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Solved How to test for failure point with inductive probe issues ?


Well-known member
Hi guys, I'm in mechanical engineering so my electrical knowledge starts and stops at Ohm's law... thanks in advance for the help and patience.

Setup :
V2.4r1 with dual SKR 1.4. From an electrical POV this is a stock config.
Basic toolhead wiring (no CANbus or toolhead PCB) using proper silicone wires
Inductive probe is wired as per these instructions.

So my clone PL-08N (formbot kit, 2020) seems to have failed closed after more than two years of use. Klipper gave me a "triggered before movement" error and the probe's LED was visibly deformed and lit up all the time.
I successfully checked continuity of the 3 wires (for the signal one, only from the toolhead-side to before the BAT85 diode) and tried to wiggle them and the connector to try and detect a half-broken wire, no can do.

So I ordered a new probe, only one I could find in stock without going through Aliexpress was a genuine Omron TL-Q5MC2 2M which apparently can be used interchangeably.
When connecting it, I can get it to trigger (signal led light up) but under mainsail it say "triggered" even when the probe's LED is off.
When having no probe connected and refreshing the endstops status rapidly it alternatively reports triggered and open...

So I think something is wrong with the board or another component.

TL;DR and actual question :

Can the Bat85 diode get damaged by a failed inductive probe ? How to test this ?
Can the board pin (or something else) get damaged by a failed inductive probe ? How to test this ?
Quick update as I'm trying to understand how the pixies work without letting the magic smoke out.

I tested the BAT85 diode and I think its working.

Here's what I did : Using the diode setting on the multimeter, it gives me 0.235V and a fain beep between the cathode (com) and the anode (V). 0L the other way around. While doing this measurement, the diode is not connected to anything (not the SKR, nor the inductive probe). I understand this means the diode is alive.

To test the board, can I safely move the inductive sensor from MCU Z to the other MCU and change the pin from "!z:p0.10" to "!P0.10" in the config ?
Alternatively is there any other pin I could use on MCU Z to connect that probe ?
Since you are using bat85 diode, you can connect it to any endstop port, and yes you can just move it from z MCU to main MCU.
Bat85 is there to protect the MCU from a probe failed in closed state with unexpectedly high voltage
Thanks, so if any end stop pin works I'll just use one on the same MCU ! Will try that asap.
It worked, thanks @Corunir ! I'm getting open and trigger status in sync with the probe's LED indicator when connecting to z1.28 (Y stop).

As far as I can tell the 0.10 pin is dead, I'm still curious as to whether this is possible through a failure of the clone PL-08 probe or if I have another underlying issue... as you said the BAT85 diode is supposed to protect the MCU... Any insight welcome :unsure: