Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 19)

Publication 1785-UM024B-EN-P - January 2003
Backup Concepts for the ControlNet PLC-5 Backup System 1-5
Switchover
Switchover is what occurs when the secondary programmable
controller assumes control of the process, thus becoming the primary
programmable controller. Several conditions can cause this to happen:
• critical failures in the primary programmable controller (e.g.,
hardware faults)
• internal conditions within the primary programmable controller
(e.g., a major fault), causing it to relinquish control of the
process to the secondary programmable controller
• external conditions recognized by the secondary programmable
controller (e.g., loss of communications with the primary
programmable controller), causing it to assume control of the
process
• a manual switchover (i.e., a forced switchover, where you cause
the secondary programmable controller to become primary, and
vice versa; you may choose to do this when testing your system
for backup performance and integrity, or as part of a system
maintenance program).
The ideal switchover time from the primary programmable controller
to the secondary programmable controller would be instantaneous.
The ControlNet PLC-5 hot backup system typically provides a
switchover time of 30-50 ms. This time is adequate to avoid a bump in
most systems, but you should determine the maximum allowable
switchover time for your applications.