Rockwell Automation 7000L PowerFlex 7000 Medium Voltage AC Drive C Frame - Marine - Manual (Page 284)

4-108 Component Definition and Maintenance
7000L-UM302B-EN-P – June 2013 7000 ―C‖ Frame - Marine
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:
Binary Code Gray Code
255 011111111 010000000
256 100000000 110000000
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.
Tachometer Feedback Board
(cont.)