Rockwell Automation Publication 7000L-UM301F-EN-P - March 2020
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Component Definition and Maintenance
Chapter 5
Positional Encoder Guidelines
When selecting a positional encoder there are certain guidelines that should be
followed for optimal performance.
1. Code Selection
: Absolute encoders can be purchased with either Gray
code or Binary output format. Gray code is a form of binary code where
only a single bit changes at a time for each sequential number or position.
The fact that only a single bit changes at a time make it easier for the
Universal Encoder Interface to read valid positional data and not
ambiguous data. If we compare the Natural Binary code to Gray code for
the transition from 255 to 2556, here is what we get:
All nine bits changed in the Binary Code while only the MSB of the Gray
code changed. In the Universal Encoder Interface there are delays created
by the frequency filter components and input hysteresis. Differences in
these delays could cause errors due to reading a bit as ON when it is
actually transitioning to OFF or vice versa. In the case of Gray code since
only one bit ever changes the ambiguity error is never more than one
count. For this reason and to reduce inrush currents, Rockwell
Automation recommends using Gray code Positional Encoders.
2. Data Polarity
: Absolute encoders typically have a High True output. If the
encoder model does not have a High/True (or Non Inverted/Inverted)
option you should assume it to be High True. In the case of a 10bit High
True encoder the zero position is represented by 0000000000. Whereas a
Low/True encoder the zero position is 1111111111. On the Universal
Encoder Interface the position data is inverted in hardware. That is a “1”
will turn on an Optocoupler producing a “0”. Therefore a High True
encoder would produce 1111111111 for the zero position. With the
POL_QRDNT jumper you can control the polarity of the input. With
the jumper installed (factory default) it is setup to accept High True
encoders and an extra inversion is done in the Universal Encoder Interface.
If you are using a Low True encoder then this jumper needs to be removed
so that the zero position is inverted by the optocouplers alone.
The other role of the POL_QRDNT jumper is to correct the data in the
event the encoder was mounted so that a CCW rotation produced
decrementing counts. If this is the case the POL_QRDNT jumper should
be configured to the opposite of what it should normally be for the data
polarity. For example if the Universal Encoder Interface is configured to
operate with High True encoders (POL_QRDNT installed), remove it to
correct for encoder mounting.
Binary Code
Gray Code
255
011111111
010000000
256
100000000
110000000
