5-20 | ni.comChapter 5 CountersFigure 5-18. X2 Encoding• X4 Encoding—Similarly, the counter increments or decrements on each edge ofchannels A and B for X4 encoding. Whether the counter increments or decrements dependson which channel leads the other. Each cycle results in four increments or decrements, asshown in Figure 5-19.Figure 5-19. X4 EncodingChannel Z BehaviorSome quadrature encoders have a third channel, channel Z, which is also referred to as the indexchannel. A high level on channel Z causes the counter to be reloaded with a specified value in aspecified phase of the quadrature cycle. You can program this reload to occur in any one of thefour phases in a quadrature cycle.Channel Z behavior—when it goes high and how long it stays high—differs with quadratureencoder designs. You must refer to the documentation for your quadrature encoder to obtaintiming of channel Z with 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. For instance, in Figure 5-20,channel Z is never high when channel A is high and channel B is low. Thus, the reload mustoccur in some other phase.In Figure 5-20, the reload phase is when both channel A and channel B are low. The reloadoccurs when this phase is true and channel Z is high. Incrementing and decrementing takespriority over reloading. Thus, when the channel B goes low to enter the reload phase, theincrement occurs first. The reload occurs within one maximum timebase period after the reloadphase becomes true. After the reload occurs, the counter continues to count as before. The figureillustrates channel Z reload with X4 decoding.Ch ACh BCounter Value 5 6 8 97 56 89 7Ch ACh BCounter Value 5 6 8 9 10 1011 1112 1213 137 568 79
