Configure An Event Task; Guidelines To Configure An Event Task; Additional Considerations For Periodic And Event Tasks - Allen-Bradley ControlLogix 1756 Reference Manual

Chapter 2 Logic Execution

Configure an Event Task

Trigger
Module Input Data State Change
Consumed Tag
Axis Registration 1 or 2
Axis Watch
Motion Group Execution
EVENT Instruction
Guideline
Place the I/O module being used to trigger an event in the
same chassis as the controller.
Limit events on digital inputs to one input bit on a module.
Set the priority of the event task as the highest priority on
the controller.
Limit the number of event tasks.
Consideration
Amount of code in the event task
Task priority
CPS and UID instructions
Communication interrupts
Motion planner
Trends
Output processing
30
An event task executes automatically based on a trigger event occurring or if a
trigger event does not occur in a specific time interval. You configure whether the
task updates output modules at the end of the task. After the task executes, it
does not execute again until the event occurs again. Each event task requires a
specific trigger.
Description
With Logix5000 controllers, a remote input module (digital or analog) triggers an event task that is based on the change of state (COS)
configuration for the module. Enable COS for only one point on the module. If you enable COS for multiple points, a task overlap of the event
task can occur.
• The ControlLogix sequence of events modules (1756-IB16ISOE, 1756-IH16ISOE) use the Enable CST Capture feature instead of COS.
• The embedded input points on the 1769-L16ER-BB1B, 1769-L18ER-BB1B, and 1769-L18ERM-BB1B modules can be configured to trigger
an event task when a COS occurs.
Only one consumed tag can trigger a specific event task. Use an IoT instruction in the producing controller to signal the production of
new data.
A registration input triggers the event task.
A watch position triggers the event task.
The coarse update period for the motion group triggers the execution of both the motion planner and the event task. Because the motion
planner interrupts all other tasks, it executes first.
Multiple EVENT instructions can trigger the same task.
For more information on event tasks, see:
• Logix5000 Controllers Common Procedures Programming Manual,
publication
• Using Event Tasks with Logix5000 Controllers,
publication

Guidelines to Configure an Event Task

Description
Placing the I/O module in a remote chassis adds more network communication and processing to the response time.
All inputs on a module trigger one event, so if you use multiple bits you increase the chance of a task overlap. Configure
the module to detect change of state on the trigger input and turn off the other bits.
If the priority of the event task is lower than a periodic task, the event task has to wait for the periodic task to complete
execution.
Increasing the number of event tasks reduces the available CPU bandwidth and increases the chances of task overlap.

Additional Considerations for Periodic and Event Tasks

Description
Each logic element (for example, rung, instruction, or structured text construct) adds to scan time.
If the event task is not the highest priority task, a higher priority task can delay or interrupt the execution of the event task.
If one of these instructions are active, the event task cannot interrupt the currently executing task. (The task with the CPS or UID.)
Incoming character processing through the serial port interrupts a task, regardless of the priority of the task.
The motion planner takes precedence over event or periodic tasks
Trend data collection takes precedence over event or periodic tasks.
You can disable output processing at the end of a task to reduce the amount of task processing time. As of RSLogix 5000 software, version 16,
the Controller Organizer displays whether outputs processing is disabled.
Rockwell Automation Publication 1756-RM094I-EN-P - September 2015
1756-PM001
LOGIX-WP003