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Bigtreetech SKR 1.3 probe issue

Posted by Focussys 
Bigtreetech SKR 1.3 probe issue
June 14, 2019 07:11AM
I recently bought a Bigtreetech SKR 1.3 board as a replacement for an Arduino and Ramps 1.4 and am having difficulty setting up the existing Z probe (LJ12A3-4_Z/BY).
The endstop pins appear to have a pullup resistor built in preventing the probe activating . The probe is normally open and 5v upon trigger.

The endstop pin shows 2.40v unconnected and this pulls the normally open probe up showing it as TRIGGERED.

The original Ramps board endstop pin shows 0.48v and shows OPEN.

I am using the existing voltage divider 15k and 10 ohms resistors and have copied everything from Marlin 1.1.9 to 2.0.
Re: Bigtreetech SKR 1.3 probe issue
June 15, 2019 03:49AM
The SKR v1.3 has +5V to the end stop supply pin but a 10k pull-up on the sense pin to +3V3 (rather than +5v), which shouldn't really matter.

You don't need the divider as the CPU pins on the SKR board are 5V tolerant (when in digital mode - NOT analog mode).

Try putting a low value pull-down resistor from the sense pin (on the 3-pin end stop connector) to GND, try say 330R resistor. That should overcome the SKR 10k pull-up resistor and provide a clean 0V/+5V to the sense pin as your probe is triggered/non-triggered.

This is assuming my thinking is right with your inductive sensor having a PNP on the +5V rail that pulls up an open collector output.
Re: Bigtreetech SKR 1.3 probe issue
June 15, 2019 11:51AM
Many thanks for coming back.

In the end because the endstop pin was pulling up I decided to try pulling it down.

I used a 560 ohm resistor from the PNP sensor output to an opto isolator. The output then went to the endstop pin and ground.
So the mini relay effect worked fine. When the sensor output triggered at 12v it powered the opto which shorted the endstop pin. #define Z_MIN_ENDSTOP_INVERTING true finished the job.
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