Example – Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual User Manual
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Manage tasks Chapter 1
System overhead
System overhead is the time that the controller spends on unscheduled communication.
•
Unscheduled communication is any communication that you do not configure through the
I/O configuration folder of the project, such as Message (MSG) instructions and
communication with HMIs or workstations.
•
System overhead interrupts only the continuous task.
•
The system overhead time slice specifies the percentage of time (excluding the time for
periodic or event tasks) that the controller devotes to unscheduled communication.
•
The controller performs unscheduled communication for up to 1 ms at a time and then
resumes the continuous task.
Continuous task
You do not assign a priority to the continuous task. It always runs at the lowest priority. All
other tasks interrupt the continuous task.
Example
The following example shows the execution of a project with three user tasks.
Task
Priority
Period
Execution time
Duration
Motion planner
N/A
8 ms (base update
rate)
1 ms
1 ms
Event task 1
1
N/A
1 ms
1…2 ms
Periodic task 1
2
12 ms
2 ms
2…4 ms
I/O task—n/a to ControlLogix and
SoftLogix controllers. See
7
5 ms (fastest RPI)
1 ms
1…5 ms
System overhead
N/A
Time slice = 20%
1 ms
1…6 ms
Continuous task
N/A
N/A
20 ms
48 ms
Rockwell Automation Publication 1756-PM005-EN-P - October 2014
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