50+ PLC Interview Questions & Answers for Freshers & Experienced Programmers

PLC is a crucial aspect of many industrial and manufacturing settings. The worldwide PLC market is estimated to skyrocket by US $3289.73 million from 2022-2027, making it a great time to embark on this career path. This blog will take you through the critical knowledge you need when preparing PLC interview questions, whether you are just starting out or already have experience as a PLC programmer.

PLC Basic Interview Questions for Freshers

This is a list of our best PLC interview questions and answers for freshers. This section will cover the PLC programming basics and some more advanced concepts you may encounter during a job interview in this field. To help you better prepare for this interview, we suggest that you take this comprehensive PLC programming course. 

Q1. What is PLC?

PLCs, or Programmable Logic Controllers, are industrial computers that control automated devices and processes. These systems combine hardware and software components that allow them to monitor inputs and regulate outputs, such as turning them on or off according to their pre-programmed logic.

Q2. What are the types of PLC? 

When it comes to Programmable Logic Controllers (PLCs), there are two main types.

• Compact PLC: It is a controller whose initial configuration cannot be altered as all the components are integrated into the unit.  
• Modular PLC: It is a controller that allows multiple expansions of the PLC system through different modules.

Q3. Do you know about redundancy in PLCs?

It is a term used when a PLC stops working as it should due to malfunction or power loss. This causes system downtime, which results in decreased productivity and slower processes. 

Q4. What are the different parts of automation?

Automation is an advanced technical process that utilizes programmed commands and functions to control processes. Its components can be divided into different parts such as:

• Input/output modules, which receive user input.
• A power supply, where the PLC operates with a DC supply. 
• CPU (Central Processing Unit) or ‘the brain’ of a computer that stores all data and instructions.
• Co-processor modules containing sets of microcomputers to support the functioning of PLCs more effectively.
• Software programs for programming certain tasks by following specific requirements. 
• Peripheral devices connecting monitors linked through other units.

Q5. How can we implement PLC automation in different industries?

The major ways to implement PLCs in industries include:

• Traffic Signals
• Driving Motors
• Button Switches
• Photo Sensors
• Water Draining Pumps
• Light Lamps

 

Q6. What are the advantages of using PLCs for automation?

It is the main component responsible for monitoring and controlling activities within factories, production lines, or machines via its input/output modules and CPU. They process inputs into outputs based on programmed algorithms with incredibly rapid response times

Q7. What are the programmable languages used in PLC?

The following are the programmable languages used in PLC.

• Instruction List Programming
• Structured Text Programming
• Functional Block Programming
• Ladder Logic Programming 
• Sequential Functional Chart

Q8. What is the position of I/O modules in PLC?

I/O modules are an essential component of PLCs. These I/O modules provide digital input and output signals to create a connection between the PLC system hardware and its associated devices. Input modules can detect high, low, or intermediate-level electrical states coming from external switches and sensors, such as temperature senders, etc.

Q9. What is the role of the power supply in a PLC system?

The power supply in a PLC system is responsible for providing the processor, I/O, or input/output modules and communication components with their necessary energy requirements. Generally, it takes an AC voltage of anywhere between 120 and 230 volts (V) or 24 V DC at its input. The output it displaces back onto the plane amounts to around 5 V DC at 2-5 A 

Q10. What are Relays?

They act as a switch that lets electricity pass from one voltage level to another. If you need to control the power of a motor running on high voltage, such as 2,300V or 4,160V, a PLC and relay can be used. The relay energizes the starter as soon as your PLC is activated and switches correctly between different voltages.

Q11. What are Differential Relays?

Differential relays play an important role in detecting faults and protecting electrical systems. They work by comparing the current entering into a system with the current leaving it. Under normal circumstances, both currents should flow at equal levels without triggering anything from differential relays. When they detect any differences between them, they trigger appropriate action required for the protection of the entire setup.

Q12. Explain Solid State Relays.

Solid state relays are electrical switching devices that use a range of components, such as BJTs, Thyristors, IGBTs, MOSFETs, and TRIACS to control the flow of energy. These relays offer significantly higher levels of efficiency than their electromechanical predecessors. 

Q13. What are IDMT Relays?

IDMT relays are largely used in electrical distribution processes for safety purposes. They work by offering a definite-time contemporary feature at higher values of fault current and an inverse time-contemporary feature when the current falls to lower levels. This helps set accurate boundaries regarding both time settings and currents that must not be exceeded before electro-protection measures are triggered.

Q14. What are Overload Safety Relays?

