Measurement Computing CIO-DAS16/M1 - Manual (Page 26)

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OUTPUT PORTS
In 82C55 mode 0 configuration, ports configured for output hold the output data written to them. This
output byte can be read back by reading a port configured for output.
INPUT PORTS
In 82C55 mode 0 configuration, ports configured for input read the state of the input lines at the moment
the read is executed. Transitions are not latched.
For information on modes 1 (strobed I/O) and 2 (bi-directional strobed I/O), please refer to an Intel or
AMD data book and see the 82C55 data sheet.
82C55 CONTROL REGISTER
BASE ADDRESS + 403
7 6 5 4 3 2 1 0
Group A Group B
MS M3 M2 A CU M1 B CL
The 82C55 can be programmed to operate in Input/ Output (mode 0), Strobed Input/ Output (mode 1) or
Bi-Directional Bus (mode 2).
Information on programming the 82C55 in mode 0 is included. Those wishing to use the 82C55 in modes
1 or 2, or who wish to program the 82C54 counter must procure a data sheet from the Intel or Harris web
site.
When the PC is powered up or RESET, the 82C55 is reset. This places all 24 lines in input mode; no
further programming is needed to use the 24 lines as TTL inputs.
To program the 82C55 for other modes, the following control code bits must be assembled into an 8-bit
byte.
MS = Mode Set. 1 = mode set active
M3 M2 Group A Function
0 0 Mode 0 Input / Output
0 1 Mode 1 Strobed Input / Output
1 X Mode 2 Bi-Directional Bus
A B CL CH Independent Function
1 1 1 1 Input
0 0 0 0 Output
M1 = 0 is mode 0 for group B. Input / Output
M1 = 1 is mode 1 for group B. Strobed Input / Output
Ports A, B, C-High, and C-Low can be independently programmed for input or output.
Group A and group B can be independently programmed in one of several modes. The most commonly
used is mode 0, input/output mode. The codes for programming the 82C55 in this mode are in Table 4-5
below.