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Fundamentals·7 min read·

Seal-in or latch: two ways to hold a motor on

Both keep an output energised after the button is released. They fail differently, and that difference is the whole reason to choose.

Short answer

A seal-in holds an output on by routing the output's own contact in parallel with the start button, so the rung keeps conducting. A latch instruction sets a bit that stays set until explicitly unlatched. The critical difference is power loss: a seal-in drops out and does not restart, while a latched bit in retentive memory can come back on.

The one-scan lagRUNG 1LateEarlyRUNG 2StartLateone scan laterrung 1 already ran

Both patterns solve the same problem: a momentary button, and an output that must stay on after you let go.

The seal-in

Put the output's own contact in parallel with the start button, and a stop contact in series after them. Press start, the coil energises, and its own contact now holds the rung true. Release start and nothing changes. Press stop and the path breaks.

The behaviour is a property of the circuit. Nothing remembers anything; the rung is simply still conducting.

The latch

An output latch instruction sets a bit. An unlatch instruction clears it. Between the two, the bit stays set whether or not the rung that set it is still true.

Where they part company

Power loss. A seal-in is holding because the rung conducts, so when power goes the output drops and, on return, the rung is false and the motor stays off. Somebody has to press start.

A latched bit in retentive memory survives. Power returns, the bit is still set, and the motor starts on its own.

That is not a bug in the latch instruction. It is the behaviour it exists to provide, and it is exactly wrong for a motor somebody might be standing next to.

Choosing

Use a seal-in for anything that moves. Use a latch for state that should survive a restart on purpose: a fault that must stay recorded, a mode selection, a batch step you do not want to lose.

Common questions

Is a seal-in safer than a latch?
Usually, yes. A seal-in depends on the rung conducting, so losing power drops the output and it does not restart when power returns. A latched bit stored in retentive memory can survive a power cycle and re-energise a motor nobody is expecting to move.
How does a seal-in circuit work?
The output's own contact is placed in parallel with the momentary start button. Pressing start energises the output, and the output's contact then holds the rung true after the button is released. A normally closed stop contact in series breaks the path.

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