Measurement Computing PC104-AC5 - Manual (Page 11)

In summary form, the registers and their function are listed on the following table.
Within each register are eight bits which may constitute a byte of data or eight
individual bit set/read functions.
Table 3-1. Board Register Functions
Configure I/O None. BASE +3
1st Port C Output 1st Port C Input BASE +2
1st Port B Output 1st Port B Input BASE +1
1st Port A Output 1st Port A Input BASE +0
WRITE FUNCTION READ FUNCTION ADDRESS
3.2 DIGITAL DATA REGISTERS
Port A
Base Address + 0
A0 A1 A2 A3 A4 A5 A6 A7
0 1 2 3 4 5 6 7
Port B Data
Base Address +1
B0 B1 B2 B3 B4 B5 B6 B7
0 1 2 3 4 5 6 7
Ports A & B may be programmed as input or output. Each is written to and read from
in bytes, but for control and monitoring, use individual bits.
When using bit set/reset and bit read functions, unwanted bits must be masked out of
reads and ORed into writes.
Port C Data
Base Address +2
CL0 CL1 CL2 CL3 CH0 CH1 CH2 CH3
C0 C1 C2 C3 C4 C5 C6 C7
0 1 2 3 4 5 6 7
Port C may be used as one 8-bit port of either input or output, or it may be split into
two 4-bit ports which may be independently input or output. The notation for the
upper 4-bit port is CH3 - CH0, and for the lower, CL3 - CL0.
Although it may be split, every read and write to port C carries eight bits of data so
unwanted information must be ANDed out of reads, and writes must be ORed with the
current status of the other port.
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