What is a PLC?

Dear All

Here i will explain you how to learn about PLC in short time, for that we have some prerequisites to understand about the PLC.


1. Digital and Analog Fundamentals
2. Boolean Gates
3. Electrical Fundamentals
4. Digital Logic Desgin

Now after having knowledge of all these fundamentals we will start from PLC, what is PLC ?

A Programmable Logic Controller (PLC) is an electronic device that takes input from the plant / machine via sensors and transmitters, executes the logic programmed in its memory and generates the useful outputs on actuators to control plant / machine.

Richard E. Morley of Bedford Associates had invented the first PLC as a consulting project for General Electric in 1968. Bedford Associates is currently named Modicon and is a supplier of PLCs.

Now have you ever think ? Why PLC ?

Any industrial machine or plant are being controlled by control and power circuits located at control and power panels respectively.

Before the invention of the PLC, control circuits were designed using hardwired relay logic.Hardwired relay logic has many disadvantages as compared to PLC logic.

There are mainly 3 part of PLC.

1. Input Module

2, Output Module

3. CPU

for the input and output module you can see that there are again 2 types of module.

1. Digital Input module

2. Analog Input module

So according to input and output signal we can connect module.

Now Input module is receive the signal from input like Switch, Proximity sensors or any digital sensors.

its senses the signals from IP module and gives signal to CPU and CPU generates one bit in its memory which called as Process Image Input (PII) and then CPU execute the program which we have already loaded in PLC and update process output Image (POI) and finally its generates signal to output module and output module is connected with final control elements like bulb, LED, Contractor or analog output.

For learning PLC SCADA and industrial automation online please watch youtube channel “Automation Revolution”.

19 Replies to “What is a PLC?”

  1. Programmable logic controller is an industrial solid-state computer that monitors inputs and outputs, and makes logic-based decisions for automated processes or machines.

    • Initially industries used relays to control the manufacturing processes. The relay control panels had to be regularly replaced, consumed lot of power and it was difficult to figure out the problems associated with it. To sort these issues, Programmable logic controller (PLC) was introduced.

    PLC Hardware 1 to 5:

    1. Input Module: Inputs are data given to PLC, classsified in
    1. Digital ip: Switch, Push Button, Proximity Sensors, etc which gives input of either 1 or 0 to PLC
    2. Analog ip: Every analog sensors like Temperature, weight, distance, etc which gives analog data to PLC
  2. Output Module: Output from PLC, classsified in
    1. Digital op: output signal either 1 or 0
    2. Analog op: variable output of 0 to 12 VDC or 4mA to 20mA DC.
  3. CPU:
    1. This component act as a brain of the system. CPU consist of Arithmetic Logic Unit, Program memory, Process image memory, Internal timers and counters, flags
    2. It receives information from I/P device, makes decisions depending upon the information and logic written and sends information through the O/P devices.
  4. Power Supply: Certain PLCs have an isolated power supply. But, most of the PLCs work at 220VAC or 24VDC.
  5. Memory: Stores the information of the status of input and output devices, and the values of timers, counters and other internal devices.
  6. Programming Device (Computer): This device is used to feed the program (logic) into the memory of the plc. First we make a program in certain PLC software then we download it to PLC.
  7. PLC Programming: It is a logic we make in Computer in PLC software. Based on that logic & conditions PLC gives output.
    1. Most commonly used programming language is Ladder diagram (LD) also known as Ladder logic
    2. Typical Ladder logic:

    Here input is push button & output is light, so when PB is pressed due to logic of PLC light will be turn ON.

    Typical PLC

    PLC can be used in almost all industrial application solutions right from small machine to large manufacturing plants. Even it caters applications of redundant systems at critical process plants.

    • Advantages of PLC:
    • They are fast and designed for the rugged industrial environment.
    • Easy to change logic i.e. flexibility
    • Low power consumption
    • They are attractive on Cost-Per-Point Basis.
    • Analog signal handling and close loop control programming
    • Reliable due to absence of moving parts
    • Easy to couple with the process computers
    • Counter, timer and comparator can be programmed
    • Capable of handling of very complicated logic operations
    • Flexibility: One single PLC can easily run many machines.

    For more

    Engineering Essentials: What Is a Programmable Logic Controller?

    PLC (Programmable Logic Controller)

    Programmable logic controller


  8. For managing the manufacturing process there was the use of relays by the industries. It is said that there should be regular replacement of the relay control panels as there is excessive consumption of power and it is difficult to identify problems associated with it.

    Define PLC?

    For the automation of various electro-mechanical processes in industries, a digital computer like Programmable Logic Controller (PLC) is used. For dealing through tough times it has been designed in such a way shielded from cold, heat, dust, and moisture etc. It is programed using the computer language and is made up of micro-processor.

    The program is downloaded to the PLC with the help of a cable and the code is written in the computer system. If you consider the non-volatile memory of the PLC you will find all the loaded programs stored over here.

    PLC Hardware

    The physical elements or the hardware of a PLC system are CPU, Memory, Input/Output, Power supply unit, and FOR MORE INFO : PLCs AND ITS FUTURE

  9. A programmable logic controller (PLC), or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembley lines , or robotics devices, or any activity that requires high reliability control and ease of programming and process fault diagnosis.

    PLC is widely used in many industries for the automation purpose.

  10. Definition: National Electrical Manufacturers Association US (NEMA)

    ü A PLC is a digital operating electronic apparatus which uses a programmable memory for internal storage of instruction for implementing specific function such as logic, sequencing, timing, counting and arithmetic to control through analog or digital input/output modules various types of machines or process.

    qInitially designed to replace relay logic boards

    qSequence device actuation

    qCoordinate activities

    qAccepts input from a series of switches

    qSends output to devices or relays

    I am giving a link of video in YOU TUBE which explain everything about the PLC


  11. A 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 can differ from office computers in the types of tasks that they perform and the hardware and software they require to perform these tasks.

    All PLCs monitor inputs and other variable values, make decisions based on a stored program, and control outputs to automate a process or machine.

  12. PLC could mean a Phase Locked Clock. This is a situation where a clock, normally a digital clock oscillator is synchronised with another where the phase of locking could be constant or variable. A related example is when two AC power stations need to have their generators locked together in frequency.
    There are many situations in frequency counters and other digital electronics where Phase Locked Clocks need to be used.
    PLC could also mean a Programmable Logic Controller and there are other persons who gave an excellent answer when applied to industrial control.

  13. A programmable logic controller, PLC, or programmable controller is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many machines, in many industries. PLCs are designed for multiple arrangements of digital and analog inputs and outputs, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory.

  14. Programmable Logic Control is an industrial control system who scan all inputs connected to PLC input side module and do process on its according to store programming in its memory and generate outputs which is also connected on its output side module i.e motors,signals etc.

    In past all above process control with the help of relays logic but due to complication and large setup and huge maintenance const involved in relay logic system. PLC replaced the relay logic system due to easy arrangements, and custom design programming,quick response time,real time environments where output must be generate in any conditions like.

    read more What is PLC and How PLC works?

  15. A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

  16. Thanks for A2A,

    PLC is Programmable Logic Controller which aims in automating electro-mechanical process. PLC will helps to generate and controls electric signals and output devices also.

    3 Points in PLC as follows:

    Input Devices

    Output Devices

    and Processors

    If you want to learn about plc programs then you can come for demo session and join course.

  17. Programable Logic Controller is used for industrial automation which uses the ladder logic as a programming language.This below picture shows the block diagram of PLC:

    ladder logic is a simple program which is very easy to understand. Other than ladder logic we can also use block diagram and instructions for programming.Circuit connections are simple in PLC.

  18. A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

    Almost any production line, machine function, or process can be greatly enhanced using this type of control system. However, the biggest benefit in using a PLC is the ability to change and replicate the operation or process while collecting and communicating vital information.

    Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to best suit your application.

  19. Here’s how I put it in one of my blog posts: “Programmable Logic Controllers (PLCs) are small industrial computers with modular components designed to automate customized control processes. PLCs are often used in factories and industrial plants to control motors, pumps, lights, fans, circuit breakers and other machinery. To understand the purpose of PLCs better….”

    If you want to see the whole post, check out: http://www.myplctraining.com/beg

  20. Programmable Logic Controller (PLC) are primary components in automation solutions. It's an industrial digital computer with complete shock resistant feature which is adapted for the control of manufacturing process. To provide malleable, robust and easily programmable controls to replace hard wire relays and timer, PLC was first developed or introduced to automobile industries.

    PLC uses in manufacturing assembly lines, robotic devices and high-reliability control activity and for the ease of program.

    And, for more detailed information and data you can visit Us Automation Controls | Wago Top Job US-Automation.

  21. It is a abbreviation  for programmable logic control,  which is very useful for electricity facilities to control than  ohters

    And there is another one called power line communication that is a kind of alternative method transmitting data.

  22. Do you mean Programmable Logic Controller? Well, if this is you’re trying to ask here is the answer. PLC is a digital computer utilize for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, light fixtures or amusement rides. PLC’s are used in many industries and machines.
    For more info please visit this website: Programmable Logic Controller In Singapore

  23. Plc programmable logic controller. It's works on ladder diagram. There different manufacturers of PLC. Programmer also have work on different software to make logic for plc. Now how this ladder diagram, program works in plc basically on logic which programmer makes it operates output of PLC. Than you can see that effect in machine or plant where plc installed.

  24. Depends on the context.
    1. Public Limited Company. That means the company is listed on the stock exchange in that specific country and can be traded on the stock market. eg BP PLC. In German, it is called AG (Aktiengesellschaft) or a listed company and may be traded on the DAX.
    2. Programmable Logic controller.

Leave a Reply

Your email address will not be published. Required fields are marked *