Chapter 7 Counters
NI 6238/6239 User Manual 7-16 ni.com
For information on 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 channelB 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.
Figure7-15 shows a quadrature cycle and the resulting increments and
decrements for X1 encoding. When channelA leads channel B, the
increment occurs on the rising edge of channelA. W hen channel B leads
channelA, 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
111Poor Good
1b 1 Many Fair Good
21 or 2 1Good Poor
321Good Good