SKR PRO V1.2, NEJE 40640 and Lightburn

OK, obviously nothing works the first try, but dang…I have research to no avail…

Have the MPCNC Primo running SKR Pro V1.2 with Marlin. Just added the NEJE 40640. I have been able to get the board talking to the laser and get a pulse to fire from the LCD screen, but I cannot get Lightburn to fire the laser. It connects and it will move the laser as if it is running the test file, but it doesn’t fire. It also doesn’t fire the laser when I hit the button test fire.

After reading the forums for wiring, I have a dedicated power supply for the laser, have the PWM board hooked up to the laser with the 4 wire cable and the two pin cable from that board hooked up to PC9 and ground

Any thoughts?

There are a couple of things to be aware of. First, there is a laser timeout to prevents the laser from continuing to fire if the laser is not moving. I believe in the latest V1 maintained version of Marlin, there is a bug in Marlin where the timeout does not play well with the laser menu. Marlin has fixed this bug, but V1 has not made a release version with the fix.

Second, starting with the latest V1 maintained version of Marlin, if you want to use inline commands (highly recommended), you need the following at the top of your laser file:

M3 I

If you have Lightburn setup for inline commands and the top of file does not have this command, the laser will not fire. This line can automatically be added to all Lightburn files by using the Start GCode section. If you haven’t already found it, inline commands are enabled in the following section:


This is on the Basic Settings page. Due to a bug in Lightburn, on some displays, you must resize the Basic Settings dialog to see this setting. Note that M03/M05 will work without the M3 I above, but the cut and engraving quality will not be as good as using inline commands.

Finally, if you just want to verify that your laser is working, you can use M42 to manipulate PC9 directly. This bypasses all laser-specific code. You can try with different S values in the range of 0 to 255 to make sure the laser varies in intensity.


Worked!!! Thank you so much.


why do you write m3 I and not m03 I? or is it the same thing?

It is the same thing. I’ve always thought that having two formats is a bit weird. It means a bit more work for the parser. There is probably some historical reason why both work.

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Okey Dokey to add to this…

I am trying to find the perfect focus. The Fire button is there, I have the laser firing when framing, but when I press it, the laser only stays on for a few seconds. It is suppose to stay on until I press the fire button again so I can change the focus. Any thoughts on why the software is turning it off after such a short time?