Chapter 7 Counters
© National Instruments 7-23 X Series User Manual
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 X Series
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 7-18 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 7-18. X1 Encoding
Ch A
Ch B
Counter Value
7 7 6 5
5
6
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