Allen-Bradley controllogix 1756-L55M12, User Manual

Page: 70 / 142

Publication 1756-UM001G-EN-P - January 2007
70 Develop Applications
Define Programs
Each program contains program tags, a main executable routine, other
routines, and an optional fault routine. Each task can schedule as
many as 100 programs.
The scheduled programs within a task execute to completion from
first to last. Programs that aren’t attached to any task show up as
unscheduled programs. You must specify (schedule) a program within
a task before the controller can scan the program.
Unscheduled programs within a task are downloaded to the controller
with the entire project. The controller verifies unscheduled programs
but does not execute them.
Define Routines
A routine is a set of logic instructions in a single programming
language, such as Ladder Diagram. 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 processor.
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.