Rockwell Automation 7000 PowerFlex Medium Voltage Air-Cooled (B Frame) Installation - ForGe Control - Manual (Page 60)

60 Rockwell Automation Publication 7000-IN007E-EN-P - June 2014
Chapter 3 Drive Installation
To reduce the effects of signal distortion at the encoder receiving electronics,
consider the following guidelines:
1. Use a low capacitance cable (< 120 pF/m [40 pF/ft]). For example, Belden
1529 A is an 18Awg 3pair cable having a capacitance of 114 pF/m
(35 pF/ft).
2. Use twisted pair cabling with a shield that covers 100% of the cable. This is
especially true in the case of quadrature encoders. It is still a better choice
for absolute encoders, although the data in these encoders will not exhibit
the same frequency spectrum as quadrature encoders, and you can use
single wire cabling. In either case, always check with the encoder
manufacturer for the recommended cable.
3. Keep cable distances as short as possible. Rockwell Automation
recommends the following cable lengths:
a. For the 20B-ENC encoders, maximum cable length is 65 m (200 ft).
Longer cable distances could cause excessive surge currents. The
operating frequency of the encoder has no bearing on this
recommended distance due to the AC Termination. However, keeping
the frequency such that the cable’s characteristic impedance is ~348 
improves the surge currents and may increase the maximum distance to
100 m (330 ft).
b. For the Universal Encoder Interface, maximum length is 200 m (650 ft)
@ 100 KHz, or to 500 m (1600 ft) at frequencies below 55 Khz.
Rockwell Automation does not recommend exceeding this distance
because the voltage drop across the cable can cause decreased power at
the encoder.
Unused Inputs
You may not require all inputs in either the quadrature or absolute encoders. For
example, the absolute encoder accepts a 12-bit encoder but also works with a
lower resolution. Likewise, quadrature encoders may not use the Z track. Follow
these guidelines for unused inputs:
1. 20B-ENC Board. Wire any unused input to the encoder power rail. This
also includes the B and B’ inputs if using a pulse encoder. Failure to do this
will result in phase loss warnings and improper operation of the encoder
feedback logic (i.e. missing counts).
2. Universal Encode Interface. When used as a quadrature encoder interface,
the same rule applies as for the 20B-ENC Board. When operating as an
absolute encoder interface, the wiring of unused inputs depends on the
position of the POL_QRDNT jumper. If the jumper is installed, wire all
unused inputs to ENC PWR, otherwise use ENC COM.