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Chapter 7
Counters
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 7-17,
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 7-17, 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 7-17.
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 7-18.
Figure 7-18.
Measurements Using Two Pulse Encoders
For information about connecting counter signals, refer to the
section.
Ch A
Ch B
Co
u
nter V
a
l
u
e
5
6
A = 0
B = 0
Z = 1
Ch Z
M
a
x Time
bas
e
8
9
0
2
1
7
4
3
Ch A
Ch B
Co
u
nter V
a
l
u
e
2
3
5
4
3
4
4
Summary of Contents for PCI-6281
Page 1: ...PCI 6281...
