Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 114)

Publication 1785-UM024B-EN-P - January 2003
E-2 Application Guidelines
Switchover Considerations The following sections provide information on factors you should
keep in mind when performing a switchover.
Why Transfer Information?
When the primary PLC-5 is operating, system outputs are a function of
current and prior I/O events. Outputs may be in the present state
because a current input condition requires a certain output state. If the
secondary system comes online and senses the input condition, it will
place its output in the same state. If a switchover occurs, so that the
secondary becomes primary, system control switches over seamlessly
because the new primary maintains the output in the present or
current state.
System outputs are, in many cases, a function of past events or states.
An output may be in the present state based on a timing event, a
counting event, or some other prior event. If the secondary comes
online and immediately assumes control, its outputs are likely to be in
an opposite state from the primary because the secondary was not in
operation while the primary was accumulating the past events. In
order to ensure seamless switchover of control, the secondary should
be refreshed with the primary’s past events before assuming control.
The ControlNet PLC-5 backup system transfers many system state
variables automatically. In addition, the secondary transfers the entire
primary data table on qualification. Upon entering Run mode, the
secondary transfers all timer accumulators, counter accumulators, PD
file type structures, and selected files (which you have specified) a
second time. This transfer places these critical state variables in the
freshest state possible. In most applications, you can transfer all
necessary information by configuring a few parameters in the
ControlNet Hot Backup Configuration file. The secondary PLC-5 uses
the configuration parameters to determine which files to transfer.