Mode 2-Bidirectional Bus - National Instruments PC-DIO-24 User Manual

Register-Level Programming
Mode 2–Bidirectional Bus
Mode 2 has an 8-bit bus that can transfer both input and output without changing the
configuration. The data transfers are synchronized with handshaking lines in port C. This mode
uses only port A; however, port B can be used in either mode 0 or mode 1 while port A is
configured for mode 2.
The control word written to the CNFG Register to configure port A as a bidirectional data bus in
mode 2 is shown as follows. If port B is configured for mode 0, then PC2, PC1, and PC0 of
port C can be used as extra input or output lines.
7 6
1 1
During a mode 2 data transfer, the status of the handshaking lines and interrupt signals can be
obtained by reading port C. The port C status-word bit definitions for a mode 2 transfer are
shown as follows.
The following are the port C status-word bit definitions for bidirectional data path (port A only).
7 6
OBFA* INTE1
Bit Name
7 OBFA*
6 INTE1
5 IBFA
PC-DIO-24 User Manual
5 4 3 2
X X X 1/0
5 4
IBFA INTE2
Description
Output Buffer Full—Low indicates that the CPU has written data
to port A.
Interrupt Enable Bit for Output—If this bit is set, interrupts are
enabled from the 82C55A for OBFA*. Controlled by bit set/reset of
PC6.
Input Buffer Full—High indicates that data has been loaded into
the input latch of port A.
1 0
1/0 1/0
Port C bits PC2,PC1,PC0
1 = input
0 = output
Port B direction
1 = input
0 = output
Group B Mode
0 = mode 0
1 = mode 1
3
INTRA
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Chapter 4
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I/O I/O I/O
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