5-18 | ni.com
Chapter 5 Counters
Period Measurement
In period measurements, the counter measures a period on its Gate input signal after the counter
is armed. You can configure the counter to measure the period between two rising edges or
two falling edges of the Gate input 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 occurring on the
Source input between the two active edges of the Gate signal.
You can calculate the period of the Gate input by multiplying the period of the Source signal by
the number of edges returned by the counter.
Period measurements return the inverse results of frequency measurements. Refer to the
Frequency Measurement section for more information.
Position Measurement
You can use the counters to perform position measurements with quadrature encoders or
two-pulse encoders. You can measure angular position with X1, X2, and X4 angular encoders.
Linear position can be measured with two-pulse encoders. You can choose to do either a single
point (on-demand) position measurement or a buffered (sample clock) position measurement.
You must arm a counter to begin position measurements.
Refer to the following sections for more information about the cDAQ chassis position
measurement options:
• Measurements Using Quadrature Encoders
• Measurements Using Two Pulse Encoders
• Buffered (Sample Clock) Position Measurement
Measurements Using Quadrature Encoders
The counters can perform measurements of quadrature encoders that use X1, X2, or X4
encoding. A quadrature encoder can have up to three channels—channels A, B, and Z.
• X1 Encoding—When channel A leads channel B in a quadrature cycle, the counter
increments. When channel B leads channel A in a quadrature cycle, the counter
decrements. The amount of increments and decrements per cycle depends on the type of
encoding—X1, X2, or X4.
Figure 5-17 shows a quadrature cycle and the resulting increments and decrements for X1
encoding. When channel A leads channel B, the increment occurs on the rising edge of
channel A. When channel B leads channel A, the decrement occurs on the falling edge
of channel A.
Figure 5-17. X1 Encoding
Ch A
Ch B
Counter Value
7 7 6 5
5
6
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