Rockwell Automation Logix5000 Controllers Tasks, Programs, and Routines Programming Manual - Manual (Page 12)

Chapter 1 Manage tasks
The number of priority levels depends on the controller.
This Logix5000 controller Has this many priority levels
CompactLogix 15
ControlLogix 15
DriveLogix 15
FlexLogix 15
SoftLogix5800 3
To assign a priority to a task, use the following guidelines.
If you want Then Notes
This task to interrupt
another task
Assign a priority number that is less than the
priority number of the other task.
• A higher priority task interrupts all
lower priority tasks.
• A higher priority task can interrupt a
lower priority task multiple times.
Another task to interrupt
this task
Assign a priority number that is greater than the
priority number of the other task.
This task to share controller
time with another task Assign the same priority number to both tasks. The controller switches back and forth
between each task and runs each task
for 1 ms.
Additional considerations
As you estimate the execution interrupts for a task, consider the following.
Consideration Description
Motion planner The motion planner interrupts all user tasks, regardless of their priority.
•
The number of axes and coarse update period for the motion group affect how long and how
often the motion planner runs.
• If the motion planner is running when a task is triggered, the task waits until the motion
planner is done.
• If the coarse update period occurs while a task is running, the task pauses to let the motion
planner run.
I/O task CompactLogix, FlexLogix, and DriveLogix controllers use a dedicated periodic task to process I/O
data. This I/O task:
•
Does not show up in the Tasks folder of the controller.
• Does not count toward the task limits for the controller.
• Operates at priority 6.
• Runs at the fastest RPI you have scheduled for the system.
• Runs for as long as it takes to scan the configured I/O modules.
As you assign priorities to your tasks, consider the I/O task.
If you want a task to Then assign one of these priorities
Interrupt or delay I/O processing 1…5
Share controller time with I/O processing 6
Let I/O processing interrupt or delay the task 7…15
12 Rockwell Automation Publication 1756-PM005-EN-P - October 2014