100+ PLC interview questions

1. What is PLC?

PLC is a combination of software and hardware that stands for Programmable Logic Controllers. It is an industrial computer used to regulate the outputs by monitoring inputs and processing the device based on its program.

2. What are the types of PLCs?

The types of PLCs are

• Modular

• Compact

3. What are the programming languages used in PLC?

The languages used in PLC are

• Function Block Diagram (FBD)

• Instruction List (IL)

• Ladder Diagram (LD)

• Sequential Function Chart (SFC)

• Structured Text (ST)

4. What are the hardware components of PLC?

Hardware components of PLCs are

• Power Supply

• Input Interface

• Output Interface

• Programming device

• Central Processing Unit (CPU)

5. What are the criteria to select a PLC?

• Number of inputs & outputs

• Memory

• Scan Time

• Communications

• Software

6. What are the major industrial control systems?



• Embedded control


  1. What is the FAT procedure for the PLC panels?

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8. What is redundancy in PLC?

The ability of a system to switch to secondary/standby system from primary equipment automatically without interfering with the current activity of a control system. Redundancy is referred to as a standby module. if a failure occurs when the primary module is working, the standby system takes the control of control system.

9. What is warm standby in PLC?

The primary processor is running the program and controlling the output states in warm standby mode. When the primary processor fails, the standby processor takes over and starts running the program.

10. What is hot standby in PLC?

Both processors run continuously in a hot standby configuration, using program scans integrated into the fiber optic link. If one processor fails and the output status remains unchanged, another processor immediately assumes control. For most applications, a hot standby configuration is recommended.

11. Difference between fixed and modular PLC

Fixed PLCs are primarily intended to perform basic functions that must be completed in the workplace. They are small and contain a power supply as well as a CPU. Modular PLCs have multiple independent systems, as well as components. It also has numerous I/O systems and it can also be repaired easily.

12. State the advantages of PLC over relay?

The advantages of PLCs are

• Smaller in size

• Easy to expand.

• Easy to program

• High level of dependability.

13. What is the timer?

The timer is the most essential PLC instruction. The function of the timer is to operate or control devices for a particular time period. We can set the timer based on our requirements.

14. What are the different types of PLC timers?

There are three different types of timers.

• On Delay Timer

• Off Delay Timer

• Retentive On Timer (RTO)

15. What is ON-Delay Timer?

After receiving a power-on signal, a timer’s output will change state after a predetermined delay. When a start input signal is received, the timer will start and the output of the timer will get turned on after a predetermined time period.

16. What is an OFF-Delay Timer?

The OFF delay means that it will change state at a predetermined time after the timer has received the shutdown signal. (Means 1 to 0) When a shutdown input signal is received, the timer will start and the output of the timer will get turned off after a predetermined time period.

17. What is Retentive On Timer?

The main purpose of the RTO is to keep or store the (accumulated) set time. It’s used when the rung state changes, the power goes out, or any other device malfunctions. The Accumulate value of the retentive timer counts from where it stopped the last time when its rung changes from false to true.

18. What is SCAN in PLC?

SCAN is the procedure that any sequential program controller operation must go through. The ladder diagram is the process of moving from the top to the bottom of the ladder. The process produces all updated outputs that match the given inputs. From the left to the right of each rung in the ladder diagram, the SCAN process takes place. The total time required for reading the input, processing the program’s logic, and updating the corresponding output in a single cycle is known as scan time. SCAN typically takes milliseconds to complete and is a continuous process.

19. What is an HMI in PLC?

The Human Machine Interface, or HMI, is the interface between humans and machines. It enables the operators to interact with or communicate with the system they are in charge of. The HMI displays a pictorial representation of the mechanical system’s status as well as direct control over its operation. The HMI’s graphical screens can be programmed to allow the operator to see all of the important statuses as well as control data. In the HMI, the various operating conditions are represented in pictures, icons, sound, and in various colors. Many HMIs use advanced touch screen technology to enhance the user’s interaction with the elements on the screen.

20. What is Ladder Logic?

Ladder Diagram is the graphical programming language Ladder Diagram is used to create software for programmable logic controllers (PLCs). This is a programming language that simulates the operation of electromechanical relays by expressing a program as a series of “coils” and “contacts.” The resulting program is the equivalent of an equation that is executed in a combinatorial manner indefinitely. image

21. What is Rung? The rungs are horizontal lines that connect the rails and logical expressions. They would reflect the wires connecting the power supply to the switching and relay elements if they were in a relay logic circuit.