Overload safety relays are critical components for keeping electrical systems running safely. Digital overload safety relays measure motor current as a way to predict the temperature of motor windings to protect them from overheating. They do this by shutting off any excessive power output before it can cause damage or failure. 

Q15. What are Reed Relays? 

Reed relays are electrical switches that use an electromagnet to control the opening and closing of two contacts. They contain a coil wound around a reed switch, with overlapping blades or ‘reeds’. When power is applied to the coil, it produces a magnetic field that draws these parts together, creating contact between them and allowing electricity to flow through the relay.

Q16. What are DC relays and AC Relays?

DC relays are made up of a coil wrapped around a ferromagnetic core with contacts that close when power is applied to the coil and flux builds in the core.

AC relays behave similarly to their DC counterparts but will pulse at whatever frequency the AC carries it through.

Q17. Explain polarized and non-polarized relays.

Polarized relays are a type of DC electromagnetic relay that uses the magnetic flux of a permanent magnet in their electromagnetic sections. It means their operating coil has polarity.

Non-polarized relays do not have a permanent magnet in their electromagnetic section. Therefore, the operating coil here does not have polarity.

Q18. What exactly are Latching Relays?

The latching relay keeps its state after being activated. Inside is an internal magnet holding contact positions even when power has been removed. This means the contact position will not change independently unless power is reapplied and changed accordingly. Therefore, once it’s ‘latched’, you can be sure that the state won’t switch again until told to do so from an external source.

Q19. What is a Rack or Chassis in a PLC system?

The hardware assembly of all the modules, such as power module, communication module, CPU, and input and output modules in modular PLC, is known as rack or chassis. It ensures even distribution of power supply to all the modules and acts as a communication link between the CPU and the various modules.

Q20. What are active backplane and passive backplane in PLC?

A PLC rack uses a backplane to provide power and interconnectivity between multiple connected modules. An active backplane has multiple slots and necessary circuitry to control module communication. 

The passive backplane has only bus connectors and little or no additional circuitry. The communication here is handled by connecting one or more expansion boards to the backplane.

PLC Interview Questions for Experienced Developers

If you are experienced with working knowledge of PLCs, then this collection of PLC interview questions is for you. 

Q21. Can you explain HMI in PLC?

Human Machine Interface, or HMI, is a way for operators to control and monitor the system they are responsible for. It provides them with an up-to-date visual overview so they know what’s going on at all times. It also allows direct manipulation of settings if something needs adjustment.

Q22. How do you test and commission a PLC system?

• Before you test and commission a PLC system, it is important to make sure that all cable connections with the PLC are secure. 
• Furthermore, confirm that they match any voltage setting specified for the PLC device and other local requirements. 
• Additionally, check if emergency stop buttons are working properly, both in terms of activation by hand or through computer programming. 
• Ensure that input/output devices connected to your machine give off signals compatible with what’s expected from them.
• Once those bases have been covered, progress on to loading software into the system.
• Then, run tests to go over its functionality against parameters already laid down during the configuration stages.

Q23. Explain Program mode, Run mode, and Test mode in PLCs.

Program Mode: It refers to a process used for new programs or updating existing ones. As soon as an administrator goes into program mode, all ladder I/O and other related outputs are put on hold so they don’t interfere with programming changes being made.

Run Mode: In Run mode, the running user program is monitored and executed as specified. The data that has been entered into memory locations can also be read or updated in this mode. Additionally, any changes made to the inputs will cause immediate action from the PLC system, according to its previously programmed logic flowchart.

Test Mode: Test mode is a programming feature that allows one to check the functionality and accuracy of an implemented program without energizing any devices connected to it. It minimizes risks associated with operating actual machinery or outputs.

Q24. How does a Latch coil work?

Latch coil will keep the output turned on even when changes are made to its input connection. In other words, the latch coil ensures that power stays continuously supplied so your instrument won’t be disrupted.

Q25. What are the advantages and disadvantages of modular PLC?

The advantages of modular PLC are:

• It is a viable solution for catering to the changing demand of processes because the modular type of PLC allows its capacity to be scaled. 
• It offers simpler troubleshooting that reduces system downtime.

The disadvantages of modular PLC are:

• It is costlier than compact PLC.
• The programming of this PLC is complex due to flexibility in its configuration.

Q26. What are the advantages and disadvantages of compact PLC?

The advantages of compact PLC are:

• Its programming is simpler because its configuration is fixed and no more internal variables can be added. 
• It is a more economical option in terms of certain application requirements of a machine.

The disadvantages of compact PLC are:

• It is restrictive for most of the application requirements as there is no possibility of expanding its capabilities.

Q27. Have you ever heard of Ladder Logic?

