National Instruments cDAQ-9172 User Manual download pdf (Page 56)

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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

example, in Figure 31, 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 31, 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 counting

as before. The figure illustrates channel Z reload with X4 decoding.

Figure 31. Channel Z Reload with X4 Decoding

Ch A

Ch B

Counter Value

5 6

A = 0

B = 0

Z = 1

Ch Z

Max Timebase

8 9 0 2 1 7 4 3

1 2 ... 51 52 53 54 55 56 57 58 59 60 61 ... 83 84

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National Instruments cDAQ-9172 User Manual Page 56