Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 101)

Programming Techniques
Chapter 5
5-20
When you do not select backup, you assign a unique Data Highway (000
8
to 376
8
) or Data Highway Plus (00
8
to 77
8
) station number to the scanner
module in the primary processor and the one in the backup processor. The
backup will continue to operate with the same station number even after
the switchover.
WARNING: To guard against personal injury and damage to
equipment, do not assign the same station number to both
scanner modules, if you have not selected backup mode.
If both scanners are assigned the same number and backup is
not selected, two stations with the same station number will
attempt to communicate on the link. This may cause
unpredictable machine motion.
Manual Switchover
The following information applies if you selected Data Highway backup
operation and configured your PLC-3 backup system for manual
switchover.
Table 5.G lists the responses that can occur with manual switchover.
Table 5.G
Manual Switchover Responses
Switchover occurs and: Then:
the processor is waiting for a response the response is ignored.
another station on the Data Highway is
initiating a message
possibly neither processor will respond to the message.
You must program the other station to respond to this
condition.
another station is communicating with the
primary processor
the other station will receive no indication that
switchover has occurred. You can, however, program a
message instruction to execute a message upon
switchover (Figure 5.7) or attempt to send commands
to the backup processor. If you can communicate with
the backup processor, you know that switchover has
not occurred.
Important: Be sure to turn off the faulted processor before initiating the
switchover to the backup processor.