Measurement Computing CIO-DAS-TEMP - Manual (Page 14)

First, program the 82C54 control register for CTR 1 to produce a single pulse for the duration of the
count period. Next, load an initial value equal to the period for which counts from the V/F converter will
be totaled See the previous explanation of initial count values for information on what value to load into
this counter. It works as a down-counter, the only choice available.
BASE + 1 Read - Counter 1 Current Count Register of the 82C54
Data 0 Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 Data 7
0 1 2 3 4 5 6 7
Will return current count.
BASE + 2 Write - Counter 2 Load Register of the 82C54
Data 0 Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 Data 7
0 1 2 3 4 5 6 7
This counter is used to sequence through multiple channels when more than one channel is read. First,
program the 82C54 control register for CTR 2 to function as a rate generator. The register at Base + 5
sets the input (prescaler) frequency for this counter.
Next, load an initial value equal to the number of clock periods between channel readings
BASE + 2 Read - Counter 2 Current Count Register of the 82C54
Data 0 Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 Data 7
0 1 2 3 4 5 6 7
Will return current count.
BASE + 3 Write - Control Register of the 82C54
BCD M0 M1 M2 RW0 RW1 SC0 SC1
0 1 2 3 4 5 6 7
The 82C54 counter is described in g reat detail in the Harris or Instel 82C54 data sheet. A brief
description of the control bits follows.
SC1 SC0 Function
0 0 Select counter 0
0 1 Select counter 1
1 0 Select counter 2
1 1 Read Command
RW1
RW0 Function
0 0 Counter latch
0 1 Read/Write least significant byte of selected counter
1 0 Read/Write most significant byte of selected counter
1 1 Read/Write least significant byte then most significant byte
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