Chapter 7 CountersNI 6238/6239 User Manual 7-18 ni.comChannel Z behavior—when it goes high and how long it stayshigh—differs with quadrature encoder designs. You must refer to thedocumentation for your quadrature encoder to obtain timing of channel Zwith respect to channels A and B. You must then ensure that channel Z ishigh during at least a portion of the phase you specify for reload. Forinstance, in Figure 7-18, channel Z is never high when channel A is highand channel B is low. Thus, the reload must occur in some other phase.In Figure 7-18, the reload phase is when both channel A and channel B arelow. The reload occurs when this phase is true and channel Z is high.Incrementing and decrementing takes priority over reloading. Thus, whenthe channel B goes low to enter the reload phase, the increment occurs first.The reload occurs within one maximum timebase period after the reloadphase becomes true. After the reload occurs, the counter continues to countas before. Figure 7-18 channel Z reload with X4 decoding.Figure 7-18. Channel Z Reload with X4 DecodingMeasurements Using Two Pulse EncodersThe counter supports two pulse encoders that have two channels—channelsA and B.The counter increments on each rising edge of channel A. The counterdecrements on each rising edge of channel B, as shown in Figure 7-19.Figure 7-19. Measurements Using Two Pulse EncodersCh ACh BCounter Value 5 6A = 0B = 0Z = 1Ch ZMax Timebase8 9 0 217 43Ch ACh BCounter Value 2 3 54 34 4
