Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 116)

Publication 1785-UM024B-EN-P - January 2003
E-4 Application Guidelines
If the secondary begins execution with the control register at a
different point than the primary, the secondary data table may diverge
from the primary since its instruction is behind the primary.
When Should the Information Be Transferred?
The primary data table is transferred at qualification. Then, selected
state variables are transferred again after the transition to Run mode to
account for transfer delays in updating the secondary. If nothing else
occurs, the primary and secondary operate independently. There are
some opportunities, however, for the PLC-5s to diverge over time. For
instance, timers in both systems run off of different system clocks.
Over time, these clocks may drift apart, causing the timer values to
diverge. You should periodically update Timers setting the
appropriate transfer bit in the transfer register. You should update, as
necessary, other variables not accounted for by the transfer register via
a message read instruction in the secondary program.
How Often Should the Information Be Transferred?
Since timers may diverge with time, you should periodically refresh
them, thereby synchronizing the secondary with the primary. Since
ControlNet bandwidth is important, it is probably adequate for you to
complete this refresh on a daily or weekly basis. A PLC program
should not simply continue to set the transfer bits to cause continuous
updating after the secondary has completed a transfer. We recommend
that you set up a daily or weekly counter that occasionally toggles the
transfer bit.
IMPORTANT
In a ControlNet Hot Backup system, do not attempt
to crossload MG, BT or CT file types. Additionally,
make sure these files are not in the range of
user-defined crossload files. This could lock-up
messages on the secondary PLC-5.