Rockwell Automation Publication 1769-UM011H-EN-P - April 2012 101
Develop Applications Chapter 7
Remember that:
• the highest priority task interrupts all lower priority tasks.
• the dedicated I/O task can be interrupted by tasks with priority levels 1...5.
The dedicated I/O task interrupts tasks with priority levels 7...15. This
task runs at the selected RPI rate scheduled for the CompactLogix system
(2 ms in this example).
• the continuous task runs at the lowest priority and is interrupted by all
other tasks.
• a lower priority task can be interrupted multiple times by a higher priority
task.
• when the continuous task completes a full scan it restarts immediately,
unless a higher priority task is running.
Define Programs
Each program contains:
• program tags.
• a main executable routine.
• other routines.
• an optional fault routine.
Each task can schedule as many as 32 programs.
The scheduled programs within a task execute to completion from first to last.
Programs unattached to any task show up as unscheduled programs. You must
specify (schedule) a program within a task before the controller can scan the
program.
Define Routines
A routine is a set of logic instructions in a single programming language, such as
ladder logic. Routines provide the executable code for the project in a controller.
A routine is similar to a program file or subroutine in a PLC or SLC controller.
Each program has a main routine. This is the first routine to execute when the
controller triggers the associated task and calls the associated program. Use logic,
such as the Jump to Subroutine (JSR) instruction, to call other routines.
You can also specify an optional program fault routine. The controller executes
this routine if it encounters an instruction-execution fault within any of the
routines in the associated program.
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