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Chapter 7
Counters
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 7-18, 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 7-18, 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. Figure 7-18 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.
The counter increments on each rising edge of channel A. The counter
decrements on each rising edge of channel B, as shown in Figure 7-19.
NI 6238/6239 User Manual
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Ch A
Ch B
Ch Z
Max Timebase
Counter Value
5
6
7
Figure 7-18. Channel Z Reload with X4 Decoding
Ch A
Ch B
Counter Value 2
3
Figure 7-19. Measurements Using Two Pulse Encoders
7-18
8
9
0
1
2
3
A = 0
B = 0
Z = 1
4
5
4
3
4
4
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