Chapter 7 Counters
X Series User Manual 7-24 ni.com
• X2 Encoding—The same behavior holds for X2 encoding except the
counter increments or decrements on each edge of channel A,
depending on which channel leads the other. Each cycle results in
two increments or decrements, as shown in Figure 7-19.
Figure 7-19. X2 Encoding
• X4 Encoding—Similarly, the counter increments or decrements on
each edge of channels A and B for X4 encoding. Whether the counter
increments or decrements depends on which channel leads the other.
Each cycle results in four increments or decrements, as shown in
Figure 7-20.
Figure 7-20. X4 Encoding
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 7-21, 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 7-21, 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
Ch A
Ch B
Counter Value
5 6 8 9 7 5 6
8
9 7
Ch A
Ch B
Counter Value 5 6 8 9 10 10 11 11 12 12 13 13 7
5 6 8 7 9
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