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Chapter 7
Counters
Method
1
1b
2
3
Position Measurement
NI 6238/6239 User Manual
Table 7-2. Frequency Measurement Method Comparison
Number of
Measurements
Counters Used
1
1
1 or 2
2
For information on connecting counter signals, refer to the
Terminals
section.
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.
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 7-15 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.
Measures High
Number of
Frequency
Returned
Accurately
1
Many
1
1
7-16
Measures Low
Frequency
Signals
Signals
Accurately
Poor
Good
Fair
Good
Good
Good
Good
Default Counter
Poor
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