Rockwell Automation 1785-CHBM ControlNet PLC-5 Hot Backup System User Manual - Manual (Page 117)

Publication 1785-UM024B-EN-P - January 2003
Application Guidelines E-5
Performance Considerations
One approach to redundant system design is to transfer as much of
the primary data table to the secondary as possible and as frequently
as possible. This approach is not required and actually lowers the
overall performance of the system. First, ControlNet bandwidth is
lowered because much more unscheduled message traffic is used to
transfer data. Secondly, because of transfer delays in getting
information to the secondary, many instructions are updated with stale
primary information when the secondary is, in fact, perfectly
synchronized with the primary.
For example, PID instructions are updated at power up of the
secondary so that the secondary PID output variable quickly catches
up with the primary output variable. The next time the PID is
executed by the primary and secondary, both PLC-5s calculate the
next output, based on the current value of the input variable which
both PLC-5s acquire from ControlNet at the same time. Since both PID
outputs are a function of the same input conditions, both PLC-5s
produce the same outputs independently. There is no need to transfer
PID variables after the initial transfers on power up.
Most variables in a PLC system can be traced to some input condition.
Once the secondary is refreshed after qualification, the secondary
tracks the primary because the secondary executes upon the same
input conditions as the primary. The main exception to continuous
transfer is with timer variables. You should periodically, but not
continuously, refresh variables.
Inhibiting or Resetting
ControlNet I/O If you want to inhibit or reset the ControlNet I/O via the ControlNet
status file, first set the inhibit bit or reset bit on the secondary PLC-5,
then set the respective bit(s) on the primary PLC-5.
Sequential Process
Application Considerations A sequential process is a set of different process states that contain
transitions between the process states.
Sequential Function Charts (SFC) are often used to define different
process states and state transitions. You can also use ladder logic to
implement the sequential process. With either approach, what is
important is that the PLC-5 experiencing qualification must be at the
same process state as the primary PLC-5 before completing
qualification and becoming the secondary programmable controller.