Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 45)

Installation
Chapter 2
2Ć33
If an automatic 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
possibly neither of the PLC-3 processors will
respond to the message. You must program
other stations on the Data Highway to recover
from this condition
another station is communicating with
the primary PCL-3 processor
the other station will receive no indication that
a switchover has occurred. You can,
however, program a MSG instruction to
execute a message upon switchover (fig.
2.13) or send commands to the backup
PLC-3 processor. If you are able to
communicate with the backup, you know that
no switchover has occurred
Run/Backup Bit
It is important to alert the proper personnel when a switchover occurs.
One way you can provide such indication is by having your program
monitor the run/backup bit (data table status section, file 0, word 3, bit 17)
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.
It is also possible to link a single PLC–3 controller to more than one Data
Highway by installing multiple 1775–KA modules in the same PLC–3. In
this configuration, each 1775–KA module connects to a different Data
Highway, and each has a unique station number on its associated highway.
However, all the 1775–KA modules in the same PLC–3 controller can
have either the same or different station numbers.
CAUTION: If such a PLC–3 station is communicating through
a PLC/PLC–2 buffer file and all of the stations’ 1775–KA
modules have the same station number, then all of these
modules will transfer data through the same buffer file. This
can cause unpredictable results if several 1775–KA modules try
to read or write to the buffer file at the same time.
When such a PLC–3 station transmits a command message to a remote
Data Highway station, the thumbwheel number specified in the PLC–3
message instruction (section titled PLC–3 Stations) determines which
1775–KA module actually transmits the command.
Multiple 1775-KA Modules in
One PLC-3