Limit Switch False Triggering Duet 3 Mini

[Topher]

Im swapping out a duet2 wifi (which was working fine) for a Duet 3 Mini wifi. I'm finding that my X axis limit switch is false triggering. I've determined it to be due to the X limit switch wires running alongside the motor wires. As an experiment, I took a spare single 12" length of switch trigger wire pair and connected to io5.in ground and trigger (my x trigger) with a spare switch board, and left it dangling. Upon homing, it did not trigger unless pressing the switch - perfect. But, when I routed the single wire along with the X-axis motor wire, then I get false triggering again. Do I need to add a small cap to this input? I've tried adding software pullup as well by including "!^io5.in" - still no go.

[Phaedrux]

Twisting the endstop and motor wiring pairs can help.

[Topher]

@phaedrux I tried twisting and thought it solved it, but nope. Its doing it on my Z now. Twisted all the pairs of limit switches. I added a 10k resistor from the 3.3v to the trigger input and put a 0.1uf ceramic cap between ground and trigger. That solved it but I dont understand, Ive NEVER had to do that before. especially with a $200 (canadian) board.

[dc42]

@leckietech what sort of endstop switches are they? I suspect they are ordinary microswitches. If so then you should use the normally-closed contacts, not the normally open contacts. If you use the normally open contacts and also route the endstop wires close to stepper motor wires, then you can expect to get false triggering because of capacitive coupling between the endstop wires and the stepper motor wires. You can avoid this by using shielded cable for the endstop wires; but it is much simpler to use the normally-closed contacts.

The Duet WiFi had lower value pullup resistors and LEDs on the endstop inputs. Some opto and Hall sensors could not sink enough current to drive them; so we use higher value pullup resistors on the Duet 3 Mini.

[Topher]

@dc42 This makes perfect sense! Thanks for the suggestion, it sounds like the perfect simple solution. Im using cherry SPDT micro switches so that will be very easy to rewire