Interview Questions

51 PLC interview questions

1. What is PLC?

Programmable logic controllers, also called programmable controllers or PLCs, are solid-state members of the computer family, using integrated circuits instead of electromechanical devices to implement control functions. They are capable of storing instructions, such as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial machines and processes.

PLCs can be thought of in simple terms as industrial computers with specially designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input/output connections to the real world).

2. What are the different modules in PLC?

  • CPU
  • Communication module
  • Digital input
  • Digital output
  • Analog input
  • Analog output
  • Pulse input

3. Explain PLC operation with controlling motor as example:

With reference to the picture:

PLCs consist of input or point modules, a central processing unit (CPU), and output or point modules. An input accepts a variety of digital or analog signals from different field devices (sensors) and converts them to a logic signal which the CPU can use. The Processor makes decisions and executes control instructions in memory based on the instructions for the program.

Output modules transform CPU control instructions into a digital or analog signal which can be used to control different field devices (actuators). Use a programming tool to enter the directions you want. These instructions specify what a given input will be achieved by the PLC. These instructions specify what a given input will be achieved by the PLC.

4. What are the different programming languages used in PLC?

There are five types of programming languages that can be used to program PLC, which include:

  1. Ladder Diagram (LD) programming language
  2. Instruction List (IL) / Statement List (SL) programming language
  3. Sequential Function Chart (SFC) / Grafcet programming language
  4. Function Block Diagram (FBD) programming language
  5. High-level programming languages (high-level), for example Visual Basic

Ladder diagrams are easy to understand because they use a graphical approach, namely using the electromagnetic-mechanical relay component symbols (coil and contact), function blocks, such as timers, counters, triggers, conditionals, and function blocks that are self-defined by the programmer.

5. Why we use PLC over relay logic?

The implementation of a system using electromechanical standard and timing relays would have made this control panel a maze of large bundles of wires and interconnections. And it is better to use PLC if an application needs requirements listed below:

  • Flexibility in control logic changes
  • Need for high reliability
  • Space requirements important
  • Data collection requirement
  • Frequent control logic change
  • Need for rapid modification
  • Need for future growth

Where system requirements call for flexibility or future growth, a programmable controller delivers returns that outweigh any initial cost advantage of a relay control. Even in a case where no flexibility or future expansion is required, the troubleshooting and maintenance aids provided by a PLC can greatly benefit a large system.

6. What is the difference between PLC and your Personal computer?

The architecture of a PLC’s CPU is essentially the same as that of a general-purpose computer; however, they are differentiated by some important characteristics. Firstly, unlike machines, PLCs are explicitly designed to survive the harsh industrial conditions

A well-designed PLC can be mounted in an environment with significant amounts of electrical noise, electromagnetic interference, mechanical vibration and humidity-free of condensation

7. What are the types of PLCs available in market?

  1. Micro PLCs: Micro PLCs are used in applications controlling up to 32 input and output devices, 20 or less I/O being the norm
  2. Small PLCs: Small PLCs controls 32 to 128 I/O
  3. Medium PLCs: 64 to 1024 I/O
  4. Large PLCs: 512 to 4096 I/O
  5. Very large PLCs: 2048 to 8192 I/O

8. Advantages of PLCs:

  • Highly reliable solid-state components
  • Small size, minimal space required
  • Service without disturbing wiring
  • Eliminate long wire/conduit runs
  • Software control relays
  • Can take output in report form.
Microprocessor-based system:
  • Higher level of performance
  • Higher quality products
  • Multifunctional capability
  • Communication capability
Programmable memory:
  • Simplifies changes
  • Flexible control
Modular architecture:
  • Installation flexibility
  • Easily installed
  • Reduces hardware cost
  • Expandability

9. How to program PLCs ?

Each PLC fabricator has its own PLC programming tools. Siemens, for example, uses Simantic S7 Manager, Allen Bradley uses RS Logix and Modicon uses programming software for PLC pro. The language used for programming is Ladder Logic (LD), Statement List (STL), Functional Block Diagram (FBD), Sequential Function Chart (SFC), Instruction List (IL), etc.

10. What is PLC scan?

PLC Scan The PLC program is executed as part of a repeat process called a scan. A PLC scan ends when the CPU reads input status. Next, you execute the application programme. The CPU then performs internal communication and diagnostic functions. Eventually, the performance status is changed by the CPU. This process repeats in run mode as long as the CPU.

The time required to complete a scan depends on the size of the program, the number of I/Os, and the amount of communication required.

11. Some important ladder relay instruction:

13. What is the purpose of a Latch coil?

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

14. What are sinking and sourcing?

A DC input module can interface with field devices in both sinking and sourcing operations, a capability that AC/DC input modules do not have.

If a device provides current when it is ON, it is said to be sourcing current. Conversely, if a device receives current when it is ON, it is said to be sinking current.

