Limit Switch Issues

[mykepredko]

@CST

Interestingly enough, the two limit switches that I said in my last reply were no longer working, are now working again. I have no explanation as no changes were made (This happened before using the test endstop).

As I indicated above, this is a marginal design. I can see there being cases where things will work one day and not work in others or if wires are moved or if a metal enclosure was on or off.

So I did a bit of research and, apparently, you are experiencing a common SKR-PRO-V1.2 Endstop problem. Doing a Google search on “SKR-PRO-V1.2 endstop” yielded a lot of results with these being somewhat representative:

Skr Pro 1.2 "infamous" endstop issue

https://www.reddit.com/r/BIGTREETECH/comments/ufbeyw/ender_5_skr_pro_12_endstops_not_working/

SKR Pro v1.2 Endstops stay triggered

Hi there, first post since I’m trying to switch from RepRap/DWC to Klipper/Fluidd! Loving it so far, can’t wait to start printing with input shaping. Strange thing though; when testing my endstops (very simple switches), connected to PB10 and PE12 (the assigned pins on SKR Pro v1.x). So signal and ground, nothing powered from the 3.3V. It works, they trigger when they should. Sadly; the software (either M119 or the Fluidd-tune endstop-thingy) doesn’t “open” the endstops again (when I release…

It seems that there are three recommended actions for the issue:

1. Remove the LEDs on the SKR-PRO-V1.2 Endstops. This will resolve the issue but there is the risk of damaging the boards and you have to know exactly what you are going after.
2. Add 1.5k resistors between +5V (Pin 1 of the Socket) and the endstop input (Pin 3 of the Socket). This is less challenging in terms of soldering but doesn’t seem to be successful 100% of the time and I’m not sure about your case of the OpenBuilds switches with the built in LEDs.
3. Buy a new board with more conventional endstop circuitry.

@CST

Forgive my ignorance, but how is a signal able to pass through without the ‘V+’ connected?

The endstop signal pin has a pull up resistor on it which holds the endstop sensor pin voltage at a high logic level until a switch closes and pulls the line down to ground. This is the typical way endstop sensors are implemented in main controller board circuitry.

Here’s an explanation with graphics:

Simple But CRUCIAL - Pull Up Resistors Explained

The issue here is that there is an LED and another resistor in the circuit which raises the endstop sensor voltage to and intermediate level, not a definite high logic level or low one.

This is why removing the LED is the first thing I recommend as it will break the path to ground and give you a definitively high voltage.

@CST

One more question for you, as this is an industrial setting for this printer, and this board is clearly not meant for that type of setting, would you have a recommendation for another board with better protection elements and reliability? I plan to begin researching a new one soon, but would like to know if there is specifics I should be looking into aside from what I know the system needs to do based on our current setup?

The problem here has nothing to do with where the SKR-PRO-V1.2 is running. The issue is simply a bad endstop pin circuit.

There are a lot of good boards out there but before I can recommend one, I need to understand your requirements. The SKR-PRO-V1.2 has six stepper motor drivers and four heaters, which is a bit unusual.

I think the best first step would be to remove the LEDs on the endstop pins and see if that resolves your problem. I think you indicated that somebody created this custom printer and there’s probably a reason why they used the SKR-PRO-V1.2 (and hopefully it’s not they had one lying around they wanted to get rid of).