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2-2

Ladder Logic (LAD) for S7-300 and S7-400

C79000-G7076-C504-02

2.1 Structure of User Programs

A user program consists of logic blocks and data blocks. Logic blocks are

blocks with a code section such as organization blocks, function blocks, or

functions.

Organization blocks (OBs) form the interface between the operating system

and the user program. Different organization blocks have different functions.

To create the LAD user program for your S7 CPU, you select the

organization blocks necessary for your specific automation task. For the most

basic task you will require the following:

S Startup (OB100, OB101)

S Scan cycle (OB1)

S Error handling (OB80 to OB87, OB121, OB122), if you do not want your

CPU to switch to STOP when an error occurs.

There are also organization blocks available to handle interrupts in the CPU

or other interrupts from the process.

For detailed information about the functions of each organization block and

the startup information provided by the CPU operating system, refer to the

Reference Manual /235/.

You can program every organization block as a structured program by

creating functions (FCs) and function blocks (FBs) and calling them in the

code section. When the blocks are called, you supply the data required for

the declared parameters.

S A function block (FB) is a logic block with “memory”. This memory

takes the form of instance data blocks assigned to the FB. The instance

DBs store all the actual parameters and static data relating to the function

block.

S A function (FC) is a logic block without “memory”, in other words

without associated instance DBs. After an FC has been processed, the

output parameters contain the calculated function values. Once the

function has been called, the user decides how the actual parameters are

used and stored.

The operating system makes the following data available:

S Peripheral I/Os

S Process image input/output

S Bit memory

S Timers

S Counters

Logic Blocks and

Data Blocks

Organization

Blocks

Functions/

Function Blocks

Data

Introduction

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