TOP PLC INTERVIEW QUESTIONS AND ANSWERS

Top PLC Interview Questions and Answers

Top PLC Interview Questions and Answers

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Programmable Logic Controllers (PLCs) are essential components in modern industrial automation. They manage machinery, systems, and processes by receiving inputs from sensors, processing data, and controlling outputs.



1. What is a PLC?


A Programmable Logic Controller (PLC) is a microprocessor-based industrial computer designed to automate processes by receiving inputs from sensors, executing control programs, and controlling outputs.

2. What are the Different Types of PLC?



  • Fixed/Integrated/Compact PLC

  • Modular PLC


3. What is the Difference Between Compact PLC and Modular PLC?



  • Compact PLC: Combines CPU, I/O modules, and power supply into one unit, offering cost-effectiveness but limited expansion options.

  • Modular PLC: Consists of separate modules for CPU, I/O, and power supply, providing scalability and flexibility.


4. What is the Difference Between Analog and Digital Signals?



  • Analog Signal: Continuous signals varying over a range (e.g., 0–10V, 4–20mA).

  • Digital Signal: Discrete signals with two states (ON/OFF, 0 or 1).


Signal handling is a core focus in PLC SCADA training.

5. What is Redundancy in a PLC Control System?


Redundancy ensures continuous operation by using backup PLC systems, enhancing reliability and minimizing downtime.

6. What are the Basic Components of a PLC System?



  • Power Supply Unit

  • Central Processing Unit (CPU)

  • Input/Output (I/O) Modules

  • Communication Unit

  • Memory Unit


7. Why Do Industries Use PLCs?



  • Faster Response Time

  • Real-Time Operation

  • Easy Troubleshooting

  • High Reliability

  • Flexible Programming

  • Support for Multiple I/O Devices


8. What is a Sensor?


A sensor detects physical changes (e.g., temperature, pressure, motion) and converts them into electrical signals for PLC processing.

9. What are the PLC Programming Languages (IEC 61131-3)?



  • Ladder Diagram (LD)

  • Functional Block Diagram (FBD)

  • Structured Text (ST)

  • Instruction List (IL)

  • Sequential Function Chart (SFC)


10. What is Ladder Diagram Programming?


It uses graphical symbols to represent control logic, resembling electrical circuit diagrams.

11. What is Structured Text Programming?


A high-level programming language using conditional statements, loops, and functions.

12. What is Functional Block Diagram (FBD) Programming?


It represents control logic with graphical blocks like timers, counters, and logic gates.

13. What is CFC in PLC?


Continuous Function Chart (CFC) is a graphical programming language for arranging and interconnecting functional blocks.

14. What is a Rack or Chassis in a PLC System?


It is a physical structure that mounts and connects PLC components like CPU, I/O modules, and power supply. It is essential in modular PLC systems.

15. What are Active and Passive Backplanes in PLC?



  • Active Backplane: Includes electronics for communication and power distribution.

  • Passive Backplane: Acts as a simple connector between modules.


16. What are Program Mode, Run Mode, and Test Mode in PLC?



  • Program Mode: Modify or download programs.

  • Run Mode: Execute control logic continuously.

  • Test Mode: Partially execute the program for testing.


17. What are Common PLC Communication Protocols?



  • RS-232/RS-485

  • Modbus RTU/TCP

  • Profinet

  • Ethernet/IP

  • DeviceNet

  • Profibus DP


18. Why is a 4-20 mA Signal Used in PLC?


The 4-20 mA signal is standard for transmitting analog data due to its noise immunity and precision.

19. What are the Types of PLC Modules?



  • Digital Modules

  • Analog Modules

  • Temperature Modules

  • Pulse/Encoder Modules

  • High-Speed Modules


20. What is an Analog Module?


It interfaces with continuous signals (e.g., voltage or current) for precise control.

21. What is a Digital Module?


It manages ON/OFF signals from switches, relays, and solenoids.

22. What is a Scan Cycle in PLC?


The scan cycle is the time taken by a PLC to monitor inputs, execute the program, and update outputs.

23. What is Sinking and Sourcing in PLC?



  • Sinking: Current flows from the load to the PLC input terminal.

  • Sourcing: Current flows from the PLC output terminal to the load.


24. What are PLC Data Types?



  • Boolean: Binary values (True/False)

  • Integer: Whole numbers

  • Real: Decimal numbers

  • String: Text data


25. Who are the Top PLC Manufacturers and What are Their Models?



  • Siemens: S7-300, S7-400, S7-1200, S7-1500

  • Allen Bradley: MicroLogix, CompactLogix, ControlLogix

  • ABB: AC500, AC800

  • Honeywell: HC900, Experion PKS

  • Mitsubishi: MELSEC FX, Q, L Series

  • Schneider Electric: Modicon M221, M241, M258, M340, M580

  • Omron: CP1H, CJ2, CS1, NX1P, NX7

  • Panasonic: FP-X, FP2


Cognidel offers specialized PLC programming courses in bangalore focusing on industrial automation, troubleshooting, and SCADA systems. With experienced instructors, hands-on training, real-world projects, and industry-aligned curricula, Cognidel ensures students are well-prepared for automation and control roles. Flexible learning options, personalized mentorship, and strong placement support make Cognidel the preferred choice for aspiring automation professionals in Bangalore.

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