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PLC – automated control for the industry

Programmable logic controllers monitor and regulate machines and manufacturing processes in industrial environments with automation technology. In the future, as autonomous systems, they could even optimize processes intelligently and independently. In our guide, we introduce you to the various systems and provide tips on selection and the right accessories.


  • What role does the programmable logic controller play in automation?
  • Advantages of SPS
  • How is a programmable logic controller structured?
  • Functions of programmable logic controllers
  • Compatibility and accessories for networking
  • The right equipment for your PLC
  • types of PLC
  • Modular PLC: A simple and individual solution for every machine system
  • Programming languages ​​for PLC

What role does the programmable logic controller play in automation?

A PLC is a digital electronic system that is equipped with programmable memory. The respective control instruction for the appropriate function can then be saved on this. Various types of machines and processes are then controlled via digital or analogue inputs and outputs.
Thanks to their great flexibility, PLCs are being used more and more frequently in many areas of industry and are replacing conventional connection-programmable controllers. Instead of hard-wired logic based on relay switches, where the control function can only be changed by laborious switching, these electronic components can be digitally programmed and take on complex control tasks in networked production processes of Industry 4.0.
Typical examples of the use of PLCs can be found in production plants, packaging machines, in the beverage and steel industries and also in the medical sector. But you also come across digital electrical controls in everyday life, for example in traffic light circuits, escalators, elevators, roller shutters and gate systems or even in heating systems.

Advantages of PLC

  • high flexibility and versatility
  • uncomplicated duplication and modification of the programming instead of cumbersome rewiring
  • simplified implementation of programming
  • less assembly effort
  • quick change of function
  • high reliability
  • low energy consumption
  • little need for space
  • Networking options with other devices and systems
  • Remote diagnosis and maintenance are possible

Programmable logic controllers only have a few disadvantages, which in individual cases speak for the use of conventional connection-programmable controllers: The costs for the components are comparatively high so the purchase of PLCs is not worthwhile for many small controllers. In addition, an appropriate technical infrastructure with digital devices and trained personnel are required for effective use. Acceptance and commissioning of systems can also be more complex since the program routines and programming have to be tested in addition to the machine components. Individual components such as safety circuits or device monitoring may become more complex.

How is a programmable logic controller structured?

The basic version of a PLC consists of a processor in the central module, signal inputs and outputs, an operating system and the interface.
The control is programmed via the application program on the computer or via a connected control panel. It is loaded onto the PLC via the interface and defines the switching of inputs and outputs. The actual operation then takes place independently of the computer and, thanks to its own PLC power supply, also autonomously.
Depending on the complexity, PLCs have a different number of analogue or digital inputs and outputs, which are linked to the machine or system via so-called sensors and actuators. At the inputs, for example, sensors such as pressure sensors, temperature or level sensors monitor the machine functions. The operating system evaluates the information collected, compares it with the parameters programmed by the user and sends the appropriate control signals to the outputs.
The actuators that control the functions are connected here. These can be contactors for switching on electric motors, electric valves for compressed air and hydraulics or drive control modules.

Functions of programmable logic controllers

PLCs can take on specific functions, such as:

  • link control,
  • Flow control,
  • as well as time, counting and arithmetic functions.

Cycle-oriented PLCs work strictly according to EVA, the basic principle of data processing with input, processing, output. The inputs are queried and control is passed to the user program. After transferring the control signals to the outputs, the process starts over.
Cyclic PLCs with interrupt processing report an alarm when the status of the connected sensor changes and then start an additional program loop adapted to the respective situation before continuing the main program.
Event-controlled PLCs process-specific pre-programmed tasks after a status change of the connected sensors.

Compatibility and accessories for networking

One of the biggest advantages of PLC is the cross-system networking with other devices and computers. Correct networking is not only important for new installations. Even PLCs based on older standards can be connected to modern devices during retrofitting using the appropriate PLC cables, plugs and adapters.
Most PLCs can be connected to a PC via a serial cable. Serial device servers are suitable for this, with which conventional serial components can be connected via LAN. Ethernet media converters connect different transmission media with each other in a compatible way. Common interfaces are RS-232 and RS-485 with DB-9 and DB-25 connectors.
However, PLC interfaces are not standardized, so depending on the device provider, other solutions are used in addition to standard serial cables, which you should find out about in advance or involve a specialist. In general, the design thus determines the requirements for the accessories for the respective PLC

The right equipment for your PLC

Your PLC gains in efficiency with the right additional equipment and can also be expanded with devices via the interface.

  • DIN rails: for optimal assembly.
  • Displays and monitors: for the direct display of operational data.
  • HMI touch panel: for multifunctional visualization, operation and diagnosis of machines and systems.
  • Smart gateway: Networking machines and systems intelligently in the IoT.

types of PLC

In terms of structure and functionality, a distinction is made between hard PLCs and soft PLCs as well as compact and slot PLCs.
Classic PLCs are hardware-based, hence their name Hard-PLC. They usually only include the most important control functions, but extensions for DIN rails and plug connections are quickly possible.
Soft PLCs take over the control in software form. They have their own operating system and often additional user software. However, they do not have their own CPU, but use the PC processor and have to share the processing power with the PC’s operating system and, if necessary, other applications. This can have an impact on the performance of the respective PLC when the load is high.
With a compact PLC, all components are housed together on a single circuit board in a single housing.
A slot PLC is used directly as a PCI plug-in card and for specific tasks such as storing production data. With these controllers, the PC replaces the PLC hardware. In contrast to a soft PLC, however, a slot PLC has a co-processor and its own operating system, with which it executes control tasks independently of the PC processor. Integrated connections enable access to decentralized actuators and sensors.

design typeHard PLCSoft PLCCompact PLCSlot PLC
notes• PLC hardware
solution • are started up quickly
• good real-time
behaviour • most important functions with expansion options
• PLC software solution
• more convenient to use
• slow start-up
• unstable at very high loads
• space-saving
• inexpensive
• suitable for small-scale tasks
• low space requirements
• they remain active thanks to their own power supply even if the PC fails
• simplified communication between PLC and PC thanks to visualization software

Modular PLC: A simple and individual solution for every machine system

Modular PLCs are not assemblies in the true sense but can be understood as a superordinate system. With modular controls, each individual functional component is housed in its own housing and on a separate control board. In this way, the PLC can be assembled individually using individual plug-in modules and easily expanded if required. An additional advantage is that in the event of a defect, only the respective assembly needs to be replaced and not the entire controller.


  • individual compilation of the required elements
  • Extension with the appropriate circuit board possible
  • for more complex regulation and control circuits

Some typical manufacturers and model series of modular PLCs


  • S7-300
  • S7-1200
  • S7-1500

Schneider Electric

⦁ M200s
⦁ M340
⦁ M580


⦁ CP1E
⦁ CP1H
⦁ CP1L

Allen Bradley

  • micro 820
  • micro 850
  • micro 870

Programming languages ​​for PLC

Various programming languages ​​can be found in the controls:

  • ST – Structured Text
  • FBD – function block language
  • LAD – Ladder Diagram
  • IL – Instruction List
  • SCL—Structured Control Language

FBD is popular with PLC beginners because it works with drag & drop and the behaviour of inputs and outputs is easy to understand. Sophisticated programming can be implemented with SCL. ST and IL are text-based, the other three graphical.

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