15. What are timers and counters?

They activate or deactivate a device after a time interval has expired or a count has reached a preset value. Commonly, timer and counter instructions are considered internal outputs. 

Timer and counter instructions, like relay-type instructions, are essential to the set of instructions for ladder diagrams.

16. What is ON delay/OFF delay timer?

An ON-delay energize timer (TON) output instruction either provides time-delayed action or measures with the duration for which some event occurs. Once the rung has continuity, the timer begins counting time-based intervals (ticks) and counts down until the accumulated time equals the preset time.

An OFF-delay energize timer (TOF) output instruction provides time-delayed action. If the control line rung does not have continuity, the timer begins counting time-based intervals until the accumulated time value equals the programmed preset value

17. What is Retentive ON-delay timer?

If the accumulated value of the timer needs to be maintained even if logic stability or device power is lost, a retentive ON-delay timer (RTO) output instruction is used. If any rung path has continuity of logic, the timer begins to count time-based intervals until the cumulative time equals the preset value.

18. What is Master Control Relay (MCR)?

A master control relay (MCR) output instruction activates or deactivates the execution of a group or zone of ladder rungs. An MCR rung is used in conjunction with an END rung (discussed later) to fence a group of rung.

When the condition of the MCR rung is TRUE it triggers the referenced output, allowing all rung outputs within the zone to be managed by their respective conditions of input rung. When the MCR output is switched off, all non-retentive (non-catched) outputs within the region are de-energised.

19. List some data manipulation instructions commonly used in ladder logic:

20. What is difference between LAN & Industrial network?

Local Area Networks: A LAN is used to link computers and their peripherals within the same building or site. For most LANs, the maximum distance in the network between two nodes is at least one mile and the transmission speed is from 1 to 20 megabaud.

Industrial Network:

A special type of LAN, the industrial network, is one which meets the following criteria:

  • Capable of supporting real‐time control
  • High data integrity (error detection)
  • High noise immunity
  • High reliability in harsh environments
  • Suitable for large installations

21. Point out features of Ladder logic program:

  • Power flows from left to right.
  • Output on the right side cannot be connected directly with the left side.
  • Contact cannot be placed on the right of the output.
  • Each rung contains one output at least.
  • Each output can be used only once in the program.
  • A particular input a/o output can appear in more than one rung of a ladder.
  • The inputs a/o outputs are all identified by their addresses, the notation used depending on the PLC manufacturer.

22. What are the factors you consider while selecting your PLC microprocessor?

  • I/O points: Each PLC processor will only be capable of working with a limited number of each type of I/O modules.
  • Memory size and Performance: Considering data or program storage and Scan time depends on the processor.
  • Speed: The required operating speed for all the I/O must be determined, with a PLC selected to match. This requires the estimation of the program size and the proportion of slow instructions
  • When a communications facility is required we need to determine whether the built‐in port is adequate for the application, or whether a separate module will be required.
  • It is essential to ensure that the selected PLC can handle the operations necessary for any particular application

23. How you check a PLC system?

  • Check that all cable connections between the PLC and the plant are complete, stable, and to the specification necessary to meet local standards.
  • Checking that all the incoming power supply matches the voltage setting for which the PLC is set.
  • Checking that all protective devices are set to their appropriate trip settings.
  • Checking that emergency stop button work.
  • Checking that all input/output devices are connected to the correct input/output points and providing the correct signals.
  • Loading and testing the software.

24. What are the major Industrial control systems?

  • PLC
  • DCS
  • Embedded control

25. Basic logic gates:

AND gate: Series operation: the output is high only when both inputs are high

OR gate: Parallel operation: the output is high when any one of the inputs is high or both are high.

NOT gate: Inverted operation: the output is high when input is low and vice versa

26. What is interlocking in ladder logic? Draw a sample:

Interlocking is a kind of application where two devices exist, and they are not required to operate simultaneously. If one of them is ON then the other should be OFF and vice versa. This control system is called interlocking.

Interlocking of two devices

27. The number of ladder logic virtual relays and input and output instructions is limited only by _____________

Memory size

28. What is the RS LINX software used for?

In general, RS Linx software is a communicator between programming devices and the controllers themselves. It acts as a link between the two and therefore passing commands is extremely useful.

This can not only configure contact drivers but also display the drivers that have already been configured and the active nodes.

RS Linx software is renowned for carrying out activities like installing, uploading, firmware upgrading, going online, sending messages and more.

29. What is an HMI?

HMI is a Human Machine Interface designed for industrial use. HMI systems can be called a “window” of a process. This window can be on a monitor or on special tools such as operator panels. An HMI system has connected an industry’s hardware and software components so that the HMI can exchange user-system information.

