l Counting the increase at each rising and falling edge of both channels when channel A
leads channel B. Counting the decrease at each rising and falling edge of both channels
when channel B leads channel A.
For more information, see Pulse measurement specifications (p. 169).
6.6.6 Pulse measurement tips
The PulseCount() instruction uses dedicated 32-bit counters to accumulate all counts over
the programmed scan interval. The resolution of pulse counters is one count or 1 Hz. Counters
are read at the beginning of each scan and then cleared. Counters will overflow if accumulated
counts exceed 4,294,967,296 (2
32
), resulting in erroneous measurements.
Counts are the preferred PulseCount() output option when measuring the number of tips
from a tipping-bucket rain gauge or the number of times a door opens. Many pulse-output
sensors, such as anemometers and flow meters, are calibrated in terms of frequency (Hz) so are
usually measured using the PulseCount() frequency-output option.
Use the LLAC4 module to convert non-TTL-level signals, including low-level ac signals, to TTL
levels for input to C terminals
Conflicts can occur when a control port pair is used for different instructions
(TimerInput(), PulseCount(), SDI12Recorder(), WaitDigTrig()). For example,
if C1 is used for SDI12Recorder(), C2 cannot be used for TimerInput(), PulseCount
(), or WaitDigTrig().
Understanding the signal to be measured and compatible input terminals and CRBasic
instructions is helpful. See Pulse input terminals and the input types they can measure (p. 72).
6.6.6.1 Input filters and signal attenuation
Terminals configured for pulse input have internal filters that reduce electronic noise. The
electronic noise can result in false counts. However, input filters attenuate (reduce) the amplitude
(voltage) of the signal. Attenuation is a function of the frequency of the signal. Higher-frequency
signals are attenuated more. If a signal is attenuated too much, it may not pass the detection
thresholds required by the pulse count circuitry. See Pulse measurement specifications (p. 169) for
more information. The listed pulse measurement specifications account for attenuation due to
input filtering.
6.6.6.2 Pulse count resolution
Longer scan intervals result in better resolution. PulseCount() resolution is 1 pulse per scan.
On a 1 second scan, the resolution is 1 pulse per second. The resolution on a 10 second scan
6. Measurements 77
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