Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 96)

Publication 1785-UM024B-EN-P - January 2003
B-16 Backup States
status tab of the ControlNet Hot Backup screen. Once the
Forced Switchover is activated, the processor writes a zero into
the Forced Switchover word of the Backup Integer File. You can
activate this switchover from either the primary or secondary
processor.
Forced Switchover is useful when you want to test edits you
have made in your application program. In this case, you make
edits to the secondary system, force a switchover, and monitor
the results of the edits. If the edits cause problems, you can
perform another Forced Switchover to give the original
processor control of the system outputs on the ControlNet
channel.
Since the Forced Switchover feature is activated from writing to
the Backup Integer File, you can activate Forced Switchovers
from external applications writing to the Backup Integer File, as
well as the application on the processor itself.
Forced Switchover does not activate for an unqualified PLC-5
ControlNet backup system or for a PLC-5 ControlNet backup
system in which the Designated Primary feature is enabled.
Unqualified Switchovers
Unqualified Switchovers occur when the primary processor of a
qualified PLC-5 ControlNet backup system is no longer in Run mode,
or when the secondary processor loses its peer-to-peer handshake
connection with the primary processor. The secondary processor then
does a switchover to become a stand-alone primary processor.
There are many reasons why an Unqualified Switchover can occur:
• you changed the primary PLC-5 system out of Run mode
• a major fault occurred on the primary PLC-5 system
• the primary PLC-5 system failed
• the primary PLC-5 system lost power
• problems with the ControlNet network caused the loss of the
handshake connection with the primary processor
Switchover Considerations
Switchovers do not occur without some potential problems:
• Divergence - when the data table values diverge between the
primary processor and secondary processor.