Marlin 2.0.x connection issues a possible fix suggestion

Hi everyone! My First post!
I really like LightBurn and have just bought a license.
I’ve had lots of success etching offline via SD card on my modified Chiron, but there seems to be a start-up comms issue with Marlin 2.0.x and LB 9.20
LB refuses to connect unless you get lucky with timing by pulling and reinserting the USB cable during the reset.

What I think is happening is this:

  1. Reset the connection in LB by right clicking devices
  2. A Marlin restart is triggered by serial port status lines
  3. LB then starts sending lots of G0 commands awaiting OK responses from Marlin
  4. Marlin hasn’t completed the boot cycle yet so doesn’t respond but the G0s keep coming
  5. Marlin serial buffer overflows with G0 and the firmware crashes so it never responds.

If you get the timing right by pulling the USB cable, sometimes you can get the comms initialised OK and Marlin responds to the G0 messages leaving LB ready to interact. From this point it works fine and you can etch live.

The problem is the USB hardware on some print controllers comes online immediately and serial RX buffers overflow or the ISR calls stack up before Marlin is ready to consume and respond to the commands.

I can actually reproduce the issue by queueing up a load of G0 commands in a terminal and sending them before Marlin 2 comes online.
Octoprint has a similar issue that can be resolved by increasing the comms retry timeout.

My theory to fix this is to add a delay to the Marlin interface in LB, so after a connection reset, LB waits for a few seconds to allow Marlin to boot and output the welcome message before starting the stream of G0 messages. 5 seconds would be a good starting point but this may need to be tweaked.

I’m a dev manager for a software company and have been using Marlin for years, I’ve added code to the official repo too, so I would be happy to send any debug info and test fixes with you.

If I roll back to an older version of LB (I went back to v6.00) the comms works just fine.

Here’s my debug log

22:32:28.610 D: Looking for port: “COM5”
22:32:28.614 D: – candidate port PID 60000 VID 4292
22:32:28.614 D: mfr: “Silicon Labs”
22:32:28.614 D: prod: “Silicon Labs CP210x USB to UART Bridge”
22:32:28.614 D: name: “COM5”
22:32:28.614 D: - found matching port name
22:32:28.614 D: Found port: PID 60000 VID 4292
22:32:28.614 D: Manufacturer: “Silicon Labs”
22:32:28.614 D: Product: “Silicon Labs CP210x USB to UART Bridge”
22:32:28.614 D: Name: “COM5”
22:32:28.622 D: Port open succeeded
22:32:28.661 D: EV: 1001 took 21 uS
22:32:28.661 D: “starting” busy: false state: 0
22:32:28.661 D: EV: 1001 took 4 uS
22:32:29.670 D: “Attempting to connect” busy: false