Error 20, job halted, Custom machine with FluidNC

[alf.condo (Alfredo Condò)]

Hello, I built my own three-axis laser machine using a Pibot mainboard and programmed the firmware with FluidNC. Everything works correctly: framing, homing, and free, repeated movement of all axes. However, when I start a job it stops after a short tim. Specifically, after finishing the first shape, it moves on to the next shape and then freezes with the following message:
-error:20
Unsupported or invalid g-code command found in block.
On or near line 0:
Job halted
Stream completed in 0:00
UPDATE:
I exported the G-Code generated by Lightburn and opened it with OpenBuild, and the error is the same:

[MikeyH (Mike Hembrey)]

Used A.I. instead of Lightburn Docs, but this should help:

Lightburn supports the M4 G-code command, which enables dynamic, variable-power laser control, but it requires specific GRBL firmware (1.1f or later) or a custom G-code device configuration to function correctly. You must use the standard “GRBL” device profile in LightBurn to use M4, as opposed to the “GRBL-M3” profile, which is for older firmwares that don’t support M4.

There may be a special .lbdev file for FluidNC controllers.

[ednisley]

@alf.condo

exported the G-Code generated by Lightburn

Showing us at least the first few lines of the offending G-Code will be helpful.

Typically, however, this is due to selecting the wrong Device Type, so that LightBurn is generating G-Code in a dialect that the controller doesn’t understand.

Similarly, enabling G-Code Clustering for a controller that can’t handle it never works.

@MikeyH likely pinned it down, but this FluidNC discussion may be relevant:

Move controls greyed out with FluidNC

I am a FluidNC developer. I am testing MillMage with FluidNC because I expect that our users will have questions. I set up a SerialUSB device using the GRBL GCode flavor, with Enable DTR on, Enable $J Jogging on, Query regularly for tool position on. Text Based on, Buffered Transfer mode, Target Buffer size 127. MillMage connects and displays startup messages in the Console window. After receiving the “Grbl 1.1f [$ for help]” signon message, MillMage appears to issue $I and $G commands to wh…

[MitchBradley]

It appears that the offending command is M4. FluidNC supports M4, but only if your config file has a “Laser:” section as described at Spindles | Wiki.js You can get more help with FluidNC via our Discord server, see Discord Server | Wiki.js . This is definitely not a problem with LightBurn; but rather a misconfiguration of FluidNC.