Chapter 9 Counters
© National Instruments Corporation 9-17 NI USB-621x User Manual
Channel Z Behavior
Some quadrature encoders have a third channel, channel Z, which is also
referred to as the index channel. A high level on channel Z causes the
counter to be reloaded with a specified value in a specified phase of the
quadrature cycle. You can program this reload to occur in any one of the
four phases in a quadrature cycle.
Channel Z behavior—when it goes high and how long it stays
high—differs with quadrature encoder designs. You must refer to the
documentation for your quadrature encoder to obtain timing of channel Z
with respect to channels A and B. You must then ensure that channel Z is
high during at least a portion of the phase you specify for reload. For
instance, in Figure 9-18, channel Z is never high when channel A is high
and channel B is low. Thus, the reload must occur in some other phase.
In Figure 9-18, the reload phase is when both channel A and channel B are
low. The reload occurs when this phase is true and channel Z is high.
Incrementing and decrementing takes priority over reloading. Thus, when
the channel B goes low to enter the reload phase, the increment occurs first.
The reload occurs within one maximum timebase period after the reload
phase becomes true. After the reload occurs, the counter continues to count
as before. The figure illustrates channel Z reload with X4 decoding.
Figure 9-18. Channel Z Reload with X4 Decoding
Measurements Using Two Pulse Encoders
The counter supports two pulse encoders that have two channels—channels
A and B.
Ch A
Ch B
Counter Value
56
A = 0
B = 0
Z = 1
Ch Z
Max Timebase
890 21743
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