I hate to bump an older thread, but I was able to get this exact hardware working (plus Octoprint) pretty well with a Laser Tree 20 W laser (3 wire)
1st, I split the 3 wire to the 2x2 wire setup as in the picture at https://us2.dh-cdn.net/uploads/db5587/original/3X/a/d/adca81742a33684d8eba3f29c8e585f933881dd8.jpeg
Hooked the red and black up to its own 12v Power source, and the white (Ground) and yellow (PC9) to the PC9 and Ground on the SKR Board.
After that I was able to fire the laser without issue using the stock firmware for the SKR using M3 S10
In Lightburn, I manually set up Marlin but I couldn’t initially see the Laser Control Commands so I could set it as inline. I had to EXPAND the settings window in order for that to appear.
After that my biggest issue was with Lightburn generating the inline codes and the laser not turning on, after a bit of trial and error, I found that I needed to send the “M3 I” command to tell Marlin to enter Inline mode
So I added
to the Lightburn gcode settings, and the laser turns off and on as it should
With this setup I am able to generate Gcode using Lightburn, and upload to Octoprint (which I am using to control the CNC) and it works flawlessly now
Hoping this helps someone else save some frustration