© National Instruments | 6-11
cDAQ-9185/9189 User Manual
Pulse versus Semi-Period Measurements
In hardware, pulse measurement and semi-period are the same measurement. Both measure the
high and low times of a pulse. The functional difference between the two measurements is how
the data is returned. In a semi-period measurement, each high or low time is considered one point
of data and returned in units of seconds or ticks. In a pulse measurement, each pair of high and
low times is considered one point of data and returned as a paired sample in units of frequency
and duty cycle, high and low time or high and low ticks. When reading data, 10 points in a
semi-period measurement will get an array of five high times and five low times. When you
read 10 points in a pulse measurement, you get an array of 10 pairs of high and low times.
Also, pulse measurements support sample clock timing while semi-period measurements do not.
Frequency Measurement
You can use the counters to measure frequency in several different ways. Refer to the following
sections for information about cDAQ chassis frequency measurement options:
• Low Frequency with One Counter
• High Frequency with Two Counters
• Large Range of Frequencies with Two Counters
• Sample Clocked Buffered Frequency Measurement
Low Frequency with One Counter
For low frequency measurements with one counter, you measure one period of your signal using
a known timebase.
You can route the signal to measure (fx) to the Gate of a counter. You can route a known timebase
(fk) to the Source of the counter. The known timebase can be an onboard timebase, such as
80 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with a known
rate.
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