flexi-bot/docs/marlin/_gcode/M042.md

tag	title	brief	author	requires	group	codes	notes	parameters	examples
m0042	Set Pin State	Set an analog or digital pin to a specified state.	thinkyhead	DIRECT_PIN_CONTROL	control	M42		tag optional since description values I true 1.1.9.1 Ignore protection on pins that Marlin is using. type bool tag optional since description values T true 2.0.5.2 Set the pin mode. Prior to Marlin 2.0.9.4 this is set with the `M` parameter. tag description 0 `INPUT` tag description 1 `OUTPUT` tag description 2 `INPUT_PULLUP` tag description 3 `INPUT_PULLDOWN` tag optional description values P true A digital pin number (even for analog pins) to write to. (`LED_PIN` if omitted) tag type pin int tag optional description values S false The state to set. PWM pins may be set from 0-255. tag type state int	pre code Turn the LED pin on M42 S1 pre code Turn on pin 33 M42 P33 S1 pre code Set pin 44 to do PWM with 50% DC M42 P44 S128

For custom hardware not officially supported in Marlin, you can often just connect up an unused pin and use M42 to control it.