Rockwell Automation 8520 9/Series CNC Integration Maintenance Manual Documentation Set - Manual (Page 671)

Section 13A
Connecting 8520 Digital Drive Systems
13A-37
A second feedback device can be attached to your machine (normally
mounted directly to the moving axis member to avoid inaccuracies caused
by the motor gearing).
When using a second feedback device, the incremental or absolute encoder
mounted on the 8500 digital servo motor provides current and velocity
loop feedback while a non-motor mounted second feedback device
provides axis position feedback.
In many cases a system may not have enough feedback ports for all the
closed loop axes and also to support additional second feedback devices.
In these cases extra positioning feedback ports are available for the 8520
digital servo module using the optional feedback module. The optional
feedback module is available only on the 9/260 and 9/290 controls that use
the 3-axis servo module. The optional feedback module is not supported by
the 4-axis servo module.
For detals on using a second feedback device with an 8500 Series motor
refer to the section covering your specific 9/Series processor or servo card.
END OF SECTION
13A.9.4
Using a Second Feedback
Device