Basic knowledge of Siemens PLCs and programming in Simatic Step7

Programmable logic controller (PLC), also known as programmable controller, is the name given to a type of computer commonly used in commercial and industrial control applications.

PLCs differ from office computers in the types of tasks they perform and the hardware and software required to perform those tasks. While specific applications vary widely, all PLCs monitor input and other variable values, make decisions based on stored programs, and control outputs to automate processes or machines.

The basic elements of a PLC include input modules or points, central processing units (CPUs), output modules or points, and programming devices. The type of input module or point used by the PLC depends on the type of input device used. Some input modules or points respond to digital inputs, also known as discrete inputs, which can be open or closed. Other modules or inputs react to analog signals.

These analog signals represent machine or process conditions as a range of voltage or current values. The primary function of a PLC’s input circuitry is to convert the signals provided by these various switches and sensors into logic signals that can be used by the CPU. The CPU evaluates the statuses of the inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of the outputs.The output modules convert the control signals from the CPU into digital or analog values that can be used to control various output devices. The programming device is used to enter and change the PLC’s program, to monitor and change the stored values. Once entered, the program and associated variables are stored in the CPU. In addition to these basic elements, a PLC system may also incorporate an operator interface device of some sort to simplify monitoring of the machine or process.

Benefits of PLC Not only are PLCs capable of performing the same tasks as hardwired controllers, they can also handle a wider range of complex applications. In addition, PLC programs and electronic communication lines replace most of the wiring required for hard-wired control. So, while field devices still need to be connected, hardwiring is less intensive, making it easier to correct errors and change applications. Following are the main advantages of PLC:     

  • Smaller physical size than hardwired solutions;
  • easier and faster changes;
  • PLC has built-in diagnostics and override functions;
  • Diagnostics can be provided centrally;
  •  Applications can be recorded immediately;
  • PLC has built-in diagnostics and override function 
  • Diagnostics can be provided centrally;

 Programming languages  :

The Contr, List and Log programming languages ​​for S7-300/400 are an integral part of the basic software.  

1.Contact diagram (CONT) It is a graphic programming language. The syntax of the instructions is reminiscent of the circuit patterns. CONTS allows you to easily follow the journey of the current between the power bars through the contacts, the complex elements and the coils.  

2.Instructions list (List) It is a textual programming language close to the machine. In a list program, the various instructions correspond, to a large extent, to the stages by which the CPU treats the program. To facilitate programming, List has been supplemented by some advanced language structures (such as, for example, block parameters and structured access to data).  

3.Logigram (log) It is a graphic programming language that uses boxes of Boole algebra to represent logical operations. Complex functions, such as mathematical functions, can be represented directly combined with logical boxes.  

4.S7-SCL (Structured Control Language) It is an advanced programming language close to Pascal, used to program programmable automata with Simatic S7. S7-SCL corresponds to the evolved textual language ST (Structured Text) defined in the CEI 61131-3 standard and was designed for certification for reusability level.

Motor Starter Example

This example shows the practical aspects of programming step 7 on the physical presence part of the system. The engine start coil (M) is in series with the normally open momentary start button, the normally closed momentary stop button, and the normally closed overload relay (OL) contacts.

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