Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 97)

Publication 1785-UM024B-EN-P - January 2003
Backup States B-17
If the output values in the data tables are different between the
primary and secondary systems, there will be a bump in the
outputs when the new primary processor takes control of the
outputs. There are features in the PLC-5 ControlNet backup
system which minimize the divergence of outputs between the
primary and secondary processors. These features are
synchronized program scans and data table crossloads.
• Switchover time - the time delay before the new primary
processor can control the system outputs on the ControlNet
channel.
There is a time period between when the old primary processor
has relinquished control of the system outputs on the ControlNet
channel and the new primary processor writes new system
outputs on the ControlNet channel. This time period is called
Switchover Time. There are two separate cases to consider:
(Note that the Switchover Times described below are the worst
case times. Actual Switchover Times may be smaller.)
• Communicated Switchover - when the processor indicates to the
secondary processor that a switchover must occur. The
following events can cause a Communicated Switchover:
– A Designated Primary switchover
– A Forced switchover
– Switching the primary PLC-5 system out of Run mode
– The occurrence of a major fault on the primary PLC-5 system
Since the peer-to-peer handshake connection is one NUT, it only
takes up to one NUT for the primary system to indicate to the
secondary system that a switchover must occur. There is up to
5ms of processing on each processor to get the switchover
indication from the primary system to the secondary system.
Thus the formula for the Communicated Switchover Time is:
Communicated Switchover Time = 10ms + NUT
So, if the NUT is 5ms, the Communicated Switchover Time is
equal to 10ms plus 5ms, or 15ms.
• Timeout Switchover - when the secondary processor loses
communication with the primary processor for a certain length
of time, it times out the primary processor, and then performs a
switchover. The following events can cause a Timeout
Switchover:
– Primary PLC-5 system failure.
– Loss of power to the primary PLC-5 system