300+ TOP PLC LAB VIVA Questions and Answers

PLC LAB VIVA Interview Questions :-

1. What is a PLC?

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Programmable Logic Controllers are industrially hardened micro computers, designed to replace electromechanical relays, used to control machines and processes.

2. Explain what are different components in PLC?

  1. Input Interface
  2. Memory Section
  3. Central Processing Unit (CPU)
  4. Programmable Language
  5. Programming tool
  6. An output Interface

3. Define Advantages of PLCs than Hard wired Relay?
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 less maintenance.

4. What is the programmable language used in PLC?
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.

5. What does Central Processing Unit (CPU) of PLC consists?
CPU is the brain of the system and consists of

  • Microprocessor: To carry out 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.

6. What is SCAN in PLC ?
The 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.

7. Difference between PLCs and Computers
Programmable Logic Controllers (PLCs) have basic architecture compared to normal general purpose computers. A normal computer can be converted to PLCs by providing a way that the computer can able to receive information or signal from the field devices such as push buttons, switches and valve positions. Computer requires a some software to process the information obtained from the input to generate an output which decide whether to close or open the valve position in the process side.

Some of the important features and characteristics that distinguish between the general purpose computers and Programmable Logic Controllers

(PLCs) are given below:

  • PLCs are designed to operate under industrial environments (PLCs have to operate under wide range of temperature conditions, humidity and other environmental conditions). They are least affected by the electrical noise and are inherent to electrical noise
  • Programming in PLCs is through Relay Ladder Logic or other easily learned language. PLCs comes with program language built in its memory. PLCs do not contain input and output devices such as keyboards, mouse, monitor, CD drives and other hard disks. It is in simple a self contained box with communication ports and set of terminals for input and output devices.
  • Unlike computers which performs numerous tasks simultaneously, PLCs execute a single program in an orderly and sequential manner from first instruction to the last instruction
  • PLCs have been designed for installation and maintenance by plant electricians. Programming in PLCs is simple (Relay Ladder Programming), it does not include any advanced code. Troubleshooting is simpler and many PLCs are designed to include fault details and written fault details on display screen.

8. PLCs Advantages or Benefits

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. Easy to Troubleshoot:
    PLCs have inbuilt diagnostics and override functions that helps the user to easily trance the software and hardware errors.

9. what is Programmable Logic Controllers (PLCs)
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, Hence 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 reliablity, simplicity, low cost, easier programming and many other functions

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