Lessons · Industrial control · the coil at the end of a rung
The coil is the action
A coil sits at the right end of a rung and turns on when the contacts to its left form a complete path. Break the path and it drops out on the next scan.
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
Coils are where logic becomes motion, light and flow. Everything on a rung exists to decide one coil.
How to think about it
One coil per rung, written once. Read the rung as a sentence: the coil is on while all of this AND any of that. If the sentence needs memory, that is a latch or a seal-in, not a plain coil.
Worked example
[Start] [/Stop] (Motor)Motor is on while Start is on and Stop is not pressed.
[Motor] (RunLamp)A coil can be read back as a contact on another rung.
Your turn
Turn the heater on while the thermostat calls.
[Thermostat] ()
Run a rung and watch the scan
The trap
A plain coil has no memory. Release Start and Motor stops on the next scan; to keep it running you need a seal-in branch or a latch.