Limit switches occasionally ignored when homing

It doesn't seem consistent when this happens but every so often when doing a homing action it will behave as though the limit switch has been triggered long before it ever touches it and makes it think home is in the wrong place. The most recent incident went like this. I click "home all" and the carriage begins moving left towards the x limit switch. It reaches and trigger the switch then begins moving forward to trigger the y limit switch. After maybe 10mm (still over 150 mm from the switch) it stops and begins homing z. Then when it tries to move backwards to begin a job it crashes into the back wall of the printer. I've checked the wiring and nothing looks out of the ordinary on our limit switches. Testing the output after this happened using M119 shows everything as normal when I trigger the switches with a finger. Any ideas why we get this intermittent issue?


  • Yes, the reason is quite simple. The effect is called cross talk where induction of other wires e.g. motors induce some voltage on the end stop line. When the board sees this it triggers end stop. A millisecond later it is back to normal signal but that is then too late. 

    Solution is make an electrical better wiring - twist sensor and motor/heater cables especially those coming close to the sensor cables. This reduces contact and also reduces the induction effect so it is less likely to get false signals from this.
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