User manual

Publication 1768-UM001C-EN-P - November 2007
Develop Applications 71
Defining Tasks
A task provides scheduling and priority information for a set of one or more
programs. You can configure tasks as continuous, periodic, or event.
• The 1768-L43 controller supports 16 tasks, only 1 of which can be
continuous.
• The 1768-L45 controller supports 30 tasks, only 1 of which 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 of
the programs assigned to the task execute in the order in which they are
grouped. A program can appear only once in the Controller Organizer and
cannot be shared by multiple tasks.
Types of Logix5000 Controller Tasks
Task Execution Task Type Description
All of the time Continuous The continuous task runs in the background. Any CPU time not allocated to
other operations (such as motion, communication, and other tasks) is used to
execute the programs in the continuous task.
• The continuous task runs constantly. When the continuous task
completes a full scan, it restarts immediately.
• A project does not require a continuous task. If used, there can be only
one continuous task.
• At a set interval, such as every 100 ms
• Multiple times within the scan of your
other logic
Periodic A periodic task performs a function at a specific interval.
• Whenever the time for the periodic task expires, the task interrupts any
lower priority tasks, executes once, and then returns control to where the
previous task left off.
• You can configure the time period from 0.1...2000 μs. The default is
10 ms. It is also controller and configuration dependent.
• The performance of a periodic task depends on the type of Logix
controller and on the logic in the task.
Immediately when an event occurs Event An event task performs a function only when a specific event (trigger) occurs.
In a 1768 CompactLogix controller, the trigger for the event task can be:
• a consumed tag trigger.
• an EVENT instruction.
• an axis trigger.
• a motion event trigger.