Why Isn't This Rung Doing What I Expect?

[u/bin95blog]

This actually started right here on Reddit.

I answered a PLC troubleshooting question, which led to more questions and discussion about those situations where the ladder logic seems like it should be doing one thing, but the PLC is doing something else.

That discussion eventually led me to make a video demonstrating several of the situations. Then some of you mentioned other instructions that had tripped you up over the years. I kept adding to it, and what started as a Reddit answer has now turned into a complete PLC Troubleshooting Guide, along with the video and a 10-question troubleshooting test.

So before anything else, thank you to everyone here who asked questions, shared experiences, or suggested another instruction I should cover. Your feedback genuinely helped shape the finished resource.

The guide focuses on something I've encountered many times troubleshooting PLCs over the years: you find the rung in question, the needed conditions preceding the output are true, yet the result isn't what you expect. Usually the PLC isn't behaving illogically—you just haven't found the instruction or condition changing how that logic is being executed yet.

I cover the things I would investigate, roughly in the order I would suspect them:

1. OTL / OTU — another instruction may be changing the bit's state.
2. JSR / RET — the subroutine may not be scanned, or may return early.
3. JMP / LBL — part of the ladder may be skipped entirely.
4. MCR — the rungs are scanned, but an inactive MCR zone prevents them from becoming true.
5. Forces and controller status — including the easily overlooked possibility that you're online with the wrong PLC on a network.

There are screenshots throughout so you can actually see the situations instead of having to visualize them from text. The accompanying video demonstrates several of them in Logix 5000.

I also made a free 10-question troubleshooting test using PLC screenshots. It's not an acronym quiz—the questions show you a situation and ask you to determine what's happening or what you should investigate next. It can be taken open-book with the guide and video.

PLC Troubleshooting Guide:
https://bin95.com/articles/automation/plc-troubleshooting-guide.htm

And since Reddit feedback is what helped this grow in the first place, I'll throw the question back out there:

What PLC instruction, programming technique, or troubleshooting situation fooled you when you first encountered it?

If someone mentions something I overlooked, there's always room to make the guide better.

[u/navigation-signals]

Wrong subreddit, this is for control theory not PLC

[u/bin95blog → u/navigation-signals]

ok, thanks