Rockwell Automation Publication 1769-UM011H-EN-P - April 2012 99
Develop Applications Chapter 7
Define Tasks
Tasks provide scheduling and priority information for programs. You can
configure tasks as continuous, periodic, or event tasks. Only one task can be
continuous.
A task can have as many as 32 separate programs, each with its own executable
routines and program-scoped tags. Once a task is triggered (activated), all the
programs assigned to the task execute in the order in which they are grouped.
Programs can only appear once in the Controller Organizer and cannot be shared
by multiple tasks.
Specify 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 CompactLogix controller uses a dedicated periodic task at priority 6 to
process I/O data. This periodic task executes at the RPI you configure for the
CompactBus, which can be as fast as once each millisecond. 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...5 take precedence over the dedicated I/O task. Tasks in this
priority range can impact I/O processing time. For example, if you use the
following configuration:
• I/O RPI = 1 ms
• a task of priority = 1...5 that requires 500 µs to execute and is scheduled to
run every millisecond
this configuration leaves the dedicated I/O task 500 µs to complete its job of
scanning the configured I/O.
Table 25 - Task Support
Controller Tasks Supported
1769-L35x 8
1769-L32x 6
1769-L31 4
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