Use Obs For Organizing Your User Program - Siemens SIMATIC S7-1200 Manual

6.1.1

Use OBs for organizing your user program

Organization blocks provide the structure for your program. They serve as the interface
between the operating system and the user program. OBs are event-driven. An event, such
as a diagnostic interrupt or a time interval, will cause the CPU to execute an OB. Some OBs
have predefined start events and behavior.
The program cycle OB contains your main program. You can include more than one program
cycle OB in your user program. During RUN mode, the program cycle OBs execute at the
lowest priority level and can be interrupted by all other types of program processing. (The
startup OBs do not interrupt the program cycle OBs because the CPU executes the startup
OBs before going to RUN mode.) After finishing the processing of the program cycle OBs,
the CPU immediately executes the program cycle OBs again. This cyclic processing is the
"normal" type of processing used for PLCs. For many applications, the entire user program is
located in a single OB, such as the default program cycle OB 1.
The CPU determines the order for handling interrupt events by a priority assigned to each
OB (Page 54).
Easy Book
Manual, 11/2011, A5E02486774-04
Programming made easy
6.1 Easy to design your user program
You can create other OBs to
perform specific functions, such
as startup tasks, for handling
interrupts and errors, or for
executing specific program code
at specific time intervals.
Use the "Add new block" dialog to
create a new OB in your user
program.
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