Ladder logic is a graphical programming language used to create software for programmable logic controllers (PLCs). The code is essentially an equation made up of “coils” and “contacts”, which operate similarly to electromechanical relays.

Q28. Can you tell me a bit more about Rungs?

A rung is essentially one line in an electrical ladder logic diagram, which looks like the steps on a set of stairs. This single line has two “legs”, where various logical elements are connected to it. These can include contacts from relays or switches as well as any additional control operations that need to be implemented with the circuit.

Q29. Have you ever heard of something called “Rail” when talking about Relay Logic Circuits? 

It looks like two vertical lines drawn on top of the page, almost like a ladder. They represent both an active and zero voltage connection for power supply: one side with higher-voltage current, while the other has low or no voltage as electricity flows from left to right through these types of circuits.

Q30. Explain the Instruction List and Structured Text. 

Instruction List is a low-level PLC programming language that contains a simple series of instructions where each instruction is made up of an operator. These operators are further made up of one or more operands. 

Structured Text is a high-level PLC programming language that uses statements to perform functions. It is a concise textual language quick to execute and read. It is used for manipulating data, barcode parsing, searching data, looping, etc.

Q31. Explain Functional Block Programming and Sequential Function Charts. 

Functional Block programming is a graphical PLC programming language that consists of various kinds of blocks with inputs and outputs. Devices connected to a PLC are linked with an input and variables are sent to output. The blocks are connected to each other, where the output of one block is used as input for another block. These define the function of a system.

Sequential Function Charts is a graphical PLC programming language that contains steps and transitions to control a program’s operational flow. Steps are actions to be performed and transitions are logical conditions necessary to be met before going ahead with the next step.

Q32. Why is 4-20 ma preferred over 0-20 ma?

A 4-20 ma signal is preferred over a 0-20 ma signal because it allows for easier detection of cable breaks by the PLC. This is because if there’s a break, the return value will be 0 ma, which differs from its input and signals an error to the PLC. 

On the other hand, when using a 0-20ma signal, any cable break would result in exactly the same output as input (0), making it difficult for the PLC to detect any issue with the cable without further analysis.

Q33. What is a Scan Cycle and what are its components?

The scan cycle refers to the systematic loop that a PLC runs through to get all of the necessary operations done effectively. It is made up of four distinct stages:

• Input scanning
• Program execution
• Output updating 
• Housekeeping tasks, such as communications or data transmission

. 

Q34. What does SIL stand for?

It stands for ‘Safety Integrity Level’ and essentially checks out safety system performance in four different levels. A higher level means fewer chances of failure but costs more due to the complexity involved in maintenance/setting up.

Q35. What is a timer instruction for PLC?

A timer instruction is a type of command on a PLC that allows you to have control over time. It uses the internal clock of your CPU to count fractions or even seconds. This preset value determines how long it will wait before acting. 

In PLC, timers may be on delay timer, off delay timer, retentive or accumulative timer, and pulse timer.

Q36. Can you tell me the difference between an ON-do away with Timer (TON) and OFF-do away with Timer (TOF)?

A TON starts counting once it has been turned on. It keeps ticking until its preset limit is reached. On the other side, TOF will start keeping track when there isn’t any input coming through. Once this accumulated amount reaches what was programmed into its settings beforehand, then the TOF will either turn off or switch to a different output based on how it is programmed. 

Q37. What are counters? Explain different types of counters.

Counters count up pulses received at their inputs until they reach a specific predefined value. The predetermined set number would signal the counter to turn its contacts on or off, as needed, for certain actions of operations/instructions to take place within a PLC system.

The different types of counters are as follows:

• Up Counter: An up counter is essentially like a digital timer or tally that counts upwards instead of down.
• Down Counter: Similar to an up counter, a down counter keeps track of numbers by counting downwards or in a decremental manner. It will typically start at the highest values and count downwards over time, either with each passing moment or after it receives a stimulus from its environment. 
• Up-Down Counter: An up-down counter is essentially a combination of both an up and down counter. It can count upwards incrementally as well as downwards decremental with each passing moment or stimulus received from its environment.

Q38. What is CRC?

A Cyclic Redundancy Check (CRC) is an error-detecting code that determines if a block of data has been corrupted. With this technique, a series of two 8-bit block check characters are generated to represent the entire block of data. These check characters are incorporated into the frame and checked at the receiving end to identify the error.

Q39. What is a Communication Protocol?

A communication protocol is a set of rules that manage how two or more devices communicate with each other, allowing them to both receive and transmit information. It effectively governs the exchange of data between different systems for it to be understood correctly by all parties involved.

Q40. Explain any three standard PLC communication protocols.

