Technical data

Basics of Designing a Program Structure

4.2 Blocks in the User Program

Programming with STEP 7

Manual, 05/2010, A5E02789666-01 73

4.2.1 Organization Blocks and Program Structure

Organization blocks (OBs) represent the interface between the operating system and the user

program. Called by the operating system, they control cyclic and interrupt-driven program

execution, startup behavior of the PLC and error handling. You can program the organization

blocks to determine CPU behavior.

Organization Block Priority

Organization blocks determine the sequence (start events) by which individual program sections

are executed. An OB call can interrupt the execution of another OB. Which OB is allowed to

interrupt another OB depends on its priority. Higher priority OBs can interrupt lower priority OBs.

The background OB has the lowest priority.

Types of Interrupt and Priority Classes

Start events triggering an OB call are known as interrupts. The following table shows the types of

interrupt in STEP 7 and the priority of the organization blocks assigned to them. Not all

organization blocks listed and their priority classes are available in all S7 CPUs (see "S7-300

Programmable Controller, Hardware and Installation Manual" and "S7-400, M7-400 Programmable

Controllers Module Specifications Reference Manual").

Type of Interrupt Organization Block Priority Class

(Default)