Allen-Bradley ControlNet PLC-5 User Manual page 55

Backup States B-13
If the even ControlNet node is the only processor to pass
qualification, it is the primary processor. However, should the
odd ControlNet node pass qualification, a switchover
immediately occurs, the odd ControlNet node becomes the
primary processor, and the even ControlNet node becomes the
secondary processor.
Once there is a qualified primary processor, you cannot change
(i.e., enable or disable) the Designated Primary feature. The first
processor that goes through qualification determined whether the
Designated Primary feature is enabled or disabled. The second
processor that goes through qualification inherits the
configuration setting from the first processor. In order to enable
or disable the Designated Primary feature for a PLC-5 ControlNet
backup system, both processors must be in Program mode. You
can then change the Designated Primary feature on one of the
processors. You should then qualify the processor on which you
have changed this feature.
• Forced Switchover
Forced Switchover allows you to switch over a qualified PLC-5
ControlNet backup system at any time. You activate a Forced
Switchover by writing a non-zero value into the Forced
Switchover word (word 31) of the Backup Integer File. 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.
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