As you’ve tried, the M3 I needs to be put at the top of the finished g-code file. You can have it inserted automatically. In lightburn, you want Device Settings/GCode in the Start text box, enter M3 I.
Note that M3 I only works if you have your output set to Inline
Good morning! I’ve entered everything into Lightburn and created the g-code. The laser switched on correctly but unfortunately always at 100%. When the code is finished the laser stays at 100%? When I plug everything in and start up the board and laser the laser immediately switches to 100% and stays on, but if I briefly unplug the 2-pin supply cable to pin pc9 it stays off? By chance I hit a metal on the outside of the laser housing and the laser immediately switched to 100%? Do I have a grounding problem? What am I doing wrong? Thanks for your help Gerhard
First, I’d like to check your g-code file. Use the upload icon in the ribbon of tools and attach a copy of your Lightburn generate g-code file to a post. The icon looks like this:
Beyond that, we need to check what that pin is doing using a multimeter. As mentioned before, a couple of wires with Dupont connections make checking pins safer.
The multimeter will allow us to check if the issue is the SKR Pro pin or the issue is in the laser. If the issue is the pin, you will need to find another PWM pin and make new firmware.
pin Pc9 I measured 1.9V Thanks for your patience
; LightBurn 1.3.01
; Marlin device profile, current position
; Bounds: X0 Y0 to X15.56 Y20.06
;USER START SCRIPT
M3 I
;USER START SCRIPT
G21
G91
; Cut @ 3000 mm/min, 1% power
M8
M05
G0 X0 Y0.067 F0
; Layer C00
G1 X5.2 F3000 I S2.5
G1 Y19.995
G1 X-5.2
G1 Y-19.995
; Cut @ 3000 mm/min, 90% power
M8
M05
G0 X10.365 Y-0.067 F0
; Layer C01
G1 X5.2 F3000 I S229.5
G1 Y19.995
G1 X-5.2
G1 Y-19.995
M9
M05
; return to starting pos
G0 X-10.365 Y0 F0
Your 1.9V does not make sense. You are testing the pin without the laser connected?
You need to test the pin at various power levels. First, the pin should be near 0V when the SKR Pro is first booted. Then you need to run your laser program at various power levels. I suggest 1%, 33%, 66%, and 100%. If this is a 5V pin, you should see voltages around 0V, 1.65V, 3.3V, and 5V. If it turns out this is a 3.3V pin, then you will see voltages around 0V, 1.1V, 2.2V, and 3.3V.
Let me know.
So now I’ve probably done it right: 100% approx. 3.3 volts, 50% approx. 1.6 volts, 1% approx. 0.7 volts on the multimeter. I then put everything together and ran it through on the machine, including the laser, switching the laser on at the beginning and then off properly at the end. Unfortunately, the rectangles aren’t connected correctly? I’m already happy about this small step 
How or what do I need to change or configure so that the lines line up exactly when lasering? What else do I need to change so that it works in Marlin mode? I can switch to Marlin, but no cards or USB are recognized? Thank you.
I’m not sure about the lines not connecting. I’ve seen this behavior with M03/M05 for control, but the problem has always been solved by using inline commands. The g-code you posted uses inline commands.
It is possible there is a mechanical issue with your machine. Try the same rectangle at lower power and much slower speed and see if that changing anything.
There is a Lightburn page outlining similar issues and solutions, but I don’t think your issues match what they are describing. Here is the page just in case.
Thanks Robert for your great help! Same thing with slow movement! I tried filling with 3000 rpm 40%, if you enlarge the photo you can see the inaccuracy of the laser! The laser also makes a different noise when lasering than before the problem? Could this possibly be caused by the PWM frequency? I’ve read a lot on the internet and in forums but haven’t found a clear solution! I also tried something with the router with good success!! The mechanics should be fine! Perhaps you can find a solution with your great general knowledge 
Thanks Robert Regards Gerhard
I’ve now tried a G-code from a USB stick. When I plug it in, the screen immediately goes down and then back up again, but I can then execute the code. Not with the SD card! Is the machine angry with me?
There is clearly some sort of hardware issue. I don’t know if the issue is the laser module or the SKR Pro board, or in the case of the USB reboot, the USB stick. Before doing anything, consider testing with an alternate USB stick to make sure the problem is not the stick.
If the issue is not the stick, there might be a partial short somewhere, or one of the three voltage regulators might be flaky. Start by cleaning up the board. Vacuum any debris and make sure the bottom side of the board is not touching anything. Next make sure all the cables and the stepper drivers are seated. If the USB stick problem still occurs, unplug all the endstops and repeat the test.
As for the PWM, it is unlikely to be the source of your issue. You do want to make sure the S value is set correctly in Lightburn:
Thanks, it was the USB stick. I tried a new one and it worked! Are the four older ones I always used defective or what? I don’t have any end stops anyway! I have no idea why the laser makes a different noise and doesn’t engrave cleanly! Thanks for your valuable time!
Hello Robert! Dear thread, what should I try next? Should I buy a new board? Re-flash it? But what kind of firmware? Or buy a different controller? Can you please help me in any way? Thanks, Gerhard
One thing we need to understand is why inserting a USB stick makes the setup go wonky. It sounds like a grounding or power supply problem to me.
In fact the laser making a different noise hints at that as well.
Double check all your power/return connections are good, and if possible put a DMM on the 5V power supply rail and do some USB insert/removals of thee problem USB sticks. What to you measure when the reset happens?
Thanks Jim, the USB stick issue has been resolved. There was a problem. 
The new one worked! I’ll follow up on the laser noise issue again! Thanks, any advice is helpful!
