Using Blocks To Structure Your Program; Organization Block (Ob) - Siemens SIMATIC S7 System Manual

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99BProgramming concepts

6.3 Using blocks to structure your program

Using blocks to structure your program

By designing FBs and FCs to perform generic tasks, you create modular code blocks. You

then structure your program by having other code blocks call these reusable modules. The

calling block passes device-specific parameters to the called block.

When a code block calls another code block, the CPU executes the program code in the

called block. After execution of the called block is complete, the CPU resumes the execution

of the calling block. Processing continues with execution of the instruction that follows after

the block call.

You can nest the block calls for a more modular structure. In the following example, the

nesting depth is 4: the program cycle OB plus 3 layers of calls to code blocks.

Organization blocks provide 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.

Calling block

Called (or interrupting) block

Program execution

Instruction or event that initiates the execution of

another block

Program execution

Block end (returns to calling block)

System Manual, 11/2011, A5E02486680-05

Start of cycle

Nesting depth

S7-1200 Programmable controller

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