If you’ve ever worked on a project that involved PLC programming, you already know how quickly small errors can turn into big, expensive disasters. One tiny oversight in logic can stall an entire production line, damage equipment, or send maintenance scrambling for hours. The good news is that most of these mess-ups are completely preventable once you know where they come from.
Let’s walk through the mistakes that drain the most money and how you can sidestep them with a little planning and a lot of clarity.
1. Skipping Proper Requirement Gathering
This is the classic one. Everyone is excited to start the project, deadlines look scary, and people jump straight into coding. Then halfway through, someone says, “Wait… that’s not how the machine is supposed to behave.”
When requirements are fuzzy, the code becomes a collection of guesses. Those guesses later turn into rework, downtime, and frustration.
How to avoid it:
Spend time upfront asking questions. What exactly should the machine do? How should it respond in abnormal conditions? Who’s going to operate it? Clear documentation saves time. Tools like system requirement templates from PLCdev can help you structure things well.
2. Ignoring Standardised Programming Practices
When a PLC project is written in a hurry, the code can look like a puzzle no one wants to solve. Missing comments, inconsistent naming, and logic scattered throughout the code always lead to costly debugging sessions later.
How to avoid it:
Stick to standards. IEC 61131-3 exists for a reason.
Comment your code like you’re explaining it to someone who will read it at 3 AM during a system breakdown. Name your variables clearly. Organise logic in clean blocks. Your future self will thank you.
3. Poor Error Handling and Safety Logic
Many expensive failures happen because error conditions weren’t handled properly. A sensor fault, a communication timeout, or an unexpected operator input can cascade into a shutdown if the logic doesn’t anticipate it.
How to avoid it:
Always build for the worst case. Include fallback states, alarms, and safe shutdown routines. If you’re working with safety circuits, the guidelines from Rockwell Automation are a solid reference.
4. Failing to Test in Realistic Conditions
Simulation is great, but nothing replicates real-world quirks like the actual machine. Rushing to deploy untested logic can lead to equipment damage or production delays. And those… are expensive.
How to avoid it:
Test in layers. First in the simulator, then in a controlled environment on the actual hardware, and finally during supervised operation. Document each test. Many engineers rely on Factory Acceptance Testing (FAT) checklists to stay thorough.
5. Not Backing Up Code or Versioning Properly
You would think this one shouldn’t still happen, but it does. A laptop crashes, a programmer forgets to save, or someone overwrites a working version. Suddenly, the whole plant is waiting while you try to reconstruct logic from memory.
How to avoid it:
Use version control systems. Even simple tools like Git paired with an online repository make life easier. GitHub has a beginner-friendly intro: GitHub
And always keep backups of the final uploaded code from every machine. Always.
6. Overlooking Operator Training
Your logic may be perfect, but if the people running the machine don’t understand the workflows, the line will still stop. Incorrect inputs or resets can trigger errors that look like programming faults.
How to avoid it:
Train your operators, create simple user guides, and offer short walk-throughs during handover. A little human-to-human knowledge transfer prevents a lot of drama on the floor.
Final Thoughts
PLC programming isn’t just about writing ladder logic or function blocks. It’s about understanding the machine, the people using it, and the tiny decisions that add up to reliable automation. The costliest mistakes usually come from rushing, skipping steps, or hoping the system will “just work.”
Slow down. Ask the right questions. Test thoroughly. Keep things clean and documented. These small habits save companies money, time, and countless headaches.