22. What is Rail?

Two rails are drawn in vertical lines at the top of the page in a ladder sketch. They will represent the active and zero voltage connections of the power supply in a relay logic circuit, where the current flows from left to right.

23. What is the counter?

The counter is a part of a mathematical function and it is an instruction that is useful for sequential counting as digital numbers. It’s a component of the mathematical function.

24. What is Up-counter?

Up counters count upwards or incrementally.

25. What is Down-counter?

Down counters count downwards or in a decremental manner.

26. What is UP-Down Counter?

A combination of an up-counter and a down-counter is an up-down counter. It has the ability to count in both directions, increasing and decreasing.

27. Name some PLC manufacturers

List of PLC Manufacturers


• Siemens

• Rockwell Automation / Allen Bradley

• Yokogawa Electric

• Honeywell

• Emerson (General Electric)

• Schneider Electric

• Omron

• Mitsubishi Electric

• Bosch Rexroth

• Delta Electronic

• Eaton

• Fanuc

• Hitachi


• Keyence

• LS Electric

• Panasonic

• Phoenix Contact

• Toshiba

28. What is Communication Protocol?

The system interacts with the two or more communication devices for receiving and transmitting various digital data or information. This communication is managed by a set of rules called ‘Communication Protocol’.

29. What are the various types of communication protocols?

Here are some standard PLC communication protocols.

• EtherNet/IP

• Profibus

• Modbus

• Interbus

• ProfiNet

• ControlNet

• DeviceNet

• DirectNet

• CompoNet


30. What are the advantages of PLC?

• It is less expensive

• It is a more dependable solution.

• The PLC is simple to connect to and communicate with the rest of the system.

• They are of a smaller size.

• It is extremely quick and precise.

• It is possible and very simple to make changes.

• Less maintenance

• Can perform complex tasks

31. What are the major disadvantages of PLC?

• Only one program can be run at a time.

• Performance is based upon climatic changes.

32. Explain the term downloading and uploading in PLC.

Uploading and downloading are the two terms used to mention the data transfer between the PLC and the system.

Uploading is the term used to refer to the process of transferring data from a PLC to a computer.

Data transfer from a PC to a PLC is referred to as downloading.

33. What are the various applications of PLC?

• Driving Motors

• Control Valve Operation

• Air Compressor

• Waste-water treatment

• Closed-loop Operations

• Crane Operation

• Traffic Signals

• Oil-filtration Process

34. What function do Rack and Chassis play in PLC systems?

The hardware assembly of the PLC system, which houses all of the hardware such as the power supply, CPU, I/Os, Extension Modules, and so on, is known as the rack and chassis.

It performs the following tasks:

• Power is distributed evenly.

• Different I/O modules are housed here.

• Serve as a link between the CPU and the I/O modules.

35. What is a sensor?

The sensor is an electronic device that detects various physical parameters and sends them to the PLC controller as input.

36. What are the types of sensors?

• Inductive Proximity Sensors

• Capacitive proximity sensors

• Optical Sensors

• Ultrasonic Sensors

• RTD (Resistance Temperature Detectors) sensors

37. What does PLC CPU consist of?

The CPU of a PLC consist of the following components

• Power Supply

• Microprocessor

• Memory

38. What is LCU?

The local control unit (LCU) is a small group of hardware systems that can perform closed-loop control in a system. The process is directly connected to the local control unit.

39. What are the classifications of PLC?

Nano has 16 I/O pins

Micro has 32 I/O pins

Elevators, car washes, and mixing machines could all benefit from nano or micro PLC.

Small: 128 I/O pins, 2 KB of memory, capable of basic to advanced machine control

Medium: Process control applications require 2048 I/O pins and 32KB of memory.

Large: 8192 I/O pins. Large PLC is used in heavy industries such as paper and pulp, chemical, automotive, and power plants.

40. What is the difference between sinking and sourcing?

Sinking and sourcing are two important concepts to understand when connecting a PLC to the outside world. These two concepts only have an impact on DC modules.

The sinking circuit connects the load to the ground (-DC ). The sourcing circuit provides a +24 volt source to the load (+DC).

