Allen-Bradley ControlNet PLC-5 User Manual page 56

B-14 Backup States
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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.
If the output values in the data tables are different between the
primary and secondary systems, there will be a bump in the
outputs when the new primary processor takes control of the
outputs. There are features in the PLC-5 ControlNet backup
system which minimize the divergence of outputs between the
primary and secondary processors. These features are
synchronized program scans and data table crossloads.
• Switchover time - the time delay before the new primary
processor can control the system outputs on the ControlNet
channel.
There is a time period between when the old primary processor
has relinquished control of the system outputs on the ControlNet
channel and the new primary processor writes new system
outputs on the ControlNet channel. This time period is called
Switchover Time. There are two separate cases to consider:
(Note that the Switchover Times described below are the worst
case times. Actual Switchover Times may be smaller.)
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Communicated Switchover - when the processor indicates to
the secondary processor that a switchover must occur. The
following events can cause a Communicated Switchover:
• A Designated Primary switchover
• A Forced switchover
• Switching the primary PLC-5 system out of Run mode
• The occurrence of a major fault on the primary PLC-5
system