Campbell Measurement and Control Module CR10 Operator's Manual page 134

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
maximum input voltage is +20 volts. A
problem, however, arises when the pulse is
actually a low frequency signal (below about 10
FIGURE 9-1. Conditioning for Long Duration Voltage Pulses
When this happens, the excess voltage is
shunted to the CR10 5 VDC supply, with the
current limited by an internal 10 Kohm resistor.
When this extra current source exceeds the
quiescent current needs of the CR10 (about 0.6
mA), the 5 VDC supply will start to rise,
upsetting all analog measurements.
Thus, pulses whose positive voltage portion
exceeds 5.6 VDC with a duration longer than 100
milliseconds need external conditioning. One
method would be to use a 4 to 5.6 V zener diode
from the signal to ground. The simplest method,
however, is to add an external 20 Kohm resistor in
series with the signal (Figure 9-1). This will limit
the current for pulses to 20 VDC to the point that it
will not upset the CR10 5 VDC supply.
LOW LEVEL AC
This configuration is used to count the
frequency of AC signals from magnetic pulse
flow transducers or other low voltage, sine
wave inputs. The minimum input voltage is 6
millivolts RMS. Input hysteresis is 11 millivolts.
The maximum AC input voltage is 20 volts
RMS. The maximum input frequency ranges
from 100 Hz at 20 mV RMS to 1000 Hz at 150
mV or greater. Consult the factory if higher
frequencies are desired.
SWITCH CLOSURE
In this configuration, the minimum switch closed
time is 5 milliseconds. The minimum switch open
time is 6 milliseconds. The maximum bounce
time is 1 millisecond open without being counted.
The 2 pulse count input channels each have
eight bit counters. Input frequencies greater
than 2000 Hz (the limit of the eight bit counter,
255 counts at the reset interval of 0.125
second) can be counted by combining two
9-2
Hz) and the positive voltage excursion exceeds
5.6 VDC.
counters on one input channel. When this
option is selected, channel 1 is used for the
pulse input. Channel 2 is not used.
Every 0.125 seconds, the CR10 processor
transfers the values from the 8 bit pulse
counters into 16 bit accumulators (max 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 location.
CAUTION: 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
the table is executed.
If a table execution was skipped because the
processor was executing the previous table
(Section 2.1) or if the user resets the time, 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