The seal-in: why Start keeps running

Press Start, let go — and the motor keeps running. Nothing in the button holds it. The circuit remembers, and the way it remembers is the most important rung in industrial control.

The problem

A start button is momentary: it's only ON while your finger is on it. Wire it straight to the motor coil and the motor dies the instant you let go. You need the circuit to remember that Start was pressed.

The trick: the output votes for itself

     Start          Stop
──┬──[ ]──┬────────[/]────────( Motor )
  │       │
  └─[Motor]┘

The rung reads: (Start OR Motor) AND not-Stop → Motor. Walk the scans:

1. You press Start. Power flows through the Start contact, through the closed Stop contact, and the Motor coil energizes.
2. You release Start. Its contact opens — but the parallel Motor contact is now closed, because Motor is ON. Power flows through the output's own contact. The rung feeds itself: the seal-in (or latch).
3. You press Stop. The XIO breaks the whole rung — both paths die, the coil drops, and with it the seal. Releasing Stop doesn't restart anything: Start is open and Motor is now open too.

The memory isn't stored anywhere special — it is the loop between the coil and its own contact, refreshed every scan.

Prove it to yourself

Run it live in the ladder logic simulator: flip Start on and off and watch the power flow switch from the Start branch to the Motor branch. That switch-over is the whole idea.

Why Stop wins

Notice the Stop contact sits outside the parallel branch — in series with everything. However the rung is being fed, Stop cuts it. That's a design rule, not an accident: stops break the seal path, always.

Why this matters

Every "press to run, press to stop" machine you've ever used is this rung. Learn to see it inside bigger programs and half of any ladder becomes familiar territory. The other half is usually timers — next lesson.