PLC (Programmable Logic Control) Interview questions


1. What is PLC?

It is a class of industrially hardened devices that provides the 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.

PLC Senses the input through I/P modules, Processes the logic through CPU and memory and gives output through output module.

2. Applications of PLC:

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. Difference between fixed and modular PLC smile:

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.

4. 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 visualization. In general, SCADA does not use a central database.

5. 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.

6. Explain Source and Sink Concept?

Sinking Source type modules gives out the current to the field digital devices while sink type modules draw current when the device is in high state
Sinking – When active the output allows the current to flow to a common ground.
Sourcing – When active, current flows from a supply, through the output device and to ground.

7. What is the 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.

8. What are the programming languages used for PLC?

  • Ladder Logic (LD),
  • Statement List (STL),
  • Functional Block Diagram (FBD),
  • Sequential Function Chart (SFC),
  • Instruction List (IL) etc

9. What is redundancy ?

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

10. What are components of redundant PLC system ?

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

11. What are the PLC ranges 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

12. What is use of RS Linx software ?

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

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

Siemens has broadly 3 PLC ranges ie Siemens S7 200 , 300 and 400.

14. What are the software used with Siemens ?

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.

15. How to program PLCs ?

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


In Mitsubishi fx-3g PLC why x8 is not used as input?