168 Rockwell Automation Publication 1756-UM535D-EN-P - November 2012
Chapter 7 Program the Redundant Controller
Example of Periodic Task Configurations
In this example, the execution time of the highest priority task (Task 1) is
significantly smaller than its period, that is, 20 ms is less than 80 ms, and the total
execution time of all the tasks is significantly smaller than the specified period of
the lowest priority task, that is, 50 ms is less than 180 ms.
Tuning the Period Specified
You may need to tune the period you specify for your periodic tasks to balance
program execution and service communication.
To check for overlaps, go online with the controller and access the Task
Properties dialog box. In the Monitor tab, note the maximum scan time. Verify
that the maximum scan time is smaller than the period you specified for the
periodic task.
Program to Obtain System
Status
For most redundant applications, you need to program to obtain the status of the
system. Program to obtain system status when you do the following:
• Program HMI to display the system status
• Precondition logic to execute based on the system status
• Use the diagnostic information to troubleshoot the system
To obtain the status of your redundant system, use a Get System Value (GSV)
instruction in your program and plan for the tags you are writing the values to.
Task Priority Execution Time Period Specified
1 Higher 20 ms 80 ms
2 Lower 30 ms 100 ms
Total execution time: 50 ms
The crossloading of data during synchronization points extends task scan times
in enhanced redundancy systems. We recommend that you balance program
execution and service communication when the system is synchronized.
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