Top PLC Interview Questions (With Sample Answers and Tips)

By Indeed Editorial Team

Published 27 July 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

A programmable logic controller (PLC) is a rugged electronic device that helps automate services across several industries. If you enjoy robotics, automation and technology, you might be interested in a career as a PLC technician or automation engineer. Reviewing popular interview questions and preparing answers can help you update your knowledge and gain confidence to attend an upcoming PLC interview. In this article, we review some of the popular PLC interview questions, discuss sample answers and share tips you can use as a guide while preparing for the interview.

Common PLC Interview Questions And Sample Answers

Here are some of the expected PLC interview questions along with example answers:

1. Describe the primary functions of each PLC component.

Employers expect PLC engineers and technicians to have a solid understanding of the various components of the device so that they can use it properly in the field. The interviewer might ask this question to test your basic knowledge of PLCs before moving on to the more complex questions. You can frame your answer in two parts. Start by listing the major components of a PLC and then explain each component's purpose.

Example: "A PLC has four major components: processor module, programming device, input-output (I/O) module and power supply module. The processor is the brain of the PLC and contains a microprocessor for implementing various control and logic functions. The purpose of the I/O module is to receive the input signals from external field devices and send the appropriate output signal to specific field devices. Engineers use the programming device to load the desired program into the processor. Finally, the power supply module supplies power to field devices."

Related: What Is A PLC Programmer? (With Skills And Qualifications)

2. Can you explain the advantages of PLCs over conventional relay-based control systems?

The interviewer might ask this question to determine if you have in-depth knowledge about relays and PLCs. PLCs are steadily replacing mechanical relays across several industrial applications due to their various benefits. Understanding the fundamental differences between these components and their way of working can help you elaborate on the distinct advantages of a PLC.

Example: "PLC systems offer increased reliability, accuracy and flexibility compared to a mechanical relay. They are easy to install as they require less wiring and are also easier to maintain than a regular relay. PLCs offer more advanced functions like data gathering, supervisory control and monitoring. Another huge advantage of a PLC over a relay is the reduced cost. As PLCs are less expensive, they help in reducing the system's overall lifecycle cost."

3. What is a remote terminal unit (RTU) and why is it used?

The remote terminal unit is a specific advanced feature in the field of automation. If the interviewer asks this question, there are chances that the RTU is critical to the role you are applying for and you might have to work with it on the job. It is a good idea to revise specific concepts mentioned in the job description so that you can make your answers relevant to the role.

Example: "The primary purpose of an RTU is to monitor and control field devices remotely. It sends the collected information to plant control systems by transmitting telemetry data. Since the functions of PLCs and RTUs overlap, there is often confusion on which device to use for a specific project. PLCs can be used for local area control while RTUs are best-suited for systems that require wide geographic telemetry."

Related: What Is Automation Engineering? Definition And Career Advice

4. Explain the differences between shared data blocks and instance data blocks?

PLC engineers often confuse the role and functionality of these two data blocks. The interviewer might ask this question to test your knowledge about these components and evaluate if you have a strong understanding of the working of PLC programming blocks without any confusion about their usage.

Example: "Shared and instance are the two types of data blocks in a PLC. The instance block is always assigned to the logical block and works similarly to a function block. It has data stored in a variable declaration table. The program does not map the shared data block to a logical block. It contains values required for the machine or plant. The program can call the shared data block directly from any execution point."

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5. Which PLC programming method do you prefer?

The interviewer might ask this question to check your experience and expertise in using different programming styles to implement PLC programs. Frame the answer by specifying your preferred programming method and its advantages. You can also demonstrate your willingness to learn by mentioning that you are enthusiastic to try other programming styles if required for the job.

Example: "In my previous role at a petrochemical plant, our team used ladder diagrams to program our PLC systems. I prefer ladder diagrams for PLC programming as it supports online editing and comments, making it easy for all team members to follow the schema. I am also eager to try other programming methods like sequential function charts, instruction lists and function block diagrams to improve my skills in PLC programming."

6. How do you determine the memory size for a PLC installation?

This question helps the interviewer know more about your decision-making skills. It helps them evaluate how you apply your knowledge to handle critical project decisions. When answering this type of question, you can describe your working style and how you make decisions on the job.

Example: "In my previous projects, I determined the PLC memory size by analysing five key factors such as the size of the control program, number of input and output points, data collection requirements, supervisory functions and scope for future expansion. I collected the values for these factors from the associated teams and then determined the PLC memory size that satisfied all requirements."

7. Consider a burglar alarm that activates if a window or motion sensor detects an unauthorised person. Can you identify the inputs and outputs for this system in PLC programming?

The interviewer might ask questions like these to evaluate your coding knowledge. You might also have a formal coding test as part of the interview to assess how well you understand the basics of PLC programming and how you apply it. Besides writing the code, you can also explain why you chose this solution.

Example: "Basically, there are two sensors in this system. The motion detector's output changes to true whenever it detects an unauthorised person. In the window sensor, the default output is always true. When someone tries to break the window, the circuit breaks, the current stops flowing and the output's status turns false. The list of inputs for this system are the master switch, motion sensor, window sensor, deactivate switch and button to stop the alarm and the outputs are the master coil, alarm and alarm coil."

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8. How do you test and commission a PLC system?

This type of question evaluates your practical knowledge. It helps the interviewer get a clear idea of your working style and previous experience handling similar systems. Try to make your answers as specific as possible and give detailed instructions on the steps you take to solve a particular challenge.

Example: "I begin by checking if all cable connections between the PLC and plant are secure and meet industry specifications and local requirements. Next, I check the incoming power supply to ensure it matches the PLC's voltage requirements. I follow it by checking the working of all protective devices and the emergency stop button. Next, I verify if all the input and output devices connect to the appropriate input and output points. Finally, I load and test the software application."

Related: 40 Manual Testing Interview Questions (With Example Answers)

9. Can you explain in simple terms the working of the timer function in a PLC?

When the interviewer asks you to explain something in 'simple terms' or 'layperson terms' during the technical interview, frame your answer so that even a non-technical person can easily understand it. Try to explain the timer's operation in simple language avoiding complicated technical terms.

Example: "When we use the timer, we instruct the PLC to wait for a specific time before doing its job. The timer function calculates the time using the internal CPU clock. There are two basic types of timer functions like the on-delay timer and off-delay timer."

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Tips To Prepare For A PLC Technical Interview

Here are a few tips that you can use to prepare for an upcoming PLC interview:

  • Read the job description carefully. You can use it to identify if the position requires prior knowledge of specific concepts and topics and prepare for them accordingly.

  • Practise your answers with a friend. Reviewing and practising your answers with a colleague or friend helps to increase your confidence levels when interviewing.

  • Review PLC programming basics. Whether you use PLC regularly for work or if it has been several months, it can be helpful to review the basic concepts and programming constructs to update your knowledge.

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