National Instruments DAQ M Series - Manual (Page 77)

Chapter 7 Counters
© National Instruments Corporation 7-15 NI 6236 User Manual
Table 7-2 summarizes some of the differences in methods of measuring
frequency.
For information about connecting counter signals, refer to the Default
Counter Terminals 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
Table 7-2. Frequency Measurement Method Comparison
Method
Number of
Counters
Used
Number of
Measurement
s 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