41. What are the functions of PLC?

PLC functions:

• Sequential computer control

• System and plant monitoring

• Position or motion control of the system

• System fault condition detection

42. What is a register in PLC?

In a PLC, the register is a memory location where data can be stored. Depending upon the register type this data can be either analog or digital.

43. State the difference between alarms and events?

Alarms are warnings of process conditions that could cause problems and require operator intervention.

Events: Events are standard system status messages that do not require a response from the operator. A detectable occurrence that may or may not is associated with an alarm is defined as an event.

44. What is Data Highway?

The technology used by Allen Bradley PLC to make communication to and from other PLCs on the network and other host computers in the same network.

45. What is Control Net?

On a single physical media link, a real-time, manage-layer network delivers high-velocity delivery of both time-critical IO facts and messaging statistics, such as upload/download of programming and configuration data and peer-to-peer messaging.

46. What is Device Net?

It is a low-value verbal link that connects various industrial devices to a network. It is based on the Controller Area Network(CAN)model.

47. What is Ethernet?

Ethernet is a wired technology used to make transmitting data in a WLAN (wired local area network) or WAN (wide area network). The communication between devices happens via a set of various rules called protocols. This network was co-developed by Digital Equipment Corp., Xerox, and Intel.

48. What is latency in communication?

The term “network latency,” also known as “lag,” refers to delays in communication over a network. The interval between the end of one verbal exchange and the start of another verbal exchange is called a latency interval. During this time, the communication will hang up and are unable to continue. Delays should be minimized.

49. Why is a 4-20mA analog signal preferable to a 0-20mA analog signal?

The following are the main reasons for using 4-20mA instead of 0-20mA:

• The value of 4mA is considered engineering zero. Due to this, 0.0mA is referred to as an invalid input. As a result, 0mA to the analog input indicates that the circuit is open and no current is flowing.

• Wire breaks are easy to find with 4-20mA.

50. What are the selection criteria to select a PLC?

• Task (system) requirement

• Cost of both hardware and software

• Reliability, stability, and flexibility

• Batch and interlock processing

• Speed of operation

• Memory size and speed

• Integration of high-level application

51. What exactly is DDE?

Microsoft’s Dynamic Data Exchange is a program that allows you to exchange data between different programs.

There are three important settings in DDE.

• Name of the application or server

• Name of the topic

• Name of the item

52. What are the functions of I/O modules?

Interfacing input and output devices in the machine or process to be controlled with electronic plug-in units.

The input module receives data from input devices (such as pushbuttons, switches, and transmitters) and sends it to the processor. Data from the processor is received by the output module, which then sends it to the output device (Relay, Valves).

Digital/Discrete:- Transmits and receives the on/off signal

Analog:- Transmits and receives signals with variable input or output.

53. What is the meaning of a universal analog input card?

Usually, in industry different cards are used depending upon the signals. But in the universal input card, we can configure different channels in a single card. We can configure RTD, Current, voltage input, and thermocouple by changing the program.

54. Write down some examples for input and output devices connected to PLC?

Digital Input: Pushbutton, Switches

Analog Input: Pressure, Temperature, Flow, Level Transmitters

Digital Output: Contactors, Solenoids

Analog Output: Speed, control valves

55. What is I/O forcing?

• Forcing I/O refers to setting the desired status of I/O in PLCs regardless of the status it receives from the field.

• We can force Inputs &Outputs in some cases when there is a problem receiving field inputs/output signals. As a result, the logic achieves the desired state.

56. What is a driver?

A driver is software that allows the external device to a computer using communication cards or com ports.

57. What are the different types of PLC programming devices?

Different types of PLC programming devices are listed.

  1. Programming with a hand-held device

  2. Terminal with a video monitor

  3. A computer with the necessary software

58. What does it mean to be constantly updated?

The CPU performs a scan of the input channels as they appear in the program instructions. Each input point is examined individually to determine its impact on the program. Before the program is instructed to execute the logic operation and output occurs, each input is scanned with a delay of 3 ms. This is called a continuous update.

59. State the purpose of Jump controls

By using jump instruction we can reduce the time taken for scan time by jumping over instruction that is not necessary for the operation of the machine. It can be accomplished by the missing of the intermediate program and being able to skip instruction while any fault occurs while production.

