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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.

The following figure shows an example of an implicit buffered semi-period measurement.

Figure 59. Implicit Buffered Semi-Period Measurement

SOURCE

GATE

Counter Value

Buffer

1 3

1 1

Counter

Armed

Starting

Edge

For information about connecting counter signals, refer to the Default Counter/Timer Routing

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.

Frequency Measurement

You can use the counters to measure frequency in several different ways. Refer to the

following sections for information about cRIO controller 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

For more information about choosing the best frequency measurement option, refer to the

Choosing a Method for Measuring Frequency and Which Method is Best? sections.

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National Instruments cRIO-904 Series User Manual

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