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1 THE STEP7 PROGRAMMING LANGUAGE STEP 7 is the standard software package used for configuring and programming SIMATIC programmable logic controllers. It is part of the SIMATIC industry software. Basic algorithm for an automation process: Programs in a CPU In a CPU, two different programs are always executing: The operating system The user program. Operating System Every CPU has an operating system that organizes all the functions and sequences of the CPU that are not associated with a specific control task. The tasks of the operating system include the following: Handling a warm restart and hot restart Updating the process image table of the inputs and outputting the process image table of the outputs Calling the user program Detecting interrupts and calling the interrupt OBs Detecting and dealing with errors Managing the memory areas Communicating with programming devices and other communications partners If you change operating system parameters (the operating system default settings), you can influence the activities of the CPU in certain areas. User Program You yourself must create the user program and download it to the CPU. This contains all the functions required to process your specific automation task. The tasks of the user program include the following: Specifying the conditions for a warm restart and hot restart on the CPU (for example, initializing signals with a particular value) Processing process data (for example, logically combining binary signals, reading in and evaluating analog signals, specifying binary signals for output, outputting analog values)
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2 Specifying the reaction to interrupts Handling disturbances in the normal running of the program STEP7 programs can be written in three types of languages; LAD (Ladder Logic Diagrams), STL (Statement List) and FBD (Function Block Diagram). LAD is like circuit diagrams to show power flow in terms of contacts and blocks. STL is an assembly like language which is the fastest of all in compiling and FBD is a language consisting of logic blocks like AND, OR and XOR gates. Objects and Object Hierarchy In the same way that the Windows Explorer shows the directory structure of folders and files, the object hierarchy for projects and libraries in STEP 7 is shown in the SIMATIC Manager. The following figure shows an example of an object hierarchy. Objects have the following functions: Carriers of object properties, Folders, Carriers of functions (for example, to start a particular application). Block Types There are several different types of blocks you can use within an S7 user program: Block Brief Description of Function Organization blocks (OB) OBs determine the structure of the user program. System function blocks
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