60. What are the different operating modes in PLC?



• Test mode

• Remote mode

61. What is program mode?

New programs are given to PLC by using program mode. We can edit and update the program that already existed in the PLC by using this mode. All the PLC outputs and ladder I/O will be in a halt position when the administrator entered this mode.

62. What is Run mode?

This mode is used to RUN a user program, so it monitors the input device and supplies power as needed. After every command is entered into the PLC program, the processor enters the RUN mode.

63. What is Test mode?

After done the programming mode, we have to check whether it is working fine or not before the program is used in real-life applications such as power plants, industries, etc. We can check the program without energizing any output in the system.

64. What is Remote mode?

we know that one of the main advantages of PLC is we can control it remotely. This mode is used to control PLC remotely with the help of a computer connecting to the PLC.

65. What is the role of EEPROM in PLC?

EEPROM is used to maintain a copy of the PLC program always. It prevents the loss of the program if any memory corruption or power loss occurs while running the plant.

66. What Is SIL?

The abbreviation SIL stands for Safety Integrity Level. A SIL is a metric that assesses the effectiveness of a safety system. It is divided into four levels. The lower the SIL level, the less likely the safety systems will fail. As the SIL level rises, so does the cost and complexity.

67. Different types of contacts

There are two types of contacts

• Normally Open (NO)

• Normally Close (NC)

68. What is NO contact?

In its normal state, a normally open contact does not flow current. The contact will be closed when energizing the NO contact and it will make the flow of current

69. What is NC contact?

A normally closed contact is one that flows current when it is in its normal state. The contact will be opened when energizing the NC contact and it will stop the flow of current.

70. What is Modbus?

It is Master-Slave based communication protocol used for receiving and sending information through serial lines between electronic devices that were developed by Modicon systems.

71. How to do commission a PLC?

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72. What is Relay Logic?

Relay logic is made up of relays that are wired together in a specific way to perform the desired switching operations.

73. What is Relay?

It’s a switch that’s controlled by electricity. The device has input terminals to control one or more control signals, in addition to a set of operational contact terminals. Any number of contacts in any contact form, including making contacts, breaking contacts, and combinations of the two, can be used in the switch.

Control panels, building automation, and manufacturing units all use this material.

74. What are the basic parts of Electrical Relay?

The Electrical relay’s basic parts and their functions are listed below

Frame: It supports the various parts of the relay.

Coil: Wire is wound around a metal core. The coil of the wire causes an electromagnetic field.

Armature: A relays moving part. The armature opens and closes the contacts. A spring attached to the armature returns it to its original position.

Contacts: The part of the switch that conducts electricity and either closes or opens a circuit

75. What is SCADA?

SCADA stands for Supervisory Control and Data Acquisition. It is a computer-based system for collecting information from various field devices such as sensors and analyzing those real-time data to monitor and control the equipment regarding the collected data.

76. Applications of SCADA

• Real-time monitoring and controlling process even from remotely • Can both analyze and calculate complex processes and maintain real-time data • Generation of process report • Historical Data Logging, Data Acquisition, Alarm generation, etc.

77. Name four editors available in the SCADA package?

• SCADA Animation Graphic Editor – Inks-cape SAGE

• Generic Logic


• UCanCode

78. Difference between SCADA and PLC?

PLC: PLC is a combination of software and hardware that stands for Programmable Logic Controllers. It is an industrial computer used to regulate the outputs by monitoring inputs and processing the device based on its program.

SCADA: SCADA stands for Supervisory Control and Data Acquisition. It is a computer-based system for collecting information from various field devices such as sensors and analyzing those real-time data to monitor and control the equipment regarding the collected data.

79. What is DCS?

DCS stands for Distributed Control System (DCS). It is regarded as the control system’s brain.

The DCS control system collects data from the field and decides how to use it. Field data can be saved for future use, used for simple process control, or combined with data from other parts of the plant to create advanced control plans.

80. What is a communication Module?

The communication module is used to transfer data between PLCs and other field devices digitally.

81. What is a loop diagram?

Instrument loop diagrams are also known as loop sheets or loop drawings. It is the most detailed type of control system diagram, containing all of the information that PFDs and P&IDs leave out.

82. What is P&ID?

The abbreviation P&ID refers to a piping and instrumentation diagram. In the process industry, it is a diagram that depicts the installed equipment. It’s similar to a system’s blueprint. It also has a logical numbering system.

