Это я не понимаю, зачем копаюсь в старых версиях Марлина и изобретаю велосипед, когда в 1.1.2 уже всё есть.
Спасибо за наводку! 
Буду разбираться. Configuration_adv.h
/**
* Spindle & Laser control
*
* Add the M3, M4, and M5 commands to turn the spindle/laser on and off, and
* to set spindle speed, spindle direction, and laser power.
*
* SuperPid is a router/spindle speed controller used in the CNC milling community.
* Marlin can be used to turn the spindle on and off. It can also be used to set
* the spindle speed from 5,000 to 30,000 RPM.
*
* You'll need to select a pin for the ON/OFF function and optionally choose a 0-5V
* hardware PWM pin for the speed control and a pin for the rotation direction.
*
* See http://marlinfw.org/docs/configuration/ ... indle.html for more config details.
*/
//#define SPINDLE_LASER_ENABLE
#if ENABLED(SPINDLE_LASER_ENABLE)
#define SPINDLE_LASER_ENABLE_INVERT false // set to "true" if the on/off function is reversed
#define SPINDLE_LASER_PWM true // set to true if your controller supports setting the speed/power
#define SPINDLE_LASER_PWM_INVERT true // set to "true" if the speed/power goes up when you want it to go slower
#define SPINDLE_LASER_POWERUP_DELAY 5000 // delay in milliseconds to allow the spindle/laser to come up to speed/power
#define SPINDLE_LASER_POWERDOWN_DELAY 5000 // delay in milliseconds to allow the spindle to stop
#define SPINDLE_DIR_CHANGE true // set to true if your spindle controller supports changing spindle direction
#define SPINDLE_INVERT_DIR false
#define SPINDLE_STOP_ON_DIR_CHANGE true // set to true if Marlin should stop the spindle before changing rotation direction
/**
* The M3 & M4 commands use the following equation to convert PWM duty cycle to speed/power
*
* SPEED/POWER = PWM duty cycle * SPEED_POWER_SLOPE + SPEED_POWER_INTERCEPT
* where PWM duty cycle varies from 0 to 255
*
* set the following for your controller (ALL MUST BE SET)
*/
#define SPEED_POWER_SLOPE 118.4
#define SPEED_POWER_INTERCEPT 0
#define SPEED_POWER_MIN 5000
#define SPEED_POWER_MAX 30000 // SuperPID router controller 0 - 30,000 RPM
//#define SPEED_POWER_SLOPE 0.3922
//#define SPEED_POWER_INTERCEPT 0
//#define SPEED_POWER_MIN 10
//#define SPEED_POWER_MAX 100 // 0-100%
#endif
Но тут то новое дело осваиваем, нужно свежие решения изучать 
