Measurement Computing CIO-DAS16/M1 - Manual (Page 25)

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Table 4-4. Digital I/O and Counter Registers
ADDRESS READ FUNCTION WRITE FUNCTION
BASE +400h Port A Input of 82C55 #1 Port A Output
BASE +401h Port B Input Port B Output
BASE +402h Port C Input Port C Output
BASE +403h None. No read back on 82C55 Configure 82C55 #1
82C54 Counter Registers
BASE +404h User Counter 0 User Counter 0 Load
BASE +405h User Counter 1 User Counter 1 Load
BASE +406h User Counter 2 User Counter 2 Load
BASE +407h None. No read back on 82C54 Counter Control
4.6.10 82C55 Digital I/O Regist ers
PORT A DATA
BASE ADDRESS + 400h
7 6 5 4 3 2 1 0
A7 A6 A5 A4 A3 A2 A1 A0
PORT B DATA
BASE ADDRESS + 401
7 6 5 4 3 2 1 0
B7 B6 B5 B4 B3 B2 B1 B0
Ports A and B can be programmed as input or output. Each is written to and read from in bytes, although
for control and monitoring purposes, individual bits are used.
Bit set/reset and bit-read functions require that unwanted bits are masked out of reads and OR’ed into
writes.
PORT C DATA
BASE ADDRESS + 402
7 6 5 4 3 2 1 0
C7 C6 C5 C4 C3 C2 C1 C0
CH3 CH2 CH1 CH0 CL3 CL2 CL1 CL0
Bit Weight Decimal
128
64 32 16 8 4 2 1
Bit Weight HEX
80
40 20 10 8 4 2 1
Port C can be used as one 8-bit port of either input or output, or it can be split into two 4-bit ports which
can be independently input or output. The notation for the upper 4-bit port is PCH3 - PCH0, and for the
lower, PCL3 - PCL0.
Although it can be split, every read and write to port C carries eight bits of data so unwanted information
must be AND’ed out of reads, and writes must be OR’ed with the current status of the other port.