PLC Interview Questions with Answers Part 2

This session is the continuation of  PLC Interview Questions with Answers Part 1 This session also contains 15 questions.The link to the PLC interview question part 1 is given below

16. Explain P&ID.

Ans. P&ID stands for piping and instrumentation diagram. It is a diagram in the process industry which shows the installed equipment. It is a like a blue print of a system. Also it has a very logical numbering.

17. What is redundancy?

Ans. The ability to switch from primary equipment to standby equipment automatically without affecting the process under control is redundancy. Redundancy means provision for the standby module. In case of failure of a module in the execution process, the standby module takes control. Hot redundancy means switching control from active processor to standby processor in less than 1 scan time.

18. What are components of redundant PLC system?

Ans. Typical component on Schneider Redundant PLCs.

The backplane used is either 4 slot o6 6 slot with-

  • 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.

19. Difference between fixed and modular PLCs?

Ans. In non-modular PLCs, the processor will have an 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.

20. Difference between PLC & DCS ?


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

PLC: The system has a processor and I / O and some functional units such as basic modules, communication modules, etc. Use a SCADA for visualisation. In general, SCADA does not use a central database.

21. Difference between PLC & Relay ?

Ans. 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.

22. Name the leading PLC brands?

Ans. Followinng are the leading PLC brands:




23. What are the PLC ranges available in Rockwell?

Ans. Following PLC ranges are available in Rockwell:

  • Pico : Non modular small PLCs.
  • Micrologix 1000, 1200 and 1500 Series.
  • SLC : SLC 5/01, 5/02, 5/03.
  • Control Logix Flex Logic and Soft PLC.

24. What is use of RS Linx software?

Ans. RS Linx software is used to perform following tasks:

  • Configure communication drivers.
  • View configured drivers and active nodes.
  • Enable communication tasks such as uploading, downloading, going online, updating firmware and sending messages.

25. What are the various PLC system in SIMATIC S7 range?

Ans. Siemens has broadly 3 PLC ranges i.e. Siemens S7 200 , 300 and 400.

26. What are the software used with Siemens?

Ans. Semanticor S7 200 PLC programming Microwin.

For S7 300 and 400 system : Simantic S7 manager.

The SCADA software used by Siemens is Win CC. Earlier Siemens use to supply COROS LS/B.

27. What is Ladder Diagram?

Ans. Ladder Diagram 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.

28. What is an HMI?

Ans. The HMI or Human Machine Interface allows operators to “interface” with the system they oversee. It provides a visual overview of the automated system’s status and direct control of its operation. An HMI’s graphical screens can be programmed to display important status and control information to the operator. Pictures, icons, sounds, and colors can all be used by HMIs to visually represent different operating conditions. And many HMIs deploy touch screen technology for user interaction with elements displayed on the screen.

29. What is the role of PLC in Automation?

Ans. 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.

30. What is role of I/O modules?

Ans. Electronic plug in units used for interfacing the I/P and O/P device in the machine or process to be controlled.

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