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Chapter 5 Register Map and Description
PCI-DIO-96 User Manual 5-4
©
National Instruments Corporation
is 1, bits 6 through 0 select the I/O characteristics of the 82C55A ports.
These bits also select the mode in which the ports are operating; that is,
mode 0, mode 1, or mode 2. When the control word flag is 0, bits 3
through 0 select the bit set/reset format of port C.
Figure 5-1. Control Word Formats for the 82C55A
D7 D6 D5 D4 D3 D2 D1 D0
Group A Group B
Mode Selection
00 = Mode 0
01 = Mode 1
1X = Mode 2
Control Word Flag
1 = Mode Set
Port A
1 = Input
0 = Output
Port C
(high nibble)
1 = Input
0 = Output
Port C
(low nibble)
1 = Input
0 = Output
Port B
1 = Input
0 = Output
Mode Selection
0 = Mode 0
1 = Mode 1
a. Control Word Flag, Mode Set (bit 7 = 1)
D7 D6 D5 D4 D3 D2 D1 D0
Control Word Flag
0 = Bit Set/Reset
Unused
Bit Set/Reset
1 = Set
0 = Reset
Bit Select
(000)
(001)
(010)
:
:
(111)
b. Control Word Flag, Bit Set/Reset (bit 7 = 0)