Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 95)

Publication 1785-UM024B-EN-P - January 2003
Backup States B-15
There are two categories for switchovers:
• Qualified Switchovers
• Unqualified Switchovers
Qualified Switchovers
Qualified Switchovers are switchovers that transition the primary
processor to the Secondary backup state and the secondary processor
to the Primary backup state. Thus after a Qualified Switchover, there is
still a qualified backup system. There are 2 Qualified Switchover
cases:
• Designated Primary
When you configure the PLC-5 ControlNet backup system and
enable the Designated Primary feature, the odd ControlNet node
of the odd-even ControlNet pair is always the primary processor
if the odd node has passed qualification. This feature is useful
when you want a particular processor to always be the primary,
whenever that processor is available.
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 clicking on the Force Switchover button on the