300+ TOP PLC Interview Questions and Answers

PLC Interview Questions for freshers and experienced :-

1. What is a PLC?

Programmable Logic Controllers are industrially hardened micro computers, designed to replace electromechanical relays, used to control machines and processes.

2. What are different components in PLC?

  1. Input Interface
  2. Memory Section
  3. Central Processing Unit (CPU)
  4. Programmable Language
  5. Programming tool
  6. An output Interface

3. Define Advantages of PLCs than Hard wired Relay?
PLCs are highly reliable, easily programmable, Small and Inexpensive, PLCs can be designed with the communication capabilities so they can converse with the local or remote computer, They can sustain in robust environment less maintenance.

4. What is the programmable language used in PLC?
The general language program consists of Ladder Diagrams. Relay logic control scheme is represented in Ladder diagrams. Alternative languages uses Boolean representation of these control schemes as base of the computer representation.

5. What does Central Processing Unit (CPU) of PLC consists?
CPU is the brain of the system and consists of

  • Microprocessor: To carry out arithmetic and logical operations
  • Memory: The area in the CPU in which the information is stored and retrieved
  • Power Supply: The electrical supply that converts the ac voltage to various DC operating voltages.
PLC Interview Questions and Answers
PLC Interview Questions

6. What is SCAN in PLC ?
The sequential operation of the controller that goes through the ladder diagram from top to bottom of the ladder. In this process it updates all the outputs corresponding to the inputs. SCAN takes place from left to right of each rung. Usually SCAN time is in milliseconds and it is a continuous process.

7. Difference between PLCs and Computers
Programmable Logic Controllers (PLCs) have basic architecture compared to normal general purpose computers. A normal computer can be converted to PLCs by providing a way that the computer can able to receive information or signal from the field devices such as push buttons, switches and valve positions. Computer requires a some software to process the information obtained from the input to generate an output which decide whether to close or open the valve position in the process side.

Some of the important features and characteristics that distinguish between the general purpose computers and Programmable Logic Controllers

(PLCs) are given below:

  • PLCs are designed to operate under industrial environments (PLCs have to operate under wide range of temperature conditions, humidity and other environmental conditions). They are least affected by the electrical noise and are inherent to electrical noise
  • Programming in PLCs is through Relay Ladder Logic or other easily learned language. PLCs comes with program language built in its memory. PLCs do not contain input and output devices such as keyboards, mouse, monitor, CD drives and other hard disks. It is in simple a self contained box with communication ports and set of terminals for input and output devices.
  • Unlike computers which performs numerous tasks simultaneously, PLCs execute a single program in an orderly and sequential manner from first instruction to the last instruction
  • PLCs have been designed for installation and maintenance by plant electricians. Programming in PLCs is simple (Relay Ladder Programming), it does not include any advanced code. Troubleshooting is simpler and many PLCs are designed to include fault details and written fault details on display screen.

8. PLCs Advantages or Benefits

  1. Higher Reliability:
    Once the program written and tested it can be easily downloaded into other PLC’s memory. It requires lesser and simpler wiring compared to conventional hard wired circuits employed. Hence reliability of the system increases significantly with PLCs
  2. More Flexibility:
    It is easier to create a new program module or change an existing program in PLC compared to hard wired circuitry system. These software program modules can be changed whenever required. Use can modify the programs in the field and if required, security can be enhanced by hardware interlocks such as key locks and software features such as passwords
  3. Lower Cost:
    PLCs were originally designed to replace relay control logic which is not economical and complex especially for large control circuits. With PLCs the cost savings have been so significant that the relay control becomes uneconomical except for some power applications. Generally if the application consists of more than half a dozen control relays, PLCs are least expensive to install
  4. Communication Capability:
    Communication capability of PLC with the other controllers and computers in the system is one of the main advantages compared to relay control circuit. Functions such as Supervisory control, data acquisition from the field, monitoring devices and process parameters associated with the field and downloading and uploading of programs can be easily possible with the PLC compared to hardwired circuits
  5. Faster Response:
    PLCs are designed for high speed and for the real time applications.Response time for PLCs are much smaller compared to relay logic circuits. The programmable controllers operates in real time i.e, an event taking carrying out at field will result in execution of operation of output
  6. Easy to Troubleshoot:
    PLCs have inbuilt diagnostics and override functions that helps the user to easily trance the software and hardware errors.

