Lessons · Industrial control · two things that can never both be on
Never both at once
An interlock uses each output's own contact to block the other, so forward and reverse can never be on together even if the logic or a stuck bit tries.
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
Two contactors closing at once is a short circuit and a fire. Interlocks are the pattern that makes 'never both' true by construction, not by hoping the inputs behave.
How to think about it
On each rung, add a normally closed contact of the other output. Add a stop to both. Wire a hardware interlock too; the program is one of two layers, never the only one.
Worked example
{ [FwdBtn] | [FwdRun] } [/Stop] [/RevRun] (FwdRun)Forward seals in, and cannot run while reverse is on.{ [RevBtn] | [RevRun] } [/Stop] [/FwdRun] (RevRun)Reverse seals in, and cannot run while forward is on.Your turn
Block the pump while the drain is open.
[PumpBtn] [DrainOpen] (Pump)
Run a rung and watch the scan
The trap
An interlock in software only is not enough: a welded contactor ignores the program. The software interlock stops the mistake; the hardware one stops the fire.