83. What is the purpose of a loop diagram?

  1. Can easily understand the data of various instruments and how these data control the output devices.
  2. Whether the tag is an open-loop or a closed-loop, the complete wiring of the loop is shown for that tag.
  3. Shows all the loop accessories like some loops have power indicators and some instruments have extra displays to show various data.
  4. Can understand all the connections including point to point along with cable details and their terminal locations

84. What is a Marshalling cabinet?

The Marshalling cabinet connects the system cabinet and the field junction boxes. From the field junction boxes to the marshaling cabinet, the main cables are run. The marshaling cabinet also terminates the prefab cables from the system cabinet IO cards.

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85. What do you know about the list of instructions?

• The Instruction List (IL) is a low-level text language with the structure of a basic mechanical assembly.

• The IEC created the IL by studying a variety of low-level languages provided by PLC manufacturers.

86. What is meant by a cause and effect diagram?

A cause and effect diagram, also known as an Ishikawa or “fishbone” diagram, is a diagram that shows the relationship between two events. It is a graphical tool for investigating and displaying the potential causes of a problem.

87. What are the advantages of a cause and effect diagram?

• It helps teams understand that there are multiple causes that contribute to an effect.

• It graphically depicts the relationship between causes and effects.

• It aids in identifying areas for improvement.

88. Name some Serial communications types?

• RS 232

• RS-485

• SPI (Serial Peripheral Interface)

• I2C (Inter-Integrated Circuit)

• USB (Universal serial bus)

• CAN (Control Area Network)

89. What is a functional block diagram?

It is a graphical representation of executable code’s sequence along with control algorithms defined by the user.

90. Name some PLC programming software

• Connected components workbench

• Machine expert basic

• TwinCAT 3

• Productivity

• Do-more

• CoDeSys

• Open PLC

• Sysmac Studio

91. What is TCP/IP protocol

TCP/IP stands for Transmission Control Protocol/Internet Protocol for both the internet and intranet. This protocol helps to connect various network devices to the internet.

92. How is PLC connected to SCADA?

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93. What is Bit?

In the PLC, a bit is an address that can be Input, Output, or Internal coil. In RSLogix 500, there are several ways to display the address of a bit. However, the standard method is


O:0/5 indicates that it is a physical output.

O:0/5 indicates that it is using Slot 0.

O:0/5 denotes the fifth output of the PLC.

Please keep in mind that the capital “O” should not be confused with the number zero.

94. What is the purpose of an Asset Management System?

An Asset Management Solution is used to reduce the downtime of an Automation System.

95. What is a Latch coil?

A latch coil instruction causes the output to remain energized even if the status of the contacts that caused the output changes. If any rung path has logic continuity, then this instruction turns the output ON and keeps it ON even if logic continuity or device control is lost.

96. What is a master control relay?

To shut down a section of an electrical system, master control relays are used. MCR is used in ladder logic to turn on one section of a programming line. An MCR option should be properly opened and closed.

97. What is a Master/Slave configuration?

It is an asymmetric model of communication. In the master/slave model, one device or process, called master, controls one or more other devices, called slaves. The Master acts as a communication hub.

98. What is CRC?

CRC stands for cyclic redundancy check. In an error detection technique, CRC is a polynomial that is used to generate a series of two 8-bit block check characters that represent the entire block of data

99. Name some software used with Siemens?

• For S7 200 PLC programming: Microwin

• For S7 300 and 400 system: Simantic S7 manager

• Wincc is the SCADA software used by Siemens

• Earlier Siemens use to supply COROS LS/B

100. Name some software used with Allen Bradley?

• Pico soft for Pico PLC programming

• RS Logic 500 for Micrologix and SLC PLC programming

• RS Logic 5000 for control Logix

101. Who is considered the father of PLC?

Richard E. Morley

102. Name four criteria by which PLCs are categorized.

• Functionality

• Number of inputs and outputs

• Cost

• Physical size

103. What are the criteria for selecting a PLC?

• Number of I/O points used

• Size of the control program

• Data-collecting requirements

• Supervisory functions required

• Future expansion

104. What is the definition of test automation?

In test automation, the software is used to perform the testing, so there are no errors like there are in manual testing.

105. State some of the automation testing tools?

Rational robot, QA run, silk performer, Load runner

106. What is a derating factor?

This is a factor that reduces the nominal value of components, which makes them safe to use in critical applications as well as in extreme environmental conditions.