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What can a PLC do? Why do we use them? (on photo: SIEMENS PLC Simatic S7-1500; credit: totallyintegratedautomation.com)
What can a PLC do? Why do we use them? (on photo: SIEMENS PLC Simatic S7-1500; credit: totallyintegratedautomation.com)

The meaning of PLC…

PLC” means “Programmable Logic Controller”, that’s clear. The word “Programmable” differentiates it from the conventional hard-wired relay logic. It can be easily programmed or changed as per the application’s requirement. The PLC also surpassed the hazard of changing the wiring.

The PLC as a unit consists of a processor to execute the control action on the field data provided by input and output modules. In a programming device, the PLC control logic is first developed and then transferred to the PLC.

So, what can a PLC actually do?

  1. It can perform relay-switching tasks.
  2. It can conduct counting, calculation and comparison of analog process values.
  3. It offers flexibility to modify the control logic, whenever required, in the shortest time.
  4. It responds to the changes in process parameters within fractions of seconds.
  5. It improves the overall control system reliability.
  6. It is cost effective for controlling complex systems.
  7. It trouble-shoots more simply and more quickly
  8. It can be worked with the help of the HMI (Human-Machine Interface) computer

There are many other things this little ‘mean’ thing can do, but one thing I’m sure – that PLCs are irreplaceable in many industry applications and control projects.

Here is an example of wired ABB’s AC500 programmable logic controllers.

Wired ABB's PLCs
Figure 1 – Wired ABB’s PLCs (photo credit: us.profinet.com)

Basic block diagram

Figure 1 shows the basic block diagram of a common PLC system.

Block diagram of a PLC
Figure 2 – Block diagram of a PLC

As shown in the above figure, the heart of the “PLC” in the center, i.e., the Processor or CPU (Central Processing Unit).

  • The CPU regulates the PLC program, data storage, and data exchange with I//O modules.
  • Input and output modules are the media for data exchange between field devices and CPU. It tells CPU the exact status of field devices and also acts as a tool to control them.
  • A programming device is a computer loaded with programming software, which allows a user to create, transfer and make changes in the PLC software.
  • Memory provides the storage media for the PLC program as well as for different data.

Size of the PLC system

Usually they are classified on the basis of their size:

  • A small system is one with less than 500 analog and digital I/Os.
  • A medium system has I/Os ranging from 500 to 5,000.
  • A system with over 5,000 I/Os are considered large.

Components of the PLC system

CPU or processor: The main processor (Central Processing Unit or CPU) is a microprocessor-based system that executes the control program after reading the status of field inputs and then sends commands to field outputs.

I/O section: I/O modules act as “Real Data Interface” between field and CPU. The PLC knows the real status of field devices, and controls the field devices by means of the relevant I/O cards.

Programming device: A CPU card can be connected with a programming device through a communication link via a programming port on the CPU.

Operating station: An operating station is commonly used to provide an “Operating Window” to the process. It is usually a separate device (generally a PC), loaded with HMI (Human Machine Software).


PLC Configurations

There are two basic configurations that commercial manufacturers offer:

1. Fixed Configuration

Fixed PLC configuration
Fixed PLC configuration

2. Modular Configuration

Modular type PLC 'SLC 500'
Modular type PLC ‘SLC 500’ (photo credit: ab.rockwellautomation.com)

PLC Applications (VIDEOs) //

Real world applications


An Application for Industrial Process Control


PLC Bottling Application


PLC application color mixing


PLC Suited To Bottling Line Application


5 guides to study PLCs //

  1. PLC – Programmable Logic Controller – Hugh Jack
  2. PLC Programming – OMRON
  3. PLC – Theory and Implementation – L.A. Bryan; E.A. Bryan
  4. Industrial Training – SCADA System and PLC – Mr. Sonu Kumar Yadav

References //

  • Industrial Automation Pocket Book – IDC Technologies
  • Overview of Programmable Overview of Programmable Logic Controllers – Dr. Fernando Rios-Gutierrez

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

10 Comments


  1. Nii Laate Lartey
    Mar 12, 2016

    Dear Edvard,
    Thanks so much for such information on PLC’s. It has enlightened my understanding on the subject. I have developed much more interest in PLC systems.


  2. vahid
    Sep 10, 2015

  3. Alok Mishra
    Jul 28, 2015

    excuse me sir can u understood me about PLC systems,it will be kind of u so please tell me,-I want to know about this topic….thank u…..


  4. Lason Suke
    Jul 23, 2015

    great article….. One question, I’m working on a renewable energy project basically solar farms which involves photovoltaic panels, battery banks,inverters and such. Do you think PLC can be applicable in control and monitoring this type of unit?


    • Jose Sanchez
      Aug 26, 2015

      Hi Lason,

      Of course!

      When your photovoltaic panel (with servomotors in one or two axes) need keep to be oriented perpendicular al the sun position you can make this with a little PLC like Micro850 series by Allen Braldey/Rockwell Automation and his realtime clock module and the correct routine for his positioning.

      If the photovoltaic panel are fixed, then this solution could not to be applied.

      Greetings!


  5. Pandharinath Jadhav
    Jul 21, 2015

    good updates


  6. Joseph Ainoo
    Jul 17, 2015

    Thanks alot my boss


  7. Ankit
    Jul 15, 2015

    Great article……helps a lot


  8. Abdullah Kaddoura
    Jul 15, 2015

    Another great article, thanks a lot mate.

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