This is standard GRBL modified for use with a 28-byj-48 motor and a pen plotter that has a servo for the pen up / down / pen change.

The servo will have 3 positions representing up, down and pen change (optional). You use normal gcode. Any time the work Z is above 0 the servo will move to the pen up position. If it is at Z 0 or below it will be in the pen down position. New: If Z is at or below -1.9 (can be changed, the default is same as the GRBL Plotter pen down defaults) the servo will move to the pen change position. See below for details.

GRBL installs as a library. Install GRBL normally as described here but using the files in this repository.

Note: you might need to change config.h as the default config is for a CoreXY machine.

If you are not using that, you need to make sure your schematic matches the pin defined in cpu_map.h. I tested the stepper motor with a ULN2003 driver.

There are 2 three (3) defined values in spindle_control.c. You can adjust these values to suite your machine. You can even reverse them to reverse the travel of your servo. Make sure the new setting does not cause the servo to hit an endpoint or it will overheat and damage itself.

Pen change (optional): Nothing too special, I just added a third position for the servo. It can be disabled by commenting out the #define PEN_SERVO_CHANGE XX line. Servo goes to the change position if Z is at or less than -1.9 (can be changed in line 91 of spindle_control.c).

#define PEN_SERVO_DOWN     19
#define PEN_SERVO_UP       22.3
#define PEN_SERVO_CHANGE   8

Grbl is still running a full range virtual Z axis with accelerations and speeds. Be aware how this will affect your pen. For example: If the Z is at 5 and you tell it to go to -5, the pen will stay up as the virtual z moves from 5 to 0. The pen will then go down. The virtual Z will continue to go down to -5 before it executes the next move. You can control these "delays" by adjusting the speed and acceleration of the Z axis and what your CAM uses a Z locations for up and down.

If you want to have the pen go to the up position at turn on, define a negative Z work offset. Send the command "G10 L2 P0 Z-2". This will trick Grbl into thinking the pen needs to be up.

Credits to bdring (repo: https://github.com/bdring/Grbl_Pen_Servo) and TGit-Tech (repo: https://github.com/TGit-Tech/GRBL-28byj-48) for basically the whole code. I just combined the two repositories and added the pen change feature.