User manual

Appendix D Register-Level Programming
© National Instruments Corporation D-5 PC-DIO-96/PnP User Manual
Figure D-1. Control Word Formats for the 82C55A
Warning: During programming, note that each time a port is configured, output
ports A and C are reset to 0, and output port B is undefined.
D2 D1 D0D5 D4 D3D7 D6

(high nibble)
1 = input
0 = output
Control Word
Flag
1 = mode set

00 = mode 0
01 = mode 1
1X = mode 2
Mode Selection

1 = input
0 = output
Port A
Port C

(low nibble)
1 = input
0 = output
Port C

0 = mode 0
1 = mode 1
Mode Selection
Group A Group B
Port B
1 = input
0 = output
Flag
D2 D1 D0D3D7


0 = bit set/reset

1 = set
0 = reset
(000)
(001)
(010)
:
:
(111)
Control Word
Bit Select
Bit Set/Reset
a. Mode Set Word Format
b. Bit Set/Reset Word Format
Unused
D6 D5
D4