Rockwell Automation Allen-Bradley DriveLogix 5730 User Manual page 17

Specifying task priorities
Each task in the controller has a priority level. The operating system uses
the priority level to determine which task to execute when multiple tasks are
triggered. You can configure periodic tasks to execute from the lowest
priority of 15 up to the highest priority of 1. A higher priority task will
interrupt any lower priority task. The continuous task has the lowest priority
and is always interrupted by a periodic task.
The DriveLogix5730 controller uses a dedicated periodic task at priority 7
to process I/O data. This periodic task executes at the RPI you configure for
the CompactBus, which can be as fast as once every 1 ms. Its total execution
time is as long as it takes to scan the configured I/O modules.
How you configure your tasks affects how the controller receives I/O data.
Tasks at priorities 1 to 6 take precedence over the dedicated I/O task. Tasks
in this priority range can impact I/O processing time. If you configure the I/
O RPI at 1ms and you configure a task of priority 1 to 6 that requires 500 µs
to execute and is scheduled to run every millisecond. This leaves the
dedicated I/O task 500 µs to complete its job of scanning the configured I/O.
However, if you schedule two high priority tasks (1 to 6) to run every
millisecond, and they both require 500 µs 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 (8 to 15).
TIP: For example, if your program needs to react to inputs and control
outputs at a deterministic rate, configure a periodic task with a priority
higher than 7 (1 through 6). 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 7 (8 through 15), such as the continuous task, so that the dedicated I/O
task is not adversely affected by your program.
The following example shows the task execution order for an application
with periodic tasks and a continuous task.
Task: Priority Level:
1 5
2 7
5 ms selected RPI 1
3 10
4 none (lowest)
Example Execution
Task Type: Time:
20 ms periodic task 2 ms
dedicated I/O task
3 ms
10 ms periodic task 4 ms
continuous task 25 ms
What is DriveLogix5730?
Worst Case
Completion Time:
2 ms
8 ms
60 ms
1-7