Arduino Mega 2560 + RAMPS voltage on GPIO pin
Connecting a 100&nbsp;Ohm resistor is definitely not safe. This load is far too high for an AtMega2560 output pin. 100&nbsp;Ohms at 5&nbsp;V (when the pin is high) is 50&nbsp;mA, whereas the recommended maximum for an AtMega2560 pin is 20&nbsp;mA (and it would be better to stay well below this maximum). You should use a higher value pull-down resistor (at least 250&nbsp;Ohms, more would be better) or find out where the stray voltage is coming from (it could be an internal pull up on the AtMega, since pin 20 is also the SDA pin which Marlin might enable).