National Instruments NI USB-621x - Manual (Page 95)

Chapter 9 Counters
© National Instruments Corporation 9-15 NI USB-621x User Manual
Table 9-3 summarizes some of the differences in methods of measuring
frequency.
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts 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.
Table 9-3. 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