Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 120)

Publication 1785-UM024B-EN-P - January 2003
E-8 Application Guidelines
How to Keep the Hot Backup Programmable Controllers in the
Same Process State
The design of the application is the most important factor to keeping
the programmable controllers in the same state. Structure the
application so that there are a low number of well-defined process
states. The more states there are in an application, the more complex
it becomes, thus increasing the chances that it will not take into
account every event that may cause a process state transition.
The transition into a different process state should be an event that
both programmable controllers receive. The application take into
account that some events should happen asynchronously on both
programmable controllers. So, the application should wait after
making one process state transition before making another one. This
allows the other PLC-5 to transition into the same process state as its
partner before the partner has transitioned to a different process state.
Some internal state variables may diverge between the two
programmable controllers. For example, a timer may timeout on one
PLC-5 where the other PLC-5 timer may not timeout until a later time.
If a process state transition was based on this timer, the programmable
controllers may diverge at least temporarily into different process
states. Through the state crossload feature, the internal state
information can be crossloaded from the primary PLC-5 to the
secondary PLC-5. Then, if the internal state variables were to diverge,
the secondary PLC-5 can match the internal state variables on the
primary PLC-5. For this to work properly, the state variables must be
placed into files that are crossloaded in the state crossload. This
requires detailed planning so that the state variables are organized in a
few data table files instead of being scattered throughout the data
table.
How to Recover When the Programmable Controllers are in
Different Process States
To recover, the programmable controllers must send their process
state information to the partner PLC-5. This should be done as a
scheduled message every NUT so that the process state information is
current. The application has to compare its own process state
information with the process state information of the partner. The
secondary PLC-5 can then change its process state information to
match the partner PLC-5.