Top 5 Mistakes New PLC Programmers Make - And How to Avoid Them

PLCs, the control devices often used in industrial environments, require special skills for operation, maintenance, and repair. PLC technicians who understand their operation will benefit from career opportunities in factory environments and other industrial settings. Inexperienced technicians commonly struggle when working with PLCs for many reasons.

Small mistakes when employing timers, counters, and general ladder logic can cause machines to behave in ways that are counterproductive or even unsafe. These challenges are normal. If you're a new technician, learning about PLCs at a reputable PLC Technician Training program helps build professional credibility and can help to start your career off with a solid foundation.

Mistake 1: Jumping Straight into Coding

Writing code is a part of PLC projects, but before writing code, it's important to have a clear goal and a list of project requirements. PLC programming starts with a project purpose and documentation to outline the expected behavior.  A clear understanding of the product or process that is being controlled is essential.

Once you have an overview of the process and an understanding of the general requirements, it becomes much easier to break the project into smaller task specific steps. Ultimately, this simplifies the process and makes for cleaner, safer, more effective code.

Mistake 2: Misunderstanding Ladder Logic Fundamentals

Having a clear understanding of the manner in which a PLC interprets a given ladder logic program is essential. This often-overlooked aspect of operation can result in significant programming errors if not managed correctly.  The ability to ensure that outputs are not being set to conflicting states by different rungs of a ladder logic program as well as ensuring the correct handling of outputs controlled or modified by subroutines, interrupts or alarm routines are essential skills to acquire as a new PLC Technician.

These problems can be both understood and resolved by testing your logic one rung at a time, using simulation tools when they're available.  Using proper notation/documentation throughout your program and labeling your contacts clearly is also essential. This adds extra steps to the process, but ultimately ensures a cleaner, more efficient, and more manageable process.

Mistake 3: Timer and Counter Misconfigurations

It's difficult for PLC technicians to understand how timers and counters work inside a system. While timers and counters may seem simple enough, the PLC's scan cycle updates values continuously. When a PLC technician incorrectly programs timers or counters, equipment may start or stop too early or behave in unpredictable ways.

Small timing errors can be difficult to detect and may lead to problems like machine downtime or unsafe conditions. This can be avoided by having a clear understanding the limits of the program elements being employed.  Program scan time puts a practical limit on the minimum timer value that can be used in an application. In addition, counter max and min values as well as counter roll-over behaviour can also have unintentional consequences if these conditions are not properly anticipated and accounted for. PLC technicians must develop good timer and counter habits, leading to more effective programs.

Mistake 4: Neglecting Signal Conditioning and Input Validation

Electrical noises and voltage spikes can cause PLCs to react unpredictably. When connecting field devices to your PLC, account for these signals. Inputs must be filtered and validated to ensure that signals from sensors are interpreted correctly.

Without filters and validations, the PLC may behave erratically. Use tools to filter tiny fluctuations, to create a less sensitive PLC. Signals must be long enough to be considered valid. This helps ensure that programs will behave predictably and consistently.

Mistake 5: Skipping Documentation, Comments, and Testing

New/Novice PLC technicians often feel rushed and excited to finish their programs. Sometimes this causes technicians to skip steps such as adding comments and documentation. Although you may understand each rung of the ladder logic, this doesn't mean that future technicians will have the same insight. When someone else needs to work on a PLC, missing documentation can make future fixes challenging. Troubleshooting the PLC can become slow and frustrating. This frustration can be minimized by taking the time to document properly, adding clear comments to explain each run, and keeping a change log when updates are made. This helps prevent confusion and makes the PLC easier to maintain.

Bonus Mistake: Ignoring Scan-Cycle Behavior

PLCs follow a scan cycle that reads inputs, updates outputs, and then repeats many times in a short period. PLC technicians who do not understand this scan cycle may struggle with timing. This can be fixed by testing the program in a slow-motion simulation that reveals what's happening behind the scenes. Once you understand the scan cycle, you can create programs that operate as intended with reliable, predictable results.

How Technicians and Students Avoid PLC Mistakes

Working hard to understand PLC programming mistakes and taking proactive steps to avoid them can make all the difference. Here are some PLC best practices to keep in mind:

1. Be open to learning: Engaging in continuous learning even after completing technical training can help ensure you stay fresh and up to date on PLC programming. Learn from others, whether through mentorship, group discussions, walk-throughs with other technicians, and more.
2. Do it by the book: If you went to a reputable training program to learn PLC best practices, then you should have been taught many methods for avoiding mistakes, like documenting and writing comments. Don't cut corners; do it as you were taught.
3. Leverage resources: Simulation software is essential for code development. Leverage these resources when they're available to ensure that you can write reliable, consistent code.

Learn PLCs With a Trusted PLC Technician Training Program

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At George Brown Polytechnic, the PLC Technician Training program prepares you to work hands-on with the latest industrial automation technologies. To learn more, call 1-888-553-5333 and speak with one of our Program Consultants.