NI cDAQ-9178 - Page 48

NI cDAQ-9178/9174 User Guide and Specifications 48 ni.com
• 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 38.
Figure 38. 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 39, 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 39, 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 39. 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 40.
Ch A
Ch B
Counter Value5 6 8 910 1011 1112 1213 137
56879
Ch A
Ch B
Co
unter Value
5
6
A = 0
B = 0
Z = 1
Ch Z
Max Timebase
8
9
021743