Chapter 8 Counters© National Instruments Corporation 8-15 NI USB-621x User ManualTable 8-2 summarizes some of the differences in methods of measuringfrequency.For information about connecting counter signals, refer to the DefaultCounter/Timer Pinouts section.Position MeasurementYou can use the counters to perform position measurements withquadrature encoders or two-pulse encoders. You can measure angularposition with X1, X2, and X4 angular encoders. Linear position can bemeasured with two-pulse encoders. You can choose either a single point(on-demand) position measurement or a buffered (sample clock) positionmeasurement. You must arm a counter to begin position measurements.Measurements Using Quadrature EncodersThe counters can perform measurements of quadrature encodersthat use X1, X2, or X4 encoding. A quadrature encoder can have upto three channels—channels A, B, and Z.• X1 Encoding—When channel A leads channel B in a quadraturecycle, the counter increments. When channel B leads channel A in aquadrature cycle, the counter decrements. The amount of incrementsand decrements per cycle depends on the type of encoding—X1, X2,or X4.Figure 8-14 shows a quadrature cycle and the resulting increments anddecrements for X1 encoding. When channel A leads channel B, theincrement occurs on the rising edge of channel A. When channel Bleads channel A, the decrement occurs on the falling edge ofchannel A.Table 8-2. Frequency Measurement Method ComparisonMethodNumber ofCountersUsedNumber ofMeasurementsReturnedMeasures HighFrequencySignalsAccuratelyMeasures LowFrequencySignalsAccurately1 1 1 Poor Good1b 1 Many Fair Good2 1 or 2 1 Good Poor3 2 1 Good Good