In a PLC communication network, the following standard protocols are used to communicate with multiple connected devices. 

• Control Net: Control Net is a type of real-time, managed layer network that delivers high-speed transmission of both time-critical input/output data and messaging information. It is an efficient way to send large amounts of data quickly without compromising its integrity.
• Device Net: A device net is an industrial network that uses a low-cost serial communication link to connect different devices on the same system. It is based on the Controller Area Network (CAN) model, which facilitates the high-speed exchange of data between multiple components.
• Ethernet: Ethernet is a wired technology that allows data to be transmitted in both Local Area Networks (LANs) and Wide Area Networks (WANs). It works by setting up a series of protocols that control how the information will move between two or more different devices. The original version is jointly developed by Digital Equipment Corporation, Xerox, and Intel.

Q41. What are the key differences between PLC and DCS?

Some of the key differences between Programmable Logic Controller (PLC) and Distributed Control System (DCS) are:

• I/O Handling: PLC handles discrete I/O signals, such as high/low, on/off, etc. DCS handles analog I/O signals, such as pressure, flow, temperature, etc.
• Programming: PLCs are programmed using graphical programming languages, such as ladder logic and function block diagrams. DCS is programmed using complex programming languages where logic is not easy to follow.
• Scalability: PLCs are scalable. They can be reconfigured to meet changing process requirements but cannot handle large-scale automation systems. DCS is highly scalable and can handle large-scale automation systems.
• Architecture: PLC architecture is more centralized and has a single processing unit to control all I/O modules. DCS has a distributed architecture that has multiple processing units connected to different field devices.

Frequently Asked PLC Programming Interview Questions

Here are some more PLC programming interview questions that will help you prepare for your interview.

Q42. What are Sinking and Sourcing?

In sinking circuits, the load is connected directly to the ground so current flows from positive (+DC ) towards negative (-DC). On the other hand, in sourcing circuits, power is provided by an external source of 24-volt direct current, which means that electricity will flow outwards to any loads/apparatus attached.

Q43. What is Uploading and Downloading in PLC?

Uploading and downloading are terms used when discussing the transfer of data between a Programmable Logic Controller (PLC) and another system. Uploading refers to sending/transferring data from the PLC into another device or computer while downloading is transferring information from the other device back into the PLC.

Q44. What is the purpose of a Sensor?

The purpose of a sensor is to detect any physical parameters and send them as input signals to the PLC controller. Sensors can be used for measuring things, such as pressure, temperature, and light, which help make further decisions or actions.

Q45. What is SCADA?

SCADA or Supervisory Control and Data Acquisition is a monitoring software used in the industries to control the hardware or devices in all remote locations. It collects field information, transfers it to the computer, and displays the information to the operator either textually or graphically.

Q46. What are some safety precautions to be taken care of while working with PLC?

Some safety precautions that one can take while working with PLC are:

• Disconnect power before making or breaking connections.
• Do not exceed PLC’s voltage and current ratings.
• Practice regular inspection and maintenance of PLC.
• Ensure that the PLC is properly grounded.

Q47. What is a Universal Analog Input Card?

A universal analog input card is a type of interface that allows for different signal types to be interpreted within one single device. It can contain multiple channels which can each be configured or programmed as needed to read various forms of data. This data includes resistance temperature detectors (RTD), currents, voltages, and thermocouples.

Q48. What are Jump Controls?

Jump controls are a special type of instruction that allows the machine to jump from one point in the program or operations sequence to another. This can be used for faster debugging and troubleshooting. It also helps with time optimization as skipped instructions reduce execution duration.

Q49. What is an RTOS? Why is it used in PLC? 

RTOS is a real-time operating system that processes data in real-time or as it is received, with consistency and predictability. It is used in PLC to meet the timing and reliability requirements of industrial processes.

Q50. What is the PLC trend chart?

A PLC trend chart is a graphical representation of data over a period of time. It is useful for monitoring variables and detecting anomalies or trends.

Q51. What are the disadvantages of PLC?

As compared to the advantages of PLC, a PLC has fewer limitations. These are:

• PLCs experience performance issues in some working environments, such as those with frequent vibrations and high temperatures. 
• PLCs cannot run on multiple programs simultaneously.

Also, get to know the difference between PLC and DCS.

Conclusion

We have listed the most common PLC interview questions for both entry-level candidates and experienced professionals. It covers topics, such as redundancy, programmable languages, advantages of using PLCs, etc. These questions will help you prepare effectively for any upcoming PLC interview. So, make sure to go over these key concepts and succeed at your next job interview. You can also refer to this guide on PLC programming for beginners to get a better understanding of this subject.