Ladder logic simulator
The classic plant-floor circuits running a real scan: pick one, flip the inputs, and watch power flow through the rungs.
The first circuit every PLC tech learns. Press Start and the motor latches itself in through its own OUTPUT BIT — release Start and it keeps running. Stop breaks the seal.
The processor sweeps on its own clock — flipping an input only writes the input table, exactly like the field. Stop it and step one sweep to watch the cycle.
Result
Scan order matters: rungs solve top to bottom, and an output written on rung 0 is already visible to rung 1 in the same sweep.
How it works
Using the tool
The classic circuits of every plant floor, running a real scan: pick one, flip the inputs, and watch the power flow light the rungs — seal-ins latching, timers accumulating, interlocks fighting, counters counting. The fastest way to make ladder logic click.
How ladder logic works
Ladder logic survives because it draws like the relay panels it replaced: power rails on the sides, rungs of contacts (conditions) feeding a coil (output). -| |- passes power when its bit is ON; -|/|- when OFF. The PLC sweeps the rungs top to bottom over and over — the scan — and three idioms cover most of a plant: the seal-in(an output's own contact keeps it latched after a momentary start), the timer (TON accumulates while its rung is true and fires a done bit at the preset), and the interlock(each output's rung examines the other, so they can never be on together). The scan here runs on its own clock, so you can stop it and step one sweep at a time — which is the only way to see the faults that exist because of the cycle rather than in spite of it: a bit that arrives one sweep late, a coil a rung above already overwrote, a pulse that fell between two scans.
New to the math? Learn the concept: PLC basics, made visual