Ladder logic

Rungs, timers, counters, seal-ins, and the scan-order problems that look like magic.

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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
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Answered
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