Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 13)

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
Additional
Considerations on page 12 .
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 13