User manual

100 Rockwell Automation Publication 1769-UM011I-EN-P - February 2013
Chapter 7 Develop Applications
However, if you schedule two high priority tasks 1…5 to run every millisecond,
and they both require 500 s or more to execute, no CPU time would be left for
the dedicated I/O task. Furthermore, if you have so much configured I/O that
the execution time of the dedicated I/O task approaches 2 ms (or the
combination of the high priority tasks and the dedicated I/O task approaches 2
ms) no CPU time is left for low priority tasks 7…15.
TIP
For example, if your program needs to react to inputs and control outputs at a
set rate, configure a periodic task with a priority higher than 6 (1…5). This
keeps the dedicated I/O task from affecting the periodic rate of your program.
However, if your program contains a lot of math and data manipulation, place
this logic in a task with priority lower than 6 (7…15), such as the continuous
task, so that the dedicated I/O task is not adversely affected by your program.
Table 25 - Multiple Tasks Example
Task Priority Level Task Type Example Execution Time Worst-Case Completion Time
1 5 20 ms periodic task 2 ms 2 ms
2 7 Dedicated I/O task
5 ms selected RPI
1 ms 3 ms
3 10 10 ms periodic task 4 ms 8 ms
4 None (lowest) Continuous task 25 ms 60 ms
030252015105 454035 50 656055
Task 1
Task 2
Task 3
Task 4
Time (ms)