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LadderLOGIC is a computer programming language primarily used for industrial and mechanical automation. Its roots trace back to the early days of electrical circuit diagrams, as it graphically resembles electrical ladder diagrams. Developed by Allen-Bradley, a brand of Rockwell Automation, LadderLOGIC serves as a foundational tool for programmable logic controllers (PLCs). This high-level language allows engineers to create complex control systems with relative ease. Its robustness and simplicity have made it a staple in the automation industry, with widespread utilization in manufacturing and production processes.

Ask the right questions secure the right LadderLOGIC talent among an increasingly shrinking pool of talent.

First 20 minutes

General LadderLOGIC app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in LadderLOGIC application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

What are the basic components of a Ladder Logic diagram?

The basic components of a Ladder Logic diagram are contacts, coils, and wires. Contacts can be normally open or normally closed. Coils represent outputs and can be output coil, output latch, or output unlatch. Wires are used to connect these components.

How would you implement a start-stop logic in Ladder Logic?

A start-stop logic can be implemented in Ladder Logic using a normally open start button, a normally closed stop button, and an output coil with a parallel branch containing a normally open contact controlled by the output coil.

What is the purpose of a seal-in (latch) circuit in Ladder Logic?

A seal-in circuit, also known as a latch, is used to maintain an output on after the activating input has been turned off. This is typically used for maintaining the state of a motor or other device.

Describe the difference between a normally open and a normally closed contact in Ladder Logic.

In Ladder Logic, a normally open contact is off when its controlling input is off and on when its controlling input is on. A normally closed contact, on the other hand, is on when its controlling input is off and off when its controlling input is on.

How would you implement a timer in Ladder Logic?

A timer in Ladder Logic can be implemented using a timer coil. The timer coil can be set to delay turning on (TON), delay turning off (TOF), or act as a retentive timer that accumulates time (RTO).

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What you’re looking for early on

Does the candidate have a solid understanding of LadderLOGIC?

Has the candidate demonstrated problem-solving skills?

Is the candidate able to communicate effectively?

Does the candidate have experience with other programming languages?

Next 20 minutes

Specific LadderLOGIC development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

What is the purpose of a counter in Ladder Logic?

A counter in Ladder Logic is used to count events, typically the number of times an input turns on. Counters can be used to control an output after a certain number of events have occurred.

How would you implement a counter in Ladder Logic?

A counter in Ladder Logic can be implemented using a counter coil. The counter coil can be set to count up (CTU) or count down (CTD).

What are the differences between a retentive timer and a non-retentive timer in Ladder Logic?

A retentive timer in Ladder Logic retains its accumulated time when the controlling input turns off, while a non-retentive timer resets its accumulated time when the controlling input turns off.

How would you implement a flashing light in Ladder Logic?

A flashing light can be implemented in Ladder Logic using a timer with a normally open contact controlled by the timer coil. The timer coil is set to delay turning on, causing the light to turn on and off at regular intervals.

What is the purpose of a master control relay in Ladder Logic?

A master control relay in Ladder Logic is used to control the power to a large portion of the circuit. When the master control relay is off, all the devices controlled by it are off, regardless of their individual states.

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At this point, a skilled LadderLOGIC engineer should demonstrate strong problem-solving abilities, proficiency in LadderLOGIC programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with LadderLOGIC.

What does this simple LadderLOGIC code do?
START
SET X
STOP
This code starts the program, sets the value of X to true, and then stops the program.
What does this LadderLOGIC code do?
START
SET X
IF X THEN Y
STOP
This code starts the program, sets the value of X to true, then checks if X is true. If X is true, it sets the value of Y to true. The program then stops.
What will be the output of this LadderLOGIC code?
START
SET X
SET Y
IF X AND Y THEN Z
STOP
This code starts the program, sets the values of X and Y to true, then checks if both X and Y are true. If they are, it sets the value of Z to true. The program then stops.
What does this LadderLOGIC code do?
START
SET X
WAIT 5
SET Y
STOP
This code starts the program, sets the value of X to true, waits for 5 seconds, then sets the value of Y to true. The program then stops.

Wrap-up questions

Final candidate for LadderLOGIC Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle LadderLOGIC application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

How would you implement a master control relay in Ladder Logic?

A master control relay can be implemented in Ladder Logic using an output coil. The output coil is connected to the power supply and controls the power to the devices in the circuit.

What are the differences between a latch and an unlatch in Ladder Logic?

In Ladder Logic, a latch is used to maintain an output on after the activating input has been turned off, while an unlatch is used to turn off an output, regardless of the state of the activating input.

How would you implement a latch and an unlatch in Ladder Logic?

A latch and an unlatch can be implemented in Ladder Logic using output latch (OTL) and output unlatch (OTU) coils. The OTL coil turns on an output and maintains it on after the activating input has been turned off. The OTU coil turns off an output, regardless of the state of the activating input.

LadderLOGIC application related

Product Perfect's LadderLOGIC development capabilities

Beyond hiring for your LadderLOGIC engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in LadderLOGIC projects, and can engage in multiple capacities.

LadderLOGIC Consultive assessments

LadderLOGIC Application Conversion Assessment LadderLOGIC Application Integration Assessment LadderLOGIC Data & Database Integration Assessment

Custom LadderLOGIC development projects

Custom LadderLOGIC App Conversion LadderLOGIC Custom App Development Process App UI Design for LadderLOGIC Facelifts

Hiring guide for LadderLOGIC Engineers

LadderLOGIC Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the LadderLOGIC engineer at this point.

LadderLOGIC Consultive assessments

Custom LadderLOGIC development projects