Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 105)

Chapter
6
6-1
Diagnosing Faults
This chapter describes how to diagnose faults in a PLC-3 backup system
using:
status indicators on the front panels of processors and on the front
panels of modules in the processors
system status words.
Also, it describes a simple diagnostic technique that can help you identify
the program section and rung number of faults when they occur.
Before discussing fault diagnosis, however, you should review what
happens when a processor encounters a major fault.
If the primary processor determines that a major fault exists while it is in
run mode, the primary processor will:
display the message DEACTIVE on the front panel of the primary
processor if a 1775-S4A or -SR is scanner number one; or display the
major fault on the front panel of the primary processor if a 1775-S5 or
-SR5 is scanner number one.
switch to program load mode
pass control to the backup processor
cause a fault message to appear on the screen of a programming
terminal, if one is being used.
Chapter Objectives
How the Backup System
Behaves When a
Major Fault Occurs