Rockwell Automation Publication 7000-UM202D-EN-P - May 2018 165
Control Component Definition and Maintenance Chapter 3
Positional Encoder Operations
Besides quadrature encoders, the universal encoder interface also accepts
positional (absolute) encoders. The interface converts parallel positional data
to a serial stream and transmits it to the DPM. The board also generates
“pseudo” quadrature differential signals, including a zero position mark,
derived from the binary data to the DPM.
There are three different positional encoder configurations available. For all of
these configurations remove the ENC_TYPE jumper. The other jumpers
configure the board for the type of positional data (gray code or natural binary)
set by CD_DQUAD and high or low true data set by POL_QRDNT.
• Gray code, low true. In this configuration the board inverts the
incoming gray code data then converts it to binary for transmission to
the DPM.
• Natural binary, low true. The board does not convert incoming data but
does invert it.
• Gray code, high true. The board converts incoming gray code data to
binary without inverting the input data.
• Natural binary, high true. The board converts positional data to the
serial stream without inverting or converting it.
Positional Encoder Guidelines
When selecting a positional encoder, follow these guidelines for optimal
performance.
1. Code Selection: Purchase absolute encoders 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 makes 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 256, we see:
All nine bits changed in the Binary Code, while only the MSB of the
Gray code changed. In the Universal Encoder Interface, the frequency
filter components and input hysteresis create delays. Differences in these
delays could cause errors due to reading a bit as ON when switching 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.
IMPORTANT Consult the factory for availability of Positional Encoder options.
Binary Code Gray Code
255 011111111 010000000
256 100000000 110000000
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