Rockwell Automation 1779-KP3R DATA HIGHWAY II - Manual (Page 29)

Installing the 1779-KP3
Communication Interface
Chapter 2
2Ć13
You can assign the same node address to more than one KP3 in the same
PLC–3 chassis as long as the KP3s are communicating over different
links.
Important: You are not required to connect these separate links by Data
Highway II bridges.
When we talk about a ‘‘backup” system, we a talking about having a
primary PLC–3 on the network and a backup PLC–3. The pairs need to
be connected in such a way that one ‘‘takes over” if something should
happen to the other (switchover).
When you configure a backup system on Data Highway II, the primary
and backup KP3s must have their thumbwheels set to the same link
address. As far as the Data Highway II is concerned, however, the backup
KP3 will assume a link address that is one number higher than the
primary KP3.
For example, if you have set the link address on both KP3s to 176 octal,
Data Highway II will list the backup KP3 as having a link address of 177
octal. This enables you to send messages to the backup KP3 from other
nodes on the link. When a switchover occurs, the backup KP3 will
become node 176 as far as the link is concerned. Node 177, the link
address of the backup KP3, will then disappear from the link.
What You Should to Monitor Possible Switchover
At the time a switchover happens and the backup KP3 becomes the
primary KP3, the new primary KP3 has no way of knowing which
messages were initiated by the former primary KP3. For this reason, you
should monitor the run/backup bit (data table status section, file 0, word
3, bit 7) in your PLC–3 ladder diagram program. In the event of a
switchover, set the done bit of each message instruction. This allows any
message being initiated during a switchover to be sent again.
Important: Alert the proper personnel when a switchover occurs. One
way you can provide this indication is by having your program monitor
the run/backup bit, and turn on alarms or lights when the status changes
from backup to run. This bit is set in the primary processor and reset in
the backup processor.
How a PLC-3 Backup System
Works on Data Highway II