Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 113)

1 Publication 1785-UM024B-EN-P - January 2003
Appendix
E
Application Guidelines
Chapter Objectives This Appendix will provide you with application examples, as well as
information on other items to consider while operating your
ControlNet PLC-5 hot backup system.
Redundancy
Considerations The PLC-5 system for redundancy is designed to allow a secondary
PLC-5 to take over I/O and system control from a primary PLC-5
which has become disabled. The main goal in designing a redundant
PLC system is to ensure that the secondary PLC-5 executes as closely
as possible to the primary system. That way, when control is
transferred from the primary to the secondary, the transfer is seamless
(or bumpless), meaning that the outputs of the system are maintained
in the same state after switchover occurs.
When the systems produce radically different outputs based on
different data table variables, divergence occurs. When divergence
occurs, outputs may change state when control is transferred from
primary to secondary. To minimize divergence, state information must
be transferred from the primary PLC-5 to the secondary PLC-5. Much,
if not all, of the information transfer between the primary and
secondary PLC-5s can be performed automatically by the secondary
PLC-5.
Depending on the design of the PLC application program, additional
information may have to be exchanged by the application program in
order for the secondary system to be in the same state as the primary.
You can accomplish this information exchange, if necessary, simply
by adding a PLC-5 message instruction in the secondary system.