Operator`s manual

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
9-2
The count is incremented when the input
voltage changes from below 1.5 volts to
above 3.5 volts. The maximum input
voltage is ±20 volts.
LOW LEVEL AC
This mode is used for counting frequency of
AC signals from magnetic pulse flow
transducers or other low voltage, sine wave
outputs.
MINIMUM AC INPUT VOLTAGE: 6 millivolts
RMS
INPUT HYSTERESIS: 11 millivolts
MAXIMUM AC INPUT VOLTAGE: 20 volts
RMS
FREQUENCY RANGE:
AC Input Voltage (RMS) Range
20 millivolts 1 Hz to 100 Hz
50 millivolts 0.5 Hz to 400 Hz
150 millivolts to 20 volts 0.3 Hz to 1000 Hz
(consult the factory if higher frequencies are
desired)
SWITCH CLOSURE
In this configuration, the minimum switch
closed time is 3 milliseconds. The minimum
switch open time is 4 milliseconds. The
maximum bounce time is 1 millisecond open
without being counted.
The 4 pulse count input channels each have
eight bit counters. Input frequencies greater
than 2550 Hz (the limit of the eight bit counter,
255 counts at the reset interval of 0.1 second)
can be counted by combining two counters on
one input channel. When this option is selected,
channel 1 or 3 is specified in parameter 2 and
the input is connected to either channel 1 or 3.
In this case, either channel 2 or 4 or both are not
used. One may use 2 sixteen bit pulse count
channels or a combination of 1 sixteen bit
counter and 2 eight bit counters. (Campbell
Scientific has a hardware and software
modification that changes the reset interval to
0.0125 seconds allowing greater input frequency
with the 8 bit counters (255/.0125=20.4 kHz).
Pulse measurements with this modification will
not work when voltage measurements are made
with the "slow" integration. Consult the factory
for additional information.)
Every 0.1 seconds, the 21X processor transfers
the values from the 8 bit pulse counters into 16
bit accumulators (maximum count is 65,535)
and the 8 bit counters are hardware reset to
zero. The pulses accumulate in these 16 bit
accumulators until the program table containing
the Pulse Count instruction is executed. At the
beginning of the execution of the Table
containing the Pulse Count instruction, the total
in the 16 bit accumulator is transferred to a
temporary RAM buffer. The 16 bit accumulator
is then zeroed. When the table execution
reaches the Pulse Count instruction, the value in
the RAM buffer is multiplied by the multiplier and
added to the offset and placed into the
designated Input Memory location. The RAM
buffer does NOT accumulate counts; it is zeroed
each time the table is executed regardless of
whether or not the pulse instruction is executed.
If all counts are necessary, it is imperative that
the Pulse Count instruction be executed (not
branched around) every time its table is
executed.
If a table execution was skipped because the
processor was executing the previous table
(Section 2.1), the value in the 16 bit accumulator
is the result of a longer than normal interval.
This value can either be used or it can be
discarded. If pulse counts are being totalized, a
missing count could be significant and the value
from the erroneously long interval should NOT
be discarded. If the pulse count is being
processed in a way in which the resultant value
is dependent upon the sampling interval, the
value from the excessive interval should be
discarded. If the value is discarded the value in
the RAM buffer from the previous measurement
will be used.
There is also an option to output the count as a
frequency (i.e., counts/execution interval in
seconds = Hz) as well as discard the result from
an excessive interval. This allows the use of a
conversion factor that is independent of the
execution interval.
When datalogger time is changed, whether
through the keyboard or with a
telecommunications program, a partial recompile
is automatically done to resynchronize program
execution with real time. The resynchronization
process resets the pulse accumulation interval
resulting in an interval whose length can be