Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 106)

Diagnosing Faults
Chapter 6
6-2
Now the backup processor is controlling outputs. If the backup processor
attempts to execute the same logic rung that caused the primary processor
to shut down, the backup processor will:
display the message DEACTIVE on the front panel of the backup
processor if a 1775-S4A or -SR is scanner number one; or display the
major fault on the front panel of the backup processor if a 1775-S5 or
-SR5 is scanner number one.
switch to program load mode
shut down the PLC-3 or PLC-3/10 system
cause a fault message to appear on the screen of a programming
terminal, if one is being used.
If a fault occurs, you can identify the nature of the fault and locate the
source of the fault by:
observing indicators on the front panel of the processor
observing indicators on each module
examining bits in status words.
Diagnosing Faults with Processor Status Indicators
The front panel of the processor has a 16-character display and several
indicators (Figure 6.1). Using these indicators you determine whether the
processor is operating normally or if a major fault exists. Table 6.A on
page 6-3 and Table 6.B on page 6-4 describe how to interpret the status
indicators on a processor when it is used as a primary or as a backup
processor respectively.
Locating a Fault