Section 8. Operation
instructions. Also, PulseCount() has the option of entering a number greater than
1 in the POption parameter. Doing so enters an averaging interval in
milliseconds for a direct running-average computation. However, use caution
when averaging. Averaging of any measurement reduces the certainty that the
result truly represents a real aspect of the phenomenon being measured.
Frequency Resolution Comparison
0.5 s Scan 5.0 s Scan
PulseCount(), POption=1 FR = 2 Hz FR = 0.2 Hz
TimerIO(), Function=2 FR = 0.0011 Hz FR = 0.00011 Hz
8.1.3.3.2 Frequency Measurement Q & A
Q: When more than one pulse is in a scan interval, what does TimerIO() return
when configured for a frequency measurement? Does it average the measured
periods and compute the frequency from that (f = 1/T)? For example,
Scan(50,mSec,10,0)
TimerIO(WindSpd(),11111111,00022000,60,Sec)
A: In the background, a 32-bit-timer counter is saved each time the signal
transitions as programmed (rising or falling). This counter is running at a fixed
high frequency. A count is also incremented for each transition. When the
TimerIO() instruction executes, it uses the difference of time between the edge
prior to the last execution and the edge prior to this execution as the time
difference. The number of transitions that occur between these two times divided
by the time difference gives the calculated frequency. For multiple edges
occurring between execution intervals, this calculation does assume that the
frequency is not varying over the execution interval. The calculation returns the
average regardless of how the signal is changing.
8.1.3.4 Switch Closure and Open-Collector Measurements
Switch closure and open-collector signals can be measured on P or C terminals.
Mechanical-switch closures have a tendency to bounce before solidly closing.
Unless filtered, bounces can cause multiple counts per event. The CR800
automatically filters bounce. Because of the filtering, the maximum switch
closure frequency is less than the maximum high-frequency measurement
frequency. Sensors that commonly output a switch closure or open-collector
signal include:
• Tipping-bucket rain gages
• Switch closure anemometers
• Flow meters
Data output options include counts, frequency (Hz), and running average.
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