A Programmable Logic Controller (PLC) is an industrial controller that checks the state of input devices and decides how to control the state of output devices based on a written program.
Get ready for your next PLC interview with our large database of frequently asked questions and their answers in the automation industry. Gain the knowledge and confidence to impress your potential employers.
Landing a job as a PLC programmer requires strong technical skills and the ability to thrive in a fast-paced industrial environment. Going into a PLC programmer interview, you can expect to answer questions that will assess your programming knowledge, troubleshooting abilities and aptitude for the unique demands of this role. Mastering the top 10 PLC programmer interview questions will help you walk into that interview with confidence armed with stories and examples that showcase your qualifications.
1. Explain PLC?
PLCs or Programmable Logic Controllers are industrial computers used for automating processes in manufacturing plants, energy facilities and other industrial settings. They monitor inputs from sensors and execute programs to control outputs like motors, valves, conveyors etc.
Key points to cover:
- PLCs are robust, reliable hardware built to withstand harsh industrial conditions
- They contain a CPU, memory, input/output modules and a power supply
- PLCs automate processes by executing ladder logic programs in a scan cycle
- They offer advantages like reduced wiring, flexible programming, and easy troubleshooting
2. What are the different parts of automation?
Industrial automation consists of various components working together:
- Sensors – Detect changes in the process and send signals to the PLC
- PLCs – Receive signals from sensors, process them per the program, and send outputs
- Actuators – Receive outputs from the PLC and perform physical actions like moving a motor
- HMIs – Allow operators to monitor and control the process
- Networks – Enable communication between the different components
3. What are the implementations of PLC automation?
PLCs can automate processes like:
- Assembly lines in manufacturing plants
- Amusement park rides
- Conveyor systems in warehouses
- Pumping stations and treatment systems in water plants
- Mixing and batching operations in food & beverage plants
- Robotic systems in auto plants
- Building controls like lighting, HVAC, access control
Almost any application that requires monitoring discrete or analog inputs to control discrete or analog outputs can leverage PLC automation
4. Role of PLC in Automation?
The PLC is the brain at the center of any automated system. Key roles include
- Reading inputs from sensors and executing logic per the program
- Communicating with HMIs, databases, and other devices on a network
- Performing actions through outputs, like running a motor, activating a solenoid
- Implementing safety features, alarms and emergency stops
- Collecting data for analytics and predictive maintenance
- Ensuring precise timing and sequencing for processes
5. What is redundancy in PLC?
Redundancy refers to the use of backup components that take over in case primary PLC components fail. This improves the overall system reliability. Methods include:
- Backup PLCs – Secondary PLCs that kick in if the main one fails
- Redundant power supplies
- Redundant networks – Additional communication paths between devices
- Redundant I/O modules
- Hot swappable components – Modules can be replaced without system shutdown
6. What is scan in PLC?
The scan cycle refers to the process where the PLC:
- Reads data from input modules
- Executes the logic program from start to end
- Updates output modules with new states
- Performs internal diagnostics and communications
- Repeats the cycle continuously
Scan time is the time taken for one complete execution of the program. It needs to be fast enough to monitor and control the process in real time.
7. How does a Latch coil work?
A latch coil is a PLC program component that works like a mechanical latch. When energized by an input, it retains its state until a reset input is received. This creates a kind of memory to store state data between program scans.
Latch coils allow PLCs to track events and store intermediate data between cycles. They are implemented using internal relays rather than physical external relays.
8. What is a draw close to the control relay?
The draw close to the control relay is a cool analogy to understand the basics of a PLC. Essentially:
- The input sensors are like the buttons to ring a bell.
- The PLC logic program is like the wiring between the buttons and the bell.
- The relay is the bell itself – it receives electrical signals and produces a physical action.
- The output devices are whatever the bell controls, like a light turning on.
So just like wiring buttons to a bell, we can wire inputs to a PLC and program it to control physical outputs. The PLC relay logic replaces controlling devices directly via hardwired relays.
9. What is your experience with PLC programming?
This open-ended question allows you to showcase your hands-on PLC programming skills. Be sure to cover:
- Specific PLC models and programming software you have worked on
- Types of industrial processes controlled
- Projects where you programmed a PLC system from scratch
- Your role in programming, testing, troubleshooting, and documenting the system
- Complex programming tasks like PID control, data manipulation, sequencing
- Communication protocols used
- How your experience translates to the company’s needs
10. How would you troubleshoot a problem with a PLC system?
This behavioral question allows you to demonstrate analytical thinking and problem-solving:
- Outline a systematic, step-by-step approach rather than ad hoc troubleshooting
- Emphasize safety precautions taken
- Discuss methods like checking I/O signals, monitoring program execution, verifying wiring
- Explain how you would isolate the problem to specific components
- Share any experience troubleshooting PLC issues in previous roles
- Highlight the importance of resolving problems quickly to minimize downtime
Thoroughly preparing for the top 10 PLC programmer interview questions will take your confidence to the next level. Be sure to have clear yet comprehensive answers ready, along with relevant examples and success stories from your past experience. This will help you stand out from the competition and land the exciting PLC programming job you want.
What is a PLC array?
In a PLC, an array is a group of data elements of the same type that are stored in memory locations that follow each other.
What is a counter in PLC programming?
It tells the PLC to keep track of how many times a certain event happens. This is done with a counter.
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FAQ
What are the 2 basic PLC programming?
What is PLC best answer?
What is a PLC interview?
What are PLC programmer interview questions?
PLC programmer interview questions refer to the interview questions used in a PLC programmer recruitment process, where hiring managers seek to understand whether candidates have the right skills, industry knowledge and technical ability to work with complex technology such as industrial computers.
How do I become a PLC programmer?
Degree in Engineering and / or Technical Certification in PLC Programming would be advantageous. 5-10 years’ experience in the Manufacturing environment, as well as a minimum of 5 years’ experience regarding PLC/Robot and Project execution within a manufacturing environment. Send cv to : [email protected]
How to write a PLC programmer resume & cover letter?
In your resume and cover letter, you can show your PLC programmer skills in several places. If you have prior experience working in PLC programming, manufacturing or electrical engineering, you can include those experiences in your professional history on your resume. You can also mention these positions in your cover letter.
