Implicit Buffered Semi-Period Measurement; Pulse Versus Semi-Period Measurements - National Instruments NI cDAQ-9181 User Manual

Chapter 5 Counters

Implicit Buffered Semi-Period Measurement

In implicit buffered semi-period measurements, on each edge of the Gate signal, the counter
stores the count in the FIFO. The STC3 transfers the sampled values to host memory using a
high-speed data stream.
The counter begins counting when it is armed. The arm usually occurs between edges on the
Gate input. You can select whether to read the first active low or active high semi period using
the CI.SemiPeriod.StartingEdge property in NI-DAQmx.
Figure 5-11 shows an example of an implicit buffered semi-period measurement.
Figure 5-11. Implicit Buffered Semi-Period Measurement
Counter Value
For information about connecting counter signals, refer to the
section.

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.
5-10 | ni.com
Counter Starting
Armed Edge
GATE
SOURCE
0
Buffer
1 2 3 1 1
3
3
3 1
1
Default Counter/Timer Routing
2 1
3
2
1
2