Chapter 7
Counters
7-16
ni.com
For information on connecting counter signals, refer to the
section.
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.
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.
Table 7-2.
Frequency Measurement Method Comparison
Method
Number of
Counters Used
Number of
Measurements
Returned
Measures High
Frequency
Signals
Accurately
Measures Low
Frequency
Signals
Accurately
1
1
1
Poor
Good
1b
1
Many
Fair
Good
2
1 or 2
1
Good
Poor
3
2
1
Good
Good
