Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 18)

Publication 1785-UM024B-EN-P - January 2003
1-4 Backup Concepts for the ControlNet PLC-5 Backup System
Equivalence Checking
Backup systems are traditionally designed in a way such that both the
primary and secondary programmable controllers are programmed
with identical applications. This allows for the controlled process to
continue with normal production operation in the event of a primary
programmable controller failure. Equivalence checks ensure that the
primary and secondary programmable controllers have identical
ladder programs, data file structures, I/O map connection, etc. You, as
the designer of the backup system, require the ability to enforce
equivalence checks between the primary and secondary
programmable controllers during the qualification process, and
periodically during normal production operation.
In some cases, you may want the secondary programmable controller
to have a completely different application than the primary
programmable controller (e.g., “safe shutdown,” limited production,
clean and wash, etc.). In that case, equivalence checking may not be
required. With the ControlNet PLC-5 hot backup system, you can
enable or disable equivalence checking as required by your
application. You must have a thorough understanding of your
application in order to make this determination.
Bumpless Transfer
Ideally, when a programmable controller switchover takes place in the
backup system, there should be no adverse effect on system
operation, and the controlled process should continue to run as
though it were still being controlled by the original primary
programmable controller. This is generally referred to as a bumpless
transfer of control.
Bumpless transfer is not required for many backup applications, but it
is highly desirable. Divergence in program scanning, I/O tables, data
tables, and long switchover times can all cause bumps when
transferring control from a primary to a secondary programmable
controller. The ControlNet PLC-5 hot backup system allows you to
choose synchronous program scan operation, as well as on-demand
crossloading of data table files you select. These options can greatly
reduce the possibility of bumps during a programmable controller
switchover.