© National Instruments Corporation 35 NI cDAQ-9178/9174 User Guide and Specifications
The count values returned are the cumulative counts since the counter armed event. That is, the sample
clock does not reset the counter. You can configure the counter to sample on the rising or falling edge
of the sample clock.
Figure 23 shows an example of buffered edge counting. Notice that counting begins when the counter
is armed, which occurs before the first active edge on Sample Clock.
Figure 23. Buffered (Sample Clock) Edge Counting
Controlling the Direction of Counting
In edge counting applications, the counter can count up or down. You can configure the counter to do
the following:
• Always count up
• Always count down
• Count up when the Counter 0 B input is high; count down when it is low
For information about connecting counter signals, refer to the Default Counter/Timer Routing section.
Pulse-Width Measurement
In pulse-width measurements, the counter measures the width of a pulse on its Gate input signal. You
can configure the counter to measure the width of high pulses or low pulses on the Gate signal.
You can route an internal or external periodic clock signal (with a known period) to the Source input of
the counter. The counter counts the number of rising (or falling) edges on the Source signal while the
pulse on the Gate signal is active.
You can calculate the pulse width by multiplying the period of the Source signal by the number of edges
returned by the counter.
A pulse-width measurement will be accurate even if the counter is armed while a pulse train is in
progress. If a counter is armed while the pulse is in the active state, it will wait for the next transition to
the active state to begin the measurement.
Refer to the following sections for more information about NI cDAQ-9178/9174 pulse-width
measurement options:
• Single Pulse-Width Measurement
• Implicit Buffered Pulse-Width Measurement
• Sample Clocked Buffered Pulse-Width Measurement
3
6
3
Counter Armed
SOURCE
Sample Clock
(Sample on Rising Edge)
Counter Value
Buffer
10763 452
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