30. How Do Fixed PLCs differ from modular PLCs?

Fixed PLCs are designed mostly to perform basic functions that are to be completed in the industry. These are compact and consist of a CPU and a power supply.

On the other hand, modular PLCs have various independent systems and parts, and they have multiple I / O devices, and they can easily be replaced.

31. Who are the main PLC manufacturers?

  • Allen-Bradley
  • ABB
  • Mitsubishi PLC
  • Honeywell
  • Hitachi
  • Motorola
  • Schneider Automation

32. What is RS 232?

RS‐232 is an asynchronous communications method. Serial communication is a way of communicating with your outside world through various types of equipment. It is called serial because the bits of data are distributed serially over a single line. A special chip called UART (Universal Asynchronous Receiver Transmitter) manages the serial ports.

33. What are the types of RS‐232 devices:

  • DTE – Data Terminal Equipment and a common example is a computer.
  • DCE – Data Communications Equipment and a common example is a modem

34. What are the Standard MODBUS Networks Employ?

ASCII Mode: Each character byte in a message is sent as 2 ASCII characters. This mode allows time interval of up to a second between characters during transmission without generating errors.

RTU Mode: There are two 4-bit hexadecimal characters in each 8-bit message byte, and the message is transmitted in a continuous stream. The greater effective density of character increases the throughput at the same baud rate over ASCII mode.

35. How to read a simple ladder logic?

  • Power flows from left to right.
  • Output on the right side cannot be connected directly with the left side.
  • Contact cannot be placed on the right of the output.
  • Each rung contains one output at least.
  • Each output can be used only once in the program.
  • A particular input a/o output can appear in more than one rung of a ladder.
  • The inputs a/o outputs are all identified by their addresses, the notation used depending on the PLC manufacturer

36. What is redundancy ?

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

37. What is master/slave configuration?

It is usually a single master device (a PLC) that talks with multiple slave devices (sensors). The master devices poll the slaves when they have the token. Slave devices only answer when asked a question. They are passive and the master can be said to be active. The slave devices just collect data and pass it to the master device when asked to do so.

38. What is CRC?

A cyclic redundancy test (CRC) is a widely used error detection function in digital networks and storage devices to detect unintentional modifications to raw data. Blocks of data accessing these structures are connected with a quick search value based on the remainder of a polynomial information section.

39. What is a Hybrid system?

Hybrid system is the combination of using two or more control system. Like the combination of discrete functions, which PLCs handled so simply and economically, with the sophisticated analog continuous control capabilities of the DCSs.

40. How you define Functional Block programming?

Data flow is the prime principle behind an FBD. In these types of programs the values pass through feature blocks from the inputs to the outputs.

A FBD program is designed using function blocks that are bound together to describe the exchange of data. The connecting lines will have a type of data that must be compatible on both ends.

41. What do you know about Instruction list?

  • Instruction List (IL) is a textual language of low level which has a similar structure to a basic machine assembler.
  • The IEC has developed IL by reviewing the many low level languages offered by PLC manufacturers.
  • The IEC has built IL by evaluating the many low level languages provided by PLC manufacturers.

42. What are the various communication interface supported by AB?

DH+, DH485, ETH, Modbus, Device Net and Control net

43. What are the types of Serial communications?

  • RS 232
  • RS-485
  • SPI (Serial Peripheral Interface)
  • I2C (Inter-Integrated Circuit)
  • USB (Universal serial bus)
  • CAN (Control Area Network)

44. Describe about Structured Text programming

  • ST is a high-level language, similar to PASCAL or BASIC.
  • ST is a separate language which was developed specifically for industrial control applications
  • Fairly straight forward and easy to learn and to use.
  • Compact formulation of a complex programming task.
  • Powerful constructs for controlling the instruction flow.
  • Useful for complex mathematic solutions.
  • Simple solutions for difficult problems in FBD, LD or IL.

45. What happens to current flow in In a current-sinking DC input module?

The current flows out of the input field device.

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

Using a 4-20 ma signal means the PLC can easily detect a cable break. This is because it returns only 0 ma. The PLC loses its ability to detect any cable breakage by using a 0-20 ma signal.

47. What are the essential documents in PLC system?

  • System overview and complete description of control operation
  • Block diagram of the units in the system
  • Full list of each input and output, destination, and number
  • I / O module wiring diagram, address recognition for each I / O point, and rack position
  • Ladder diagram with rung description, number, and function.

48. What are the softwares used with Siemens?

  • For 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

49. What are the 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

50. How are communication protocols distinguished?

Communication protocols are distinguished with the number nodes supported, the speed of communication and total network.

51. Explain the Time Stamp available in S7-200 PLC?

This warning message shows that the project time marks do not match the task time marks in the PLC. This warning message shows that the project time marks do not match the task time marks in the PLC. However, the programs may be functionally identical and still have different timestamps.


Instrumentation Engineer

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