Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 43)

Installation
Chapter 2
2Ć31
Using Manual Switchover
After you select the BACKUP OPERATION for a rev. D. (or later)
1775–KA module, you may choose to use your PLC–3 backup system for
manual switchover. In manual switchover, you must initiate the
switchover by changing the position of a switch in a backup cable. (Refer
to the PLC–3 Programmable Controller Backup Concepts Manual, pub.
1775–803, for more details.) You must be sure to turn off the faulted
PLC–3 processor before you begin the switchover, however.
If a manual switchover occurs:
And: Then:
the PLC-3 processor is waiting for a
response
the response is ignored
another station on the Data Highway is
initiating a message
you may not receive a response from
either PLC-3 processor. You must
program other stations on the Data
Highway to recover from this condition
you are only communicating with the
primary PLC-3
the other stations on the Data Highway
will receive time-out errors for
messages they send after the primary
goes deactive and before switchover
occurs.
You can program the MSG instruction to execute a message upon
switchover or you can send commands to the backup PLC–3 processor
(Figure 2.13). As long as you get responses from the backup processor,
switchover has not yet occurred.