PLC Interview Questions and Answers

PLC Interview Questions and Answers : Today we are sharing an useful article on basic PLC Questions useful for your exam preparation.

PLC Interview Questions and Answers

Basic Programmable Logic Controllers MCQ

The PLC was invented by.

a) Bill Gates

b) Dick Morley

c) Bill Landis

d) Tod Cunningham

The first company to build PLCs was .

a) General Motors

b) Allen Bradley

c) Square D

d) Modicon

Classify the following as automatic control, manual control, discrete control or continuous control.

a) A sensor is used to turn on and off the lights of a room.

b) A temperature sensor is used to adjust the room temperature and maintain it at a set point.

c) A user starts the bottle filling machine, places the bottle on the conveyor belt to fill it with the required liquid.

d)  The luggage system at the airport.

e)  A factory that makes red pens and has no human employees.

Answer :

The part that monitors the inputs and makes decisions in a PLC is the CPU.

a. True

b. False

c. None of the above

One of the following is an input device

a. Motor

b. Light

c. Valve

d. Sensor

Which one of the following is not a PLC manufacturer

a. Siemens

b. Mitsubishi

c. Microsoft

d. ABB

Solenoids, lamps, motors are connected to:

a. Analog output

b. Digital output

c. Analog input

d. Digital input

In a PLC “I” is used for output and “Q” is used for input

a. True

b. False

c. None of the above

PLC stands for programmable logo controller

a. True

b. False

c. None of the above

To increase the number of inputs and outputs of the PLC, one can use expansion modules.

a. True

b. False

c. None of the above

An example of discrete (digital) control is:

a. Varying the volume of a music system

b. Turning a lamp ON or OFF

c. Varying the brightness of a lamp

d. Controlling the speed of a fan

A solenoid is an example of an output device.

a. True

b. False

c. None of the above

Which of the following statements is not correct?

a) The PLC rung output [-( )-] is a discrete output instruction or bit in memory.

b) Each rung of the ladder logic represents a logical statement executed in software – inputs on the right and outputs on the left.

c) Input and output instructions in ladder logic do not directly represent the switches and actuators.

d) PLC input instructions are logical symbols associated with voltage at the input module terminals.

Which of the following statements is correct?

a) Ladder logic is a PLC graphical programming technique introduced in the last 10 years.

b) A ladder logic program is hard to analyze because it is totally different when compared with the equivalent relay logic solution.

c) The number of ladder logic virtual relays and input and output instructions is limited only by memory size.

d) The number of contacts for a mechanical relay is limited to number of coils on the relay.

Which of the following statements is NOT correct?

a) The status of each input can be checked from one location and outputs can be forced on and off.

b) All symbols in the RLL represent actual components and contacts present in the control system.

c) PLCs are not as reliable as electromechanical relays in RLL.

d) Input (-| |-) and output (- ( ) -) instruction symbols in the ladder logic represent only data values stored in PLC memory.

Which of the following statements is NOT correct?

a) If a problem in a PLC module occurs, the module can be changed in a matter of minutes without any changes in wiring.

b) Outputs can be paralleled on the same rung.

c) The physical wires between the input and output field devices and the PLC input and output modules are the only signal wires required in the PLC system.

d) The cost and size of PLCs have increased significantly in the last 10 years.

Which of the following statements about a single pole double throw relay is NOT true?

a) It is called an SPDT type of relay.

b) It has one common contact.

c) It has two positions (NC and NO).

d) It has a center off position.

Which of the following statements about a single pole double throw relay is true?

a) Insulators are used in the armature to isolate the electrical switching contacts from the rest of the relay components.

b) The NC contact and the pole are in contact when the relay is off.

c) It has just one coil.

d) All of the above.

Which of the following statements about RLL is NOT true?

a) NO contact symbol has two parallel lines to indicate an open contact.

b) RLL stands for Relay Ladder Logic.

c) NC contact symbol has the same two parallel lines with a line across them to indicate closed contacts.

d) The right power rail is positive or the high side of the source, and the left power rail is the power return or ground.

The _____ is moved toward the relay electromagnet when the relay is on.

a) Armature

b) Coil

c) NO contact

d) NC contact

When a relay is NOT energized:

a) There is an electrical path through the NO contacts

b) There is an electrical path through the NC contacts

c) Neither the NO or the NC contacts have an electrical path

d) Both the NO and the NC contacts have an electrical path

Which of the following RLL applications is not normally performed in early automation systems?

a) On/off control of field devices

b) Logical control of discrete devices

c) On/off control of motor starters

d) Proportional control of field devices

Current flows into the _____

a) Input terminal of a sinking DC input module

b) Input terminal of a sinking output field device

c) Output terminal of a sinking input field device

d) All of the above

In a current sinking DC input module _____

a) The current flows out of the input field device

b) Requires that a AC sources be used with mechanical switches

c) The current flows out of the input module

d) Currents can flow in either direction at the input module

AC output field devices can interface to _____

a) AC output modules

b) Relay output modules

c) Both a and b

d) Neither a or b

What one item in the output module circuit above should be changed to make it correct.

a) The battery polarity

b) Output module should be sourcing

c) Field device should be sinking

d) Current flow direction

What one item in the input module circuit above should be changed to make it correct.

a) The battery polarity

b) Input module should be sinking

c) Field device should be sinking

d) Current flow direction

When _____ contacts are actuated, they disrupt the power supply through them.

a. normally open type

b. normally closed type

c. both a. and b.

d. none of the above

The type of memory which is fast and temporarily stores the data which are immediately required for use is called as______.

a. HDD

b. ROM

c. RAM

d. SSD

How is the speed of operation of conventional relay system as compared to digital controllers?

a. very slow

b. very fast

c. same

d. almost similar

The capability of convention relay systems for complex operations is ____ that of the PLCs .

a. poor than

b. excellent than

c. as good as

d. unpredictable as

How is the noise immunity of PLCs to electrical noises as compared to that of conventional relay controllers?

a. poor

b. excellent

c. as good as noise immunity of conventional relay controllers

d. unpredictable

_____ of PLCs can be done in very little time.

a. Programming

b. Installation

c. Commissioning

d. All of the above

PLC can be _____ in plant to change the sequence of operation.

a. only programmed

b. only reprogrammed

c. programmed and reprogrammed

d. able to give a set point

The PLC is used in _______.

a. machine tools

b. automated assembly equipment

c. moulding and extrusion machines

d. all of the above

Which of the following can be the output of PLC?

1. Relay coils

2. Solenoids

3. Indicators

4. Motors

5. Lamps

6. Alarms

Select correct option

a. Only (1), (2), (3) and (4)

b. Only (3), (4), (5) and (6)

c. Only (1), (2), (3) and (5)

d. All the (1), (2), (3), (4), (5), and (6)

Which of the following cannot be an input that is given to the PLC?

a. Manual switches

b. Relays c. Sensors

d. None of the above

PLC Questions and Answers

Explain what are different comonents in PLC?

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

Explain Advantages of PLC 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. etc…

Explain 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.

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

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.

Explain 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.

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

PLC’s 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.

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.

User 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.

What is Progrmmable 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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