Hello,
Thanks for your answer .
i may spend a bit of time on the set up page ;D
it s not a laser but more of a plasma cutter i guess ;p
32=1 ====> laser on
32=0 ======> spindle on
If i turn the $32 by 1, i don t think i ll have any signal because the spindle will be off and my wire on the motherboard itself is connected to the spindle.
electronic wise, i m using an arduino UNO and the wire on the motherboard is connected to the Spindle because GRBL isn t specially made for the plasma ( i don t need speed control on the router / no need of Z axes, i just on a signal on and off )
So i kind of connect the wire into the spindle to be able to get a signal .
Now if we have a look on the Gcode GRBL is starting the spindle
M4 - start the spindle counterclockwise at the S speed (so i m using this command to have a signal on the torch . )
i have been able to have some result by changing the position of the M4 / M5 manually . but i don t want to have to modify the Gcode everytime by hand 
My machine isn t perfect far away from that specially for the circle . but as a fabricator i just fix it
thanks for your time
Florian
