I didn’t use it yet but as @robertbu says you must define/enable it in your marlin’s Configuration_adv.h file, rebuild Marlin then reflash your board.
According to Marlin’s Gcode documentation, M8(air assist on) is carried by COOLANT_FLOOD_PIN(not the MIST one)
So when i’ll have time to upgrade to automatic air assist, i’ll use PF9(right next to PC9 i use for my laser) and GND to control :
- a 3V-12VDC to 250VAC Solid State Relay with a soket for using it with my compressed air diaphragm pump(80L/sec with low psi) in a classical power ON/OFF mode.
or/and - a 3V-12VDC to 12V-24VDC Solid State Relay with a 24V Air solenoid valve for using it with my air compressor(high psi) in a Air ON/OFF mode with compressor still ON.
See below the PF9 pin in red and GND in black.
I use Marlin 2.0.7.2 and in my configuration_adv.h i would have make that changes:
#define COOLANT_CONTROL
#define COOLANT_FLOOD_PIN PF9 // coolant flood / air assist
/**
- Coolant Control
- Add the M7, M8, and M9 commands to turn mist or flood coolant on and off.
- Note: COOLANT_MIST_PIN and/or COOLANT_FLOOD_PIN must also be defined.
*/