PLC interview questions



1. What is PLC?


PLC means Programmable Logic Controller. It is a class of industrially hardened devices that provides hardware interface for input sensors and output control element. The field I/p include element like limit switches, sensors, push button and the final control elements like actuator, solenoid/control valves, drives, hooters etc.

2. Role of PLC in Automation ?


PLC plays most important role in automation. All the monitoring as well as the control actions are taken by PLCs. PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.

3. Difference Between Fixed and Modular PLCs ?


In non modular PLCs the processor will have inbuilt power supply and I/Os in one unit. The modular PLC, will have separate slots for components like Power supply, I/O modules. You can select the I/Os or power supply as per the need.

4. What is meaning of scan time in PLC?


Scan time is the Time required to read the I/P, Process the logic and update the output in one cycle.

5. What is redundancy ?


The capacity to switch from primary equipment to standby equipment automatically without affecting the process under control.

6. What are components of redundant PLC system ?


  • Power Supply
  • Controller with built-in Modbus Plus and Modbus ports
  • Optional dual cable Modbus Plus
  • Optional fiber optic Modbus Plus
  • CHS Hot Standby module
  • Dual cable Remote I/O Head

7. What is ladder diagram ?


This is a programming language, which expresses a program as a series of “coils” and “contacts”, simulating the operation of electromechanical relays. The resultant program is the equivalent of an equation, which is executed continuously in a combinatorial manner.

8. Difference between PLC & Relay ?


  • PLC can be programmed whereas a relay cannot.
  • PLC works for analog I/Os such as PID loops etc. whereas a relay cannot
  • PLC is much more advanced as compared to relay.
  • Modifications in relay base circuit is difficult compared to PLCs

9. Difference between PLC & DCS ?


DCS: The system uses multiple processors, has a central database and the functionality is distributed. That is the controller sub system performs the control functions, the history node connects the data, the IMS node gives reports, the operator station gives a good HMI, the engineering station allows engineering changes to be made.

PLC: The system has processor & I/O’s and some functional units like basic modules, communication modules and so on. Uses a SCADA for visualization. Generally the SCADA does not use a central database

10. What is role of I/O modules?


Electronic plug in units used for interfacing the i/p and o/p device in the machine or process to be controlled..

11. Scan cycle of PLC?


PLC’s scan cycle follows following path Scan cycle of PLC

  • nput Image Updation
  • Process Logic Execution