Rockwell Automation 1756-L55M12, Selection Manual, Page: 56 / 88

54
1756-SG001G-EN-P — March 2004
How a Logix
System Uses Tasks
A Logix controller uses three types of tasks. Use the following table to choose the
appropriate type of task for each section of your logic.
Event tasks
Several factors affect the closed-loop event throughput:
y
input module type, filter time, and response time
y
backplane size and loading
y
controller type, configuration, and loading
y
output module type and response time
If you want to execute a section of your
logic:
Then use this type of task: Description:
all of the time
Continuous Task
The continuous task runs in the background. Any CPU time not
allocated to other operations (such as motion, communications, and
periodic or event tasks) is used to execute the programs within the
continuous task.
y
The continuous task runs all the time. When the continuous task
completes a full scan, it restarts immediately.
y
A project does not require a continuous task. If used, there can be
only one continuous task.
y
at a constant period (e.g., every
100 ms)
y
multiple times within the scan
of your other logic
Periodic Task.
A periodic task performs a function at a specific period.
y
Whenever the time for the periodic task expires, the task interrupts
any lower priority tasks, executes one time, and then returns
control to where the previous task left off.
y
You can configure the time period from 0.1 ms to 2000 msec. The
default is 10 ms. It is also controller and configuration dependent.
y
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 Task
An event task performs a function only when a specific event (trigger)
occurs.
The trigger for the event task can be:
y
change of a digital input
y
new sample of analog data
y
certain motion operations
y
consumed tag
y
EVENT instruction
Whenever the trigger for the event task occurs, the event task:
y
interrupts any lower priority tasks
y
executes one time
y
returns control to where the previous task left off