Name Description
7 OBFA* Output buffer full for Port A. Low indicates that the CPU has
written data out to Port A.
6 INTEA Interrupt enable bit for Port A. If this bit is high, interrupts are
enabled from the 8255A for Port A. Controlled by bit set/reset of
PC6.
5,4 I/O Extra I/O status line 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 that an interrupt request
is asserted.
2 INTEB Interrupt enable bit for Port B. If this bit is high, interrupts are
enabled from the 8255A 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 out to Port B.
0 INTRB Interrupt request status for Port B. When INTEB is high and
OBFB* is high, this bit is high, indicating that an interrupt request
is asserted.
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 ACKB* is not provided when Port C is read.
OBFA*PC7
PC6
PC5
PC4
PC3
PC2
PC1
PC0
I/O
INTRA
ACKB*
OBFB*
INTRB
I/OGroup A
Group B
ACKA*