Plc Interview Questions and Answers

Plc Interview Questions

  • 1) What is PLC?
  • 2) What are different components Of PLC?
  • 3) What are various applications of PLC?
  • 4) Who are leading suppliers of PLC?
  • 5) What is redundancy in PLC?
  • 6) What is an HMI in PLC?
  • 7) What is SCAN in PLC?
  • 8) List the advantages of PLC over Relays?
  • 9) Explain scan cycle of PLC?
  • 10) What is role of I/O modules in PLC?
  • 11) What is LD in PLC?
  • 12) What is MCR in PLC?
  • 13) How Do Fixed PLCs differ from modular PLCs?
  • 14) Which programmable languages are used in PLCs?
  • 15) Differentiate between PLCs and DCS.
  • 16) Differentiate between PLCs are Relays
  • 17) Which PLC ranges are available in Rockwell?
  • 18) What is the RS LINX software used for?
  • 19) How are PLCs more advantageous than hard-wired Relay?
  • 20) Which one would you prefer: 4-20 ma or 0-20 ma? Explain your answer.

1) What is PLC?

PLC is a digitalized computer used in industries to manufacture robotic and automobile devices. It is used in the automation of certain electromechanical processes is which is used as a device that controls electricity and is like a transducer for converting electrical energies to mechanical energies. In order to work efficiently and to make the program easy, the language of PLC is designed logically and their language resembles the ladder logic diagrams.

2) Differentiate between PLCs and DCS.

Here are a few considerable differences between the two:

  • PLCs were invented much earlier than DCS.
  • PLCs are used for electrochemical processors while DCS are used for Pneumatic/Single Loop Controls.
  • PLCs have generally fixed scan time while the scan time for DCS is adjustable.
  • PLCs are used for discrete controls and DCS are used for regulatory controls.

3) Differentiate between PLCs are Relays

The difference between PLC and Relay is given below:

  • PLCs are much cheaper compared to relays.
  • The PLCs are more efficient compared to relays.
  • They are used more.
  • PLCs can be repaired easily but it is hard to repair Relays.

4) Which one would you prefer: 4-20 ma or 0-20 ma? Explain your answer.

Use of a 4-20 ma signal results in easy detection of a cable break by the PLC. This is because it only returns 0 ma. On using a 0-20 ma signal, the PLC loses its ability to detect any cable break. Since the return value is 0 ma which is the same as input, the PLC thinks that the signal is working perfectly. That is why 4-20 ma is preferred over 0-20 ma

5) Which PLC ranges are available in Rockwell?

The PLC ranges available in Rockwell are as follows:

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

6) What is the RS LINX software used for?

RS Linx software is generally a communicator between programming devices and the controllers themselves. It acts as a link between both and hence it is extremely useful to pass commands. It can not only configure communication drivers but also view the already configured drivers and the active nodes. Tasks, which enable communication, are its forte. RS Linx software is renowned for performing tasks such as downloading, uploading, updating firmware, going online, sending messages, and more.

7) Which programmable languages are used in PLCs?

Each computer or rather, all kinds of software have their own programming language. These languages are unique and command the computer to produce outputs. These languages may or may not be the same for all computers.

The programming languages used in PLCs are:

  • Instruction List Programming
  • Structured Text Programming
  • Functional Block Programming
  • Ladder Logic Programming
  • Sequential Functional Chart

8) What is SCAN in PLC?

SCAN is the procedure, which any sequential operation of the program controller must go through. It is the process of the operation going from the top to the bottom of the ladder in the ladder diagram. The process results in all updated outputs, which correspond to the given inputs. The process of SCAN takes place from the left to the right of each rung in the ladder diagram. Scan time is basically the total time needed to read the input, process the logic of the program and update the corresponding output in a single cycle. Generally, the time for SCAN is of the order milliseconds and it is a continuously running process.

 

 

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