Updated September 7, 2026
Programmable Logic Controller is a class of industrially hardened devices that act as an interface for input and output sensors. The i/p field elements include switches, sensors, and push buttons, and final elements such as actuators, solenoid/control valves, drives, hoppers, etc. PLC has an important role when it comes to automation. PLC handles all tasks related to monitoring and control. The main job of a PLC is to sense inputs through input modules, process the logic in the CPU, and provide the necessary outputs through output modules.
Now, if you are looking for a job related to PLC, you need to prepare for the 2026 PLC Interview Questions. Every interview is indeed different depending on the job profile. Here, we have prepared important PLC Interview Questions and Answers to help you succeed in your interview.
PLC Interview Questions and Answers for Freshers
In this 2026 PLC Interview Questions article, we shall present the 10 most important and frequently asked PLC interview questions. These interview questions are divided into two parts, as follows:
Part 1 – PLC Interview Questions (Basic)
This first part covers basic PLC Interview Questions and Answers.
Q1. Explain the role of I/O Modules.
Answer:
To control a machine’s process, input and output devices are used to interface with electronic plug-in units. The input modules receive data from input devices such as pushbuttons, Switches, and Transmitters, which then send it to the processor. The output module receives all data from the processors and sends it to output devices such as relays and valves. The devices can be as follows:
Analog: It is used to send and receive variable input and output signals.
Digital/Discrete: Used for sending and receiving on/off signals.
Q2. What is SCADA? Name any four editors who are available in the SCADA package?
Answer:
Supervisory Control and Data Acquisition (SCADA) is a computerized hardware and software-based project that will provide a single integrated view. This view has all control and information resources, enabling engineers, supervisors, and operators to view and interact with the workings of the entire operation. It provides a graphical representation of their production processes.
The four editors are as follows:
- Inkscape SAGE – SCADA Animation Graphic Editor
b. Generic Logic
c. TRACE MODE
d. UCanCode
Let us move to the next PLC Interview Questions.
Q3. Define SCADA and list its benefits.
Answer:
Supervisory Control and Data Acquisition (SCADA) is a hardware- and software-based system that provides a single integrated view. This view provides all control and information resources, enabling engineers, supervisors, and operators to view and interact with the entire operation. It provides a graphical representation of their production processes.
The benefits of SCADA are:
- It preserves your capital investment.
- It helps in integrating your business systems.
- In order to reduce your operating and maintenance costs, SCADA is very beneficial.
- SCADA helps in improving the quality of your product
- When you use SCADA, productivity will be maximized.
- It is reliable and robust.
- It provides the ability to view and adjust the points and set them remotely.
- It helps in predictive maintenance and allows proactive planning rather than reactive planning.
It also offers higher reliability and greater flexibility, enabling all work at lower costs.
Q4. Explain the difference between PLC and DCS?
Answer:
These are common PLC interview questions. The difference between PLC and DCS is explained below:
- PLC: A system with a processor and an input-output system, comprising functional units such as basic modules, communication modules, etc. It uses SCADA for visualization and does not use a central database.
- DCS: DCS mainly uses multiple processors with a central database, and the functionality is distributed evenly. Here, the control subsystem performs the control functions. The history node is used to connect to data; also, the IMS node provides reports, and the operator station provides a good HMI. In addition, the engineering station allows engineering changes to be made.
Q5. Explain the role of a rack or chassis in the PLC system.
Answer:
A hardware assembly that helps in housing the processor, communication, and input-output modules. It works in the following ways:
- It distributes power evenly.
- It contains different input and output modules.
- It acts as a communication link between the I/O module and the CPU.
In different PLC systems, there are many different slots for chassis that are available. If a user wants additional chassis, they can be connected by using chassis interconnecting cables.
Part 2 – PLC Interview Questions (Advanced)
Let us now have a look at the advanced PLC Interview Questions and Answers.
Q6. What are the non-fatal errors that are available in Siemens PLC?
Answer:
The errors that are compiled and runtime errors are known as non-fatal errors. Non-fatal errors ought to degrade the performance of your PLC. It does not hamper the execution of the program or any updating in the I/O modules.
- Runtime Errors: These are the errors that are detected at runtime. These are reflected in special memory bits of your program, which the user can monitor and evaluate.
- Compile Errors: These are the errors that are detected at compile-time, and they are detected at download. When this error is encountered, the compilation halts, and the previous program is retrieved.
Q7. Explain the difference between instance data blocks and shared data blocks.
Answer:
The block that is assigned to any logical block. It works as a function block. The instance block usually has data that is stored in function blocks in the variable declaration table.
A shared data block, on the other hand, is never assigned to any logical block. It usually contains values required by the plant or the machine that is going to be installed. It is called directly at any point in the program.
Let us move to the next PLC Interview Questions.
Q8. List the advantages of PLC over Relays?
Answer:
PLC is beneficial over Relays with the following aspects
- PLC consumes less power than Relays
- When necessary, they can also be high-consumption PLCs.
- PLCs can be easily changed and expanded through programs.
- Whenever system requirements change, the relay wiring needs to be updated. This is not the case in PLC.
- There is no hard-wired requirement in a PLC.
Q9. What is Simplicity?
Answer:
These are the most popular PLC Interview Questions asked in an interview. SCADA has a package named Simplicity. Some servers are responsible for taking care of data, its collection, and distribution. It is based on client-server architecture, having servers and viewers. The viewers who are there cannot connect to servers; hence, they cannot access the collected data in full or perform control actions.
Q10. Where is SCADA used?
Answer:
SCADA is used in various industrial processes, such as chemical plants, power generation and distribution, nuclear processes, and metallurgical processes. It can also be used to process nuclear waste and reactors, among other applications.
Recommended Articles
This has been a guide to the list of PLC Interview Questions and Answers. Here we have listed the 10 most useful sets of interview questions to help the job seeker crack the interview with ease. You may also look at the following articles to learn more –
