Technical data

Basics of Designing a Program Structure
4.2 Blocks in the User Program
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 77
Deselected Interrupt OBs
If you assign priority class 0 or assign less than 20 bytes of local data to a priority class, the
corresponding interrupt OB is deselected. The handling of deselected interrupt OBs is restricted as
follows:
• In RUN mode, they cannot be copied or linked into your user program.
• In STOP mode, they can be copied or linked into your user program, but when the CPU goes
through a restart (warm start) they stop the startup and an entry is made in the diagnostic
buffer.
By deselecting interrupt OBs that you do not require, you increase the amount of local data area
available, and this can be used to save temporary data in other priority classes.
Cyclic Program Processing
Cyclic program processing is the "normal" type of program execution on programmable logic
controllers, meaning the operating system runs in a program loop (the cycle) and calls the
organization block OB1 once in every loop in the main program. The user program in OB1 is
therefore executed cyclically.