Top PLC SCADA Interview Questions and Answers

1. What is PLC?

A Programmable Logic Controller (PLC) is a microprocessor-based industrial computer used 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: Integrates CPU, I/O modules, and power supply in a single unit, making it cost-effective but limiting expansion.
Modular PLC: Consists of separate modules for CPU, I/O, and power supply, offering flexibility for expansion and scaling.

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

Analog Signal: Continuous signals that can vary over a range (e.g., 0–10V, 4–20mA).
Digital Signal: Discrete signals with two states (ON/OFF, 0 or 1).

These signal types are fundamental concepts covered in PLC and SCADA training programs.

5. What is Redundancy in a PLC Control System?

Redundancy in PLC systems involves using a backup PLC to ensure continuous operation if the primary system fails, ensuring high system reliability and reducing 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

These components form the foundation of PLC programming courses.

7. Why Do Industries Use PLCs for Process Control?

Faster Response Time
Real-Time Operation
Easy Troubleshooting
High Reliability
Flexibility in Programming
Support for Multiple I/O Devices

These benefits are thoroughly explored in PLC SCADA training programs.

8. What is a Sensor?

A sensor detects physical changes (e.g., temperature, pressure, or motion) and converts these into electrical signals, which are processed by a PLC.

9. Which Programming Languages Are Used in PLC?

PLC programming is standardized by IEC 61131-3, which includes:

Ladder Diagram (LD)
Functional Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL)
Sequential Function Chart (SFC)

10. What is Ladder Diagram Programming?

Ladder Diagram (LD) uses graphical symbols to represent control logic, resembling an electrical circuit diagram.

11. What is Structured Text Programming?

Structured Text (ST) is a high-level programming language with conditional statements, loops, and function definitions.

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

In Functional Block Diagram (FBD) programming, control logic is represented using graphical blocks like timers, counters, and logic gates.

13. What is CFC in PLC?

Continuous Function Chart (CFC) is a graphical programming language that allows users to create control logic by freely arranging functional blocks.

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

A rack or chassis is the physical structure where PLC modules (CPU, I/O, power supply) are mounted and connected. This setup is especially relevant in modular PLC systems.

15. What are Active and Passive Backplanes in PLC?

Active Backplane: Contains additional electronics to support communication and power distribution.
Passive Backplane: Simply connects modules without additional electronics, focusing on providing physical connectivity.

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

Program Mode: Used to modify and download the PLC program.
Run Mode: The PLC executes the program continuously.
Test Mode: Allows partial execution of the PLC program for testing or simulation.

These modes are critical concepts in PLC and SCADA training.

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 the standard used for transmitting analog data between sensors and PLCs. It is preferred for its noise immunity and accuracy.

19. What are the Types of PLC Modules?

Digital Modules
Analog Modules
Temperature Modules
Pulse/Encoder Modules
High-Speed Modules

These modules play a vital role in PLC operations and are thoroughly covered in PLC programming courses.

20. What is an Analog Module?

Analog modules interface with continuous signals (e.g., voltage or current) and are used for more precise control and measurement in PLC systems.

21. What is a Digital Module?

Digital modules are used to manage simple ON/OFF signals, such as switches, relays, or solenoids, commonly found in field devices.

22. What is a Scan Cycle in PLC?

The scan cycle refers to the time it takes for a PLC to monitor inputs, execute its logic, and update outputs. This cycle repeats continuously throughout the operation.