Rockwell Automation Publication 1789-UM002K-EN-P - January 2015 245
Windows Considerations Appendix D
A periodic task detects an overlap and sets the Overlap fault bit in the controller
if a periodic tasks fails to run at all during its assigned time slot or if a periodic
tasks starts later than scheduled and cannot complete before the start of the next
period. This diagram shows periodic task intervals, when a task actually starts,
and what is considered an overlap condition.
If two controllers in the same virtual chassis each have high priority periodic tasks
and the tasks become active at the same time, the Windows operating system tries
to switch between the tasks at whatever quantam is set within the Windows
operating system. The quantam varies based on the performance boost setting for
the process. With no performance boost, the quantam is 20 ms for the Windows
workstation. Typically a SoftLogix controller finishes the entire scan of a periodic
task before using a whole quantam.
To use the whole quantam, a thread has to be ready to execute the whole time. If a
thread stops and makes any type of I/O call, (such as disk drive, DRAM
memory, and so forth), the thread gets switched by the Windows operating
system and the CPU executes the next thread that is ready to run. This applies to
the SoftLogix controller because the controller references different tags in a
program scan, which are DRAM I/O operations. Therefore, the Windows
operating system switches back and forth many times between two periodic tasks
that are executing at the same time and at the same priority level, with the
switching happening in the microsecond range.
Periodic Task Intervals
Actual Task Execution
Overlap
Condition
Overlap
Condition
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