9. what is Programmable Logic Controllers (PLCs)
Programmable Logic Controllers (PLCs):

  • Programmable Logic Controllers (PLCs) are the micro processor controller devices situated at the remote sites of the plant or at the field of the process systems. PLCs purpose is to monitor the process parameters located at the field and adjust the outputs based on the inputs received by the PLCs. PLCs will operate on any systems which will have input field devices that are On-Off type (discrete or digital ) or analog input devices. Similarly it will operate on the field devices which will be discrete or analog output type.
  • In simple words, Hence PLC acts as interface between the input and output devices at the process side of the industries. It monitors the inputs receive from the input devices and takes necessary output control functions by executing the programs stored in its memory.
  • The term Logic is used in Programmable Logic Controllers because all the program inside the PLC will be of Logic programming (eg: Ladder diagrams ).
  • In industrial applications hard wired relay logics were replaced by Programmable Logic Controllers due to its reliablity, simplicity, low cost, easier programming and many other functions

10. What are the main advantages of PLC?

  1. Modular design
  2. Increased reliability
  3. Lower cost
  4. Compact size
  5. Ease of programming
  6. Rugged construction and designed to withstand vibrations, temperature, humidity and noise
  7. Ability to communicate with computer

11. What Is The Need Can Be Satisfied In Designing An Industrial Grade Lcu?

Flexibility of changing the control configuration Ability to use the controller without being a computer expert. Ability to bypass the controller in case it fails. So that the process still can be controlled manually. Ability of the LCU to communicate with other LCUs and other elements in system.

12. What Is Lcu?

Local control unit is the small collection of hard ware in the system that can do closed loop control. Local control unit interfaces directly to the process.

13. What Is The Significance Of Dcs?

DCS is used for interfacing and computing functions and also provides the means of communication between the other devices. It consists of local control unit, low level interface, high level interface, shared communication facility Etc.

14. Advantages Of Plcs Than Hard Wired Relay?

PLCs are highly reliable, easily programmable, Small and Inexpensive, PLCs can be designed with the communication capabilities so they can converse with the local or remote computer, They can sustain in robust environment less maintenance.

15. What Is Ladder Logicdiagram?

Ladder Logic Diagram is a symbolic language used in PLC. This diagram consist of series of symbols interconnected by lines to indicate the flow of current through the various devices. In ladder logic diagram the sides of the ladder forms the power sources and current flows through the various –logic input devices that form the rungs of the ladder.

16. What Are The Major Areas Of Application Of Plc?

  • Sequence control, timing, counting, and data calculation
  • Batch or continuous process control
  • Precise position/motion control
  • Open loop or feedback control, process data acquisition and display

17. What Are The Advantages Of Plc Over Relays?

  1. They have to be hard-wired to perform a specific function
  2. No hard-wire requirement
  3. When the system requirement change, the relay wiring has to be changed or modified
  4. Can be easily changed or expanded through program
  5. Higher power consumption
  6. Lower power consumption

18. Define The Functions Of Plc-input And Output Modules:

  1. Input interface modules accept signals from the machine or process devices (120Vac) and convert them into signals (5Vdc) that can be used by the controller.
  2. Output interface modules convert controller signals (5Vdc) in to external signals (120Vdc) used to control machine or process.

19. List The Potential Benefits Of Scada?

  • Educed site visits and callouts
  • Predictive maintenance which allows proactive rather than reactive planning
  • The ability to view and adjust set points remotely
  • Reliability and robustness
  • Maximize productivity
  • Improve product quality
  • Reduce your operating and maintenance costs
  • Integrate with your business systems
  • Preserve your capital investment

20. Define Some Of The Application Areas Of Scada?

Industrial process: chemical plants, power generation and distribution, Metallurgy etc and nuclear process: reactors, nuclear waste etc.

21. Define Scada ?

Supervisory Control and Data Acquisition (SCADA) system is computerised hardware and software based project that provides a single integrated view of all control and information resources, enable engineers, supervisors, operators to view and interact with the working of an entire operation through graphical representation of their product process.

22. What Is Scada? Give Any Four Editors Available In Scada Package?

Supervisory Control and Data Acquisition (SCADA) system is computerised hardware and software based project that provides a single integrated view of all control and information resources, enable engineers, supervisors, operators to view and interact with the working of an entire operation through graphical representation of their product process.

Four editors:

  1. Inks-cape SAGE – SCADA Animation Graphic Editor
  2. Generic Logic
  4. UCanCode


CIMPLICITY is a SCADA package. It is based on a client server architecture consisting of servers and viewers. Servers are responsible for the collection and distribution of data. Viewers connect into servers and have full access to the collected data for viewing and control actions.

24. Explain the classification of PLC?

There are five major classification of PLC based on number of inputs and outputs, cost, functionality and physical size. They are

    1. Nano : 16 I/O pins
    2. Micro : 32 I/O pins
    3. Nano or micro PLC could used on applications such as elevators, car washes or mixing machines.
    4. Small : 128 I/O pins,2Kbytes memory, capable of simple to advanced level of machine control
    5. Medium: 2048 I/O pins, 32Kbytes memory, used for process control applications
    6. Large: 8192 I/O pins, 750 Kbytes memory, mostly used to control individual production process or entire plants like paper and pulp, chemical and automotive and power plants.

25. List out the programming languages in PLC?

  • Ladder Logic Diagram
  • Sequential Function Charts
  • Instruction List
  • Function Block
  • Structured Text Diagram

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