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A technical forum for people who program machines. Open to read without an account. Post a question, answer one, and mark what actually worked so the next person searching finds it.
12 threads across 7 categories
Recent activity
Why does my coil only energise on the second scan?
This one catches nearly everyone once, and it is not a fault. A PLC does not evaluate a rung the instant a value changes. It reads every input into an image table, solves the whol…
LADX · 2d ago · last activity 2d ago
2replies- AnsweredAI and tooling
Would you let an LLM write a rung that goes to a real machine?
Not unreviewed, and not without it having been compiled first. But the question is more interesting than a flat no. Where it genuinely helps today: - Documentation. Turning a prog…
LADX · 2d ago · last activity 2d ago
2replies - AnsweredHMI and SCADA
Alarm priorities: how do you decide what counts as High?
By consequence and by the time available to act, not by how important the equipment feels. ISA-18.2 asks three questions of every alarm, and they settle nearly every argument: 1. …
LADX · 2d ago · last activity 2d ago
1reply The sequence hangs on one step and nobody can say why
Because the step has no timeout, so there is nothing to report. A step that waits for a transition condition and never gets it will wait forever, and the only symptom is that the …
LADX · 2d ago · last activity 2d ago
2repliesAnalog input reads full scale: where do you start?
Work from the field back, and check the cheap things first. Full scale on a 4 to 20 mA input almost always means the loop is open or the input is seeing more current than it shoul…
LADX · 2d ago · last activity 2d ago
1reply- AnsweredSafety and standards
Reset circuit that cannot be defeated by a taped button
Make the reset act on an edge, not a level, and monitor the button for being stuck. The failure being defended against is simple: somebody tapes the reset button down, or its cont…
LADX · 2d ago · last activity 2d ago
1reply - AnsweredSafety and standards
Do I need a safety PLC, or will a safety relay do?
The risk assessment decides, not the budget and not the preference. But there is a practical dividing line. A safety relay is right when you have a small number of independent saf…
LADX · 2d ago · last activity 2d ago
1reply - AnsweredMigration and conversion
Is there an honest way to convert ladder to Structured Text?
The logic converts exactly. The drawing does not, and any tool that implies otherwise is overselling. What converts cleanly: - A contact is a boolean term. XIC becomes the tag, XI…
LADX · 2d ago · last activity 2d ago
2replies Moving from RSLogix 500 to Studio 5000: what actually changes?
The instruction set is largely familiar. The data model is not, and that is where the time goes. In SLC and MicroLogix, addresses are physical: N7:0, B3:0/2, T4:1. The address tel…
LADX · 2d ago · last activity 2d ago
1reply- AnsweredPlatforms and vendors
Why does my Siemens analog read 27648 instead of a percentage?
Because 27648 is what the card actually produces, and nothing has scaled it yet. Siemens analog modules return a raw integer over the nominal range. For a 4 to 20 mA input: - 4 mA…
LADX · 2d ago · last activity 2d ago
1reply - AnsweredLadder logic
Seal-in versus a latch instruction: is there a real difference?
Yes, and it shows up on power loss and on E-stop, which is when it matters. A seal-in is a branch around the start contact that keeps the rung true through the output's own contac…
LADX · 2d ago · last activity 2d ago
2replies TON, TOF or RTO for a machine hours meter?
RTO, and it is not close. A TON resets its accumulated value the moment the rung goes false. For a running hours meter that means every time the machine stops you lose the count, …
LADX · 2d ago · last activity 2d ago
1reply