Campbell Scientific CR7 Measurement and Control System - Manual (Page 144)

SECTION 13. CR7 MEASUREMENTS
13-20
13.7 PULSE COUNT MEASUREMENTS
Many pulse output type sensors (e.g.,
anemometers and flow-meters) are calibrated in
terms of frequency (counts/second). For these
measurements the accuracy is related directly
to the accuracy of the time interval over which
the pulses are accumulated. Variation in the
pulse sampling interval DOES NOT effect those
cases where the pulse measurement is
independent of time, i.e., where the total pulse
count is of interest instead of frequency.
The Pulse Count Instruction (#3) causes the
pulse channel counters to be read and reset to
zero every time the program table is executed.
The CR7 operating system checks the program
tables every 0.1 seconds to determine whether
or not a table should be executed. If any of the
3 tables requires execution and contains a
Pulse Count Instruction, ALL active pulse
channels in the specified I/O Module are read
and reset to zero before the table is executed.
Reading the pulse channels immediately
instead of when the Pulse Count Instruction is
encountered in the program sequence
eliminates variation in the pulse sampling
interval caused by variable program execution
times.
If the table execution is delayed, for example by
lengthy output processing, the pulse channel
counters are not read until the next execution
interval occurs. Whenever a table execution
overrun occurs, a decimal point appears below
the colon that separates the ID and Data field of
the display. Frequency data taken during an
overrun is invalid because the pulse sampling
interval is extended. The configuration code
(Parameter 4) entered in the Pulse Counting
Instruction allows measurements taken during
overruns to be discarded and replaced by the
count obtained during the previous, correct
sampling interval.
PULSE COUNT MEASUREMENTS USING
MULTIPLE I/O MODULES
Pulse channels contained in multiple I/O
Modules are read and reset based on the
program table priority. Within a specific
program table, the channels are reset according
to the priority of the I/O Module, i.e., Module 1
first and Module 4 last. For example, if Table 1
and 2 have simultaneous execution intervals, all
the active pulse channels in the I/O Modules
referenced by Pulse Count Instructions in Table
1 are read and reset first, followed by the I/O
Modules referenced in Table 2. Within Table 1
however, pulse channels contained in I/O
Module 1 are read first, followed by I/O Module
2, etc. All Pulse Counting Instructions
referencing the same I/O Module should be
contained in the same program table.
Approximately 0.6ms is required to read and
reset the pulse channels contained in one I/O
Module. Thus different I/O Modules having the
same pulse sample intervals are offset in real
time by 0.6ms per I/O Module. The pulse
sample interval associated with an I/O Module
is constant however, as long as the
programming rules listed below are observed.
1. If possible place all Pulse Count
Instructions in the same program table.
2. If more than one program table contains
Pulse Count Instructions, give Table 1 the
fastest execution interval and make the
other execution intervals even multiples of
Table 1's interval.
If these rules are violated a 0.6ms variation
could exist in an I/O module's sample interval.
For example, suppose Table 1 resets the pulse
counters in I/O Module 1 every 3 seconds and
Table 2 resets the pulse counters I/O Module 2
every second. When Table 1 is executed the
pulse counters in I/O Module 2 will be reset
0.6ms later than when only Table 2 is executed.
The frequencies must exceed 1.67 kHz before
the measurement is affected by a 0.6ms
variation in the pulse sampling interval.