Lessons · Industrial control · latch, seal-in, or plain coil
Latch, seal-in, or plain coil
A seal-in is a rung that stays true. A latch is a bit that stays set. The difference is where the release lives.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A seal-in keeps its stop in the rung, in series, in sight. A latch puts the release somewhere else in the program, where it can be missed.
How to think about it
For anything with a stop, prefer the seal-in: what stops it is written next to what starts it. Reach for a latch when the state genuinely has to outlive the rung.
Worked example
{ [Start] | [Motor] } [/Stop] (Motor)The seal-in. Start on scan 2, Stop held on scan 3, Start pressed again on scan 4. Motor: 0 1 0 0 -- the stop is right there in the rung.Your turn
Which pattern keeps the stop condition inside the rung?
the
Run a rung and watch the scan
The trap
Many shops require the unlatch on the very next rung. It is a convention, not a rule of the language, and it exists because the pair is one idea.