Campbell 21X Operator's Manual page 95

SECTION
9.
INPUT/OUTPUT INSTRUCTIONS
The count is incremented when the input
voltage changes from below
1.5
volts to
above 3.5
volts. The
maximum input
voltage is
t20
volts.
LOW LEVEL AC
This mode is used for counting freqqency of
AO signals from magnetic pulse flow
transducers or other low voltage, sine wave
outputs.
MINIMUM AC INPUT VOLTAGE: 6 millivolts
RMS
INPUT
HYSTERESIS:
11
millivolts
MAXTMUM
AC INPUT VOLTAGE: 20 volts
RMS
FREQUENCY RANGE:
AC Input Voltage
(RMS)
Range
20
millivolts 1
Hz
to
100
Hz
50
miflivolts
0.5
Hz
to
4OO
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.
f
n
this case, either
channel2
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 (2551.0125=20.4kHz\.
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
f
rom 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
i
the RAM buffer is multiplied
by
the multiplier
added to
the offset and placed into the
designated Input Memory
location. The
RAM
butJer
does NOT accumulate counts: it
is
each time the
table is executed regardless
of
whether or
not
the pulse instruction
is
executed.
lf
allcounts are
necessary, it is imperative that
the Pulse Count instruction be executed (not
branched around) every time
its
table
is
executed.
lf
a
table execution was skipped because the
processor was executing the previous table
(Section 2.1), the value
in
the
16
bit accumu
is
the result of a longer than normal interval.
This value can either be used or
it
can
be
discarded.
lf pulse counts
are being totalized,
a
missing count could be significant and the value
from the erroneously long interval should NOT
be
discarded.
lf
the pulse count is being
processed in
a
way
in
which the resultant value
is dependent upon the sampllng interval, the
value from the excessive interualshould be
discarded.
lf the value is discarded the value
in
the RAM buffer from the previous
will be
used.
There
is
also an option to output the count as
a
frequency (i.e., counts/execution interual
in
seconds
= Hz) as well as discard the result f
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
is automatically done
to resynchronize program
execution with real
time. The
resynchronization
process resets the pulse accumulation interval
resulting
in
an interualwhose length can
be
9-2