30 Mostly asked PLC Interview Questions

1. What is Programmable Logic Controllers (PLCs)?

Ans. Programmable Logic Controllers (PLCs):

Programmable Logic Controllers (PLCs) are the micro processor controller devices situated at the remote sites of the plant or at the field of the process systems. PLCs purpose is to monitor the process parameters located at the field and adjust the outputs based on the inputs received by the PLCs. PLCs will operate on any systems which will have input field devices that are On-Off type (discrete or digital ) or analog input devices. Similarly it will operate on the field devices which will be discrete or analog output type.

In simple words, PLC acts as interface between the input and output devices at the process side of the industries. It monitors the inputs receive from the input devices and takes necessary output control functions by executing the programs stored in its memory.

The term Logic is used in Programmable Logic Controllers because all the program inside the PLC will be of Logic programming (eg: Ladder diagrams).

In industrial applications hard wired relay logics were replaced by Programmable Logic Controllers due to its reliability, simplicity, low cost, easier programming and many other functions.

2. What are the applications of PLC?

Ans. PLC can be used in almost all industrial application solutions right from small machine to large manufacturing plants. Even it caters applications of redundant systems at critical process plants.

3. Explain what are different components in PLC?

Ans. Following are the different components in PLC:

Input Interface. Memory Section. Central Processing Unit (CPU). Programmable Language. Programming tool. An output Interface.

4. What are the two basic sections of PLC?

Ans. The two basic sections of PLC are:

CPU The Input/Output interface system

5. What are the three major components of CPU?

Ans. The three major components of CPU are:

Processor Memory system System power supply

6. Explain what is the programmable language used in PLC?

Ans. The general language program consists of Ladder Diagrams. Relay logic control scheme is represented in Ladder diagrams. Alternative languages uses Boolean representation of these control schemes as base of the computer representation.

7. Explain what does Central Processing Unit (CPU) of PLC consists?

Ans. CPU is the brain of the system and consists of:

Microprocessor: To carryout arithmetic and logical operations. Memory: The area in the CPU in which the information is stored and retrieved. Power Supply: The electrical supply that converts the ac voltage to various DC operating voltages.

8. Explain what is SCAN in PLC ?

Ans. SCAN is the process in which a sequential operation of the controller that goes through the ladder diagram from top to bottom of the ladder. In this process it updates all the outputs corresponding to the inputs. SCAN takes place from left to right of each rung. Usually SCAN time is in milliseconds and it is a continuous process.

9. What are the PLCs Advantages or Benefits?

Ans. Following are the PLCs Advantages or Benefits:

Higher Reliability: Once the program written and tested it can be easily downloaded into other PLC’s memory. It requires lesser and simpler wiring compared to conventional hard wired circuits employed. Hence reliability of the system increases significantly with PLCs.

More Flexibility: It is easier to create a new program module or change an existing program in PLC compared to hard wired circuitry system. These software program modules can be changed whenever required. Use can modify the programs in the field and if required, security can be enhanced by hardware interlocks such as key locks and software features such as passwords.

Lower Cost: PLCs were originally designed to replace relay control logic which is not economical and complex especially for large control circuits. With PLCs the cost savings have been so significant that the relay control becomes uneconomical except for some power applications. Generally if the application consists of more than half a dozen control relays, PLCs are least expensive to install.

Communication Capability: Communication capability of PLC with the other controllers and computers in the system is one of the main advantages compared to relay control circuit. Functions such as Supervisory control, data acquisition from the field, monitoring devices and process parameters associated with the field and downloading and uploading of programs can be easily possible with the PLC compared to hardwired circuits.

Faster Response: PLCs are designed for high speed and for the real time applications.Response time for PLCs are much smaller compared to relay logic circuits. The programmable controllers operates in real time i.e, an event taking carrying out at field will result in execution of operation of output.

Easy to Troubleshoot: PLCs have inbuilt diagnostics and override functions that helps the user to easily trance the software and hardware errors.

10. Explain Advantages of PLCs than Hard wired Relay?

Ans. PLCs are highly reliable, easily programmable, Small and Inexpensive, PLCs can be designed with the communication capabilities so they can converse with the local or remote computer. They can sustain in robust environment with less maintenance.

11. How to program PLCs ?

Ans. Every PLC manufacturer have their own software for programming the PLC. For example Siemens uses Simantic S7 Manager, Allen Bradley uses RS Logix and Modicon uses PLC pro programming software. The programming language used is Ladder Logic (LD), Statement List (STL), Functional Block Diagram (FBD), Sequential Function Chart (SFC), Instruction List (IL) etc.

12. What is the meaning of scan time in PLC?

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

13. Why 4-20 ma and not 0-20 ma?

Ans. When you use a 4-20 ma signal, the PLC will be able to detect a cable break, because it returns 0 ma.

When you use a 0-20 ma signal, the PLC won’t be able to detect a cable break, because it returns 0 ma and the PLC will think there is nothing wrong with the signal.

14. What Is SIL?

Ans. SIL stands for Safety Integrity Level. A SIL is a measure of safety system performance. It has four levels. Higher the SIL level, lower the probability of failure for the safety systems. The cost and complexity also increases as SIL level increases.

15. Name different types of timers used in PLC.

Ans. In PLC, timers may be On delay timer, Off delay timer, Retentive or accumulative timer & Pulse timer.

The basis of every time is ON DELAY timer. Using only On delay we can arrange various other types of times.

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 6?


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 2?

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:


Allen Bradley Modicon Texas Instruments General Electric European:

Siemens Festo Telemechanique Japanese:

Toshiba Omron Fanuc Mistubishi

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.