Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 118)

Publication 1785-UM024B-EN-P - January 2003
E-6 Application Guidelines
It is also important that the secondary PLC-5 remains at the same state
with the primary PLC-5, or it may diverge into a different process
state. When both programmable controllers remain at the same state
and the primary PLC-5 faults or fails, the secondary PLC-5 would
properly assume control of the sequential process in the correct
process state.
Not all applications follow a sequential process. In these applications,
the outputs are based on the inputs and require no special internal
process states and require no special considerations.
What Causes a ControlNet PLC-5 to Get Into an Incorrect State
There are two times a PLC-5 can get into a different process state than
that of the primary PLC-5:
• during qualification - the PLC-5 being qualified may slip into a
different state than the primary PLC-5
• the secondary PLC-5 may randomly diverge into a different state
than the primary PLC-5
Because of the complexity of the process, qualification is the most
likely reason this could happen.
During step 3, some of the crossloaded data in the complete data
table crossload may not match the data in the primary PLC-5. This
happens if the data in the primary PLC-5 changed since being
crossloaded into the qualifying PLC-5. In this case, the qualifying
PLC-5 would now execute its program, and based on some of its
outdated data table values would transition into the wrong process
state.
EXAMPLE
This example assumes that both programmable
controllers have the same data table structure which
allows automatic data table crossloading at
qualification. During qualification, the following
process occurs:
1. You transition the desired secondary PLC-5 from Program to Run
mode.
2. A complete data table crossload occurs.
3. The qualifying PLC-5 transitions into Run mode.
4. A state data table crossload is performed, including Timer and
Counter accumulators, entire PD structures and any user-defined
files.
5. The qualification completes and the PLC-5 becomes a qualified
secondary PLC-5.