PLC troubleshooting trainer
A machine with a hidden field fault and the ladder online: work the panel, compare what the PLC sees with the machine, and name it.
The complaint
Operator: “I press Start and nothing happens. Nothing. It worked fine yesterday.” Work the panel and watch what the logic sees.
Result
Nothing ruled out yet — the PLC view below is live.
What the PLC sees — output bits
A lit coil is an order, not proof the machine moved. Under each coil is what that device is physically doing — when the two disagree, the break is past the output card.
Call it — name the component and how it failed
Open contact, broken wire or loose terminal — the PLC never sees it actuate.
How it works
Using the tool
A machine with a hidden fault, and everything a real tech gets: the operator's complaint, the panel, the machine in front of you, and the ladder online. The logic is healthy — something in the field is lying to it or ignoring it. Work the panel, compare what the PLC sees with what the machine does, and name the fault.
The method
The method is one comparison, made twice. Input side: do something on the panel and watch its contact in the online ladder — if the contact ignores you, the PLC never received it (dead button, broken wire, loose terminal). Output side: compare each output bit with what the machine physically does — bit ON with a dead device is a burned coil or tripped overload; bit OFF with a running device is a welded contactor feeding it around the PLC.
And when every bit and every device agree, nothing is broken — read the values: presets, setpoints, counts. Someone may have edited the program. The logic in these scenarios is always healthy; the world around it is not. That is the normal state of a plant.
New to the math? Learn the concept: Ladder logic patterns