The Top 10 PLC Programmer Interview Questions and How to Answer Them

Landing a job as a PLC programmer requires mastery of both technical skills and the ability to impress potential employers. With competition fierce for these lucrative roles, candidates need to enter interviews ready to highlight their experience and expertise.

This article will explore the 10 most common PLC programmer interview questions, providing tips and sample responses to help you ace the interview

1. Explain PLC?

PLCs or Programmable Logic Controllers are industrial digital computers used to automate processes and machinery They contain a processor, memory, input/output modules, and a power supply PLCs are programmed using special software, either in ladder logic or other programming languages.

They keep an eye on sensor inputs all the time, run logic based on the program, and manage outputs to make a machine or process run itself. PLCs offer benefits like flexibility, ease of programming, robustness, and built-in communication capabilities.

Sample Response “PLCs are industrial computers that provide real-time control and monitoring of industrial equipment like assembly lines robotic arms conveyor systems, and more. They contain memory, a processor, input/output modules and are programmed in languages like ladder logic. PLCs offer sophisticated capabilities like PID control, motion control, sequential control, and more. They are highly robust for industrial environments. Compared to traditional relays, PLCs make it easy to modify and update control logic.”

2. What are the different parts of automation?

This question tests your understanding of industrial automation as a whole. Get ready to talk about the important parts, such as sensors, controllers, networks, human-machine interfaces (HMIs), and more.

Sample Response: *”The key components of an industrial automation system are:

• Sensors – Devices that measure a physical property like temperature, pressure, level, flow. They provide raw data to the control system.

• Actuators – Devices like motors or valves that manipulate the process based on commands from the controller.

• Controllers – Devices like PLCs that execute control logic and provide commands to actuators.

• Networks – Enable communication between components. Examples are Ethernet, Modbus, Profinet.

• HMIs – Allow operators to monitor and control the process, usually through a graphical interface.

• Software – For programming, data collection, visualization, analysis. Examples are SCADA, MES, HMI development platforms.

Additional components can include safety systems, data historians, telemetry devices, and more.”*

3. What are the implementations of PLC automation?

For this question, focus on real-world examples of PLC applications to showcase the breadth of your experience. Be sure to highlight various industries and process types.

Sample Response: *”PLCs are hugely versatile and used for all types of industrial automation tasks, including:

• Assembly lines and robotic cells in manufacturing

• Conveyor and material handling systems in logistics facilities

• Mixing vessels, reactors, and distribution systems in chemical plants

• Blowers, pumps, and compressors in wastewater treatment

• Extruders, rollers, and cutters in food processing plants

• Oil rig equipment monitoring and emergency shutdown in petrochemical facilities

• Ride control and show action sequencing in theme parks

• Power transformer monitoring and grid management in utilities

• Gate control, signaling, and switching in railways and metro systems”*

4. Role of PLC in Automation?

Emphasize how central PLCs are to industrial automation across sectors. Outline their key responsibilities – controlling equipment, communicating with devices, executing programs, collecting data, and more.

Sample Response: *”PLCs play an absolutely vital role in industrial automation systems across manufacturing plants, utilities, oil and gas facilities, and more. Their core responsibilities include:

• Reading inputs from sensors and instruments

• Executing the control program logic continuously and in real-time

• Communicating with HMIs, data historians, drives, and other equipment on the network

• Controlling field devices like motors, valves, pumps through digital or analog signals

• Monitoring process variables and production metrics

• Performing precision functions like PID control and motion control

• Generating alarms and emergency responses

• Collecting and storing data for analytics and predictive maintenance

5. What is redundancy in PLC?

Redundancy is an important concept for the reliability of PLC systems. Convey your understanding of how duplicate components prevent downtime and process disruptions.

Sample Response: *”Redundancy in PLC systems refers to having backup components that can take over in case the primary PLC fails. This prevents downtime and maintains continuous process operation.

Some key ways redundancy can be implemented include:

• Backup PLCs – Having hot standby or dual-redundant PLCs that run asynchronously.

• Redundant power supplies – Critical for maintaining PLC operation.

• Redundant communication modules/networks – Prevents loss of connectivity.

• Redundant I/O modules – Backup modules can take over I/O handling.

• Redundant controllers for critical equipment like emergency shutdown valves.

All components are designed for automatic failover. The redundant PLCs share data so the secondary can seamlessly assume control if needed. This level of redundancy ensures maximum reliability and uptime for critical industrial processes.”*

6. What is scan in PLC?

Use this question to demonstrate your knowledge of one of the fundamental operations of a PLC system – the scan cycle.

Sample Response: *”The scan cycle refers to the process a PLC uses to continuously monitor inputs, execute the ladder logic program, and update outputs. It involves three key steps:

1. Input Scan – The PLC scans all the input modules and updates the memory with the status of field input devices like sensors and switches.

2. Program Execution – The PLC executes the ladder logic program sequentially, using the latest input data. Logic commands like timers and counters are updated.

3. Output Scan – Based on the program execution, the PLC updates the status of output modules which connect to field devices like motors and valves.

This scan happens continuously, many times per second, to ensure real-time control. A typical scan rate may be 10-100 ms depending on the PLC model and program complexity.”*

7. How does a Latch coil work?

Latch coils are a commonly used PLC programming element, so interviewers often ask questions to verify candidates understand their operation and applications.

Sample Response: *”A latch coil, also called a seal or set coil, is a PLC programming element that self-holds its ON state until reset. When the input condition is met, the latch coil turns ON. Even when the input condition goes away, the coil stays ON until reset by another signal.

Latch coils are useful in applications like:

• Holding equipment in a certain mode like auto/manual control state

• Keeping valves open or motors running until intentionally stopped

• Sealing alarms ON until acknowledged

• Setting flip-flops and control relays

The reset signal is applied to the unlatch coil associated with the latch coil. This resets the latch and turns the output OFF.”*

8. What is a draw close to the control relay?

This tests knowledge of draw close relays which force outputs OFF in case of PLC power failure. Being able to explain this safety concept is impressive.

Sample Response: *”A draw close to the control relay is a safety component used in PLC circuits to force outputs to turn OFF in case of PLC power failure.

Normally the PLC outputs remain in their last state if power is lost, which can be dangerous for industrial equipment. Draw close relays use mechanical contacts that are held open by a solenoid energized by the PLC.

If PLC power fails, the solenoid is de-energized, closing the contacts and shorting the output terminals which shuts OFF the connected field devices. This provides a fail-safe operation.”*

9. Explain analog vs. digital signals.

This tests foundational knowledge of electrical signals. Be ready to clearly explain the difference between discrete on/off digital signals vs. variable analog signals.

Sample Response:

*”Digital signals represent discrete on/off states, 1 or 0. They provide binary information. Analog signals are continuous, representing a range of values. Key differences include:

• Digital signals are either ON or OFF while analog signals vary continuously.

• Digital is used for discrete states and alarms while analog measures real-world values.

• Analog signals use 0-10V, 4-20mA while digital uses 24VDC, 120VAC, Ethernet, etc.

• Analog requires ADCs and DACs to interface with digital controllers.

• Analog provides higher information density but is slower and prone to electrical noise.

• Examples of analog IO are temperature, pressure, speed. Examples of digital IO are switches, buttons, valve position.”*

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