TOP PLC SCADA INTERVIEW QUESTION AND ANSWERS

Top PLC SCADA Interview Question and Answers

Top PLC SCADA Interview Question and Answers

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What is PLC?


Programmable Logic Controller (PLC) is a microprocessor or microcontroller-based industrial computer used to automate processes. It receives inputs from sensors, processes data based on pre-programmed instructions, and controls outputs.



2. What are the different types of PLC?



  • Fixed/Integrated/Compact PLC

  • Modular PLC





3. Difference Between Compact PLC and Modular PLC?



  • Compact PLC: Processor, I/O, and power supply are combined in a single unit with a fixed number of I/O connections. It is cost-effective and ideal for smaller applications.

  • Modular PLC: Consists of separate modules (CPU, I/O, Power supply) mounted on a rack. Offers flexibility for expansion and customization.





4. What is Analog and Digital Signal?



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

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





5. What is Redundancy in PLC Control System?


Redundancy involves having a backup PLC (secondary CPU) operating alongside the primary PLC. It ensures uninterrupted operation if the primary PLC fails, increasing reliability and reducing downtime.



6. What are the Basic Components of PLC Systems?



  • Power Supply Unit

  • Central Processing Unit (CPU)

  • Input/Output (I/O) Modules

  • Communication Unit

  • Memory Unit





7. Why is PLC Used in Industrial Control Processes?



  • Faster Response Time

  • Programmability

  • Real-time Operation

  • Communication Capabilities

  • Easier Troubleshooting

  • Increased Reliability

  • Support for High Input/Output Numbers





8. What is a Sensor?


A sensor converts physical properties (e.g., temperature, motion, pressure) into electrical signals for interpretation by PLC systems.



9. Which Programming Languages are Used in PLC?


Standardized by IEC 61131-3, PLC languages include:

  1. Ladder Diagram (LD)

  2. Functional Block Diagram (FBD)

  3. Structured Text (ST)

  4. Instruction List (IL)

  5. Sequential Function Chart (SFC)





10. What is Ladder Diagram Programming?


It’s a graphical PLC programming language resembling a ladder. Vertical rails represent power flow, and horizontal rungs represent control logic.



11. What is Structured Text Programming?


A high-level text-based programming language combining conditional statements and logic for process control.



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


Graphical language using logic blocks (e.g., timers, counters, arithmetic functions) interconnected to form control logic.



13. What is CFC in PLC?


Continuous Function Chart (CFC) is a flexible, graphical programming language for arranging control blocks without strict sequence restrictions.



14. What is a Rack or Chassis in PLC?


A rack provides a physical framework for mounting and connecting PLC modules (CPU, I/O, power supply, etc.) via a backplane bus system.



15. What is Active and Passive Backplane?



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

  • Passive Backplane: Only provides physical connections without additional electronic functions.





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



  • Program Mode: Edit and download programs (I/O scanning stops).

  • Run Mode: Executes the program in real-time.

  • Test Mode: Partial execution for troubleshooting.





17. What are Common PLC Communication Protocols?



  • RS-232/RS-485

  • Modbus RTU/TCP

  • Profinet

  • Ethernet/IP

  • DeviceNet

  • Profibus DP





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


It’s an industry-standard analog signal for transmitting sensor data reliably over long distances with reduced noise interference.



19. What are the Types of PLC Modules?



  • Digital Module

  • Analog Module

  • Temperature Module

  • Pulse/Encoder Module

  • High-speed Module





20. What is an Analog Module?


Processes continuous signals, typically with voltage (0–10V) or current (4–20mA) sensing capabilities.



21. What is a Digital Module?


Handles discrete ON/OFF signals from devices like switches and outputs like relays or lamps.



22. What is a Scan Cycle?


The scan cycle is the time taken by a PLC to read inputs, process the program logic, and update outputs in real-time.



23. Explain Sinking and Sourcing in PLC Systems?



  • Sinking: Current flows from the load to the PLC (negative path).

  • Sourcing: Current flows from the PLC to the load (positive path).





24. What are PLC Data Types?


Common PLC data types:

  • Boolean: TRUE/FALSE (1/0)

  • Integer: Whole numbers

  • Real: Decimal numbers

  • String: Text data





25. Name Popular PLC Manufacturers and Their Models:



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

  • Allen Bradley (AB): 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


 

 

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