Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 46)

Hardware Description
Chapter 5
5Ć9
The servo positioning assembly provides a means for detecting and
responding to faults in your servo positioning system.
Since the servo positioning assembly is part of a PC system, diagnostic
information about fault conditions detected by the servo positioning
assembly can be block transferred to the PC processor.
At the PC processor, you can use the ladder diagram program to respond
to diagnostic information about fault conditions in any way you feel is
appropriate for your application. This may include turning off machinery,
turning on alarms, or generating report printouts. Furthermore, with an
Allen-Bradley Data Highway network, you can send this diagnostic
information to a computer or other Allen-Bradley PC processors.
The servo positioning assembly provides specific fault responses if certain
critical connections are broken.
Loss of Feedback
The 1771-ES expander continuously monitors the tachometer and encoder
feedback. If it senses an imbalance between these signals, it holds the
velocity command output signal at zero and disables the servo drive
through the drive disable circuit. Therefore, if the cable from either the
encoder or the tachometer breaks, the 1771-ES expander will disable the
servo drive.
Hardware Stop
You must connect a set of normally open contacts of your master control
relay between the hardware stop input terminal and the input power
supply common terminal. Normally, the master control relay would be
energized, pulling the hardware stop input low. This allows the module to
enable the servo drive.
However, if the master control relay de-energizes for any reason (such as
extreme overtravel limit or emergency stop), the hardware stop input goes
high. This forces the module to hold the velocity command output signal
at zero and disable the servo drive by turning off the drive disable circuit.
Therefore, if a connection in the hardware stop circuit breaks, the
1771-ES expander will disable the servo drive.
Fault Responses