Справочник Пользователя для National Instruments PC-DIO-24

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.

1

1

X

X

X

1/0

1/0

1/0

7

6

5

4

3

2

1

0

 

 1 = input

 0 = output

 

 1 = input

 0 = output

 

 0 = mode 0

 1 = mode 1

Port C bits PC2,PC1,PC0

Port B direction

Group B Mode

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

5

4

3

2

1

0

OBFA*

INTE1

IBFA

INTE2

INTRA

I/O

I/O

I/O

7

OBFA*

Output Buffer Full—Low indicates that the CPU has written data
to port A.

6

INTE1

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.

5

IBFA

Input Buffer Full—High indicates that data has been loaded into
the input latch of port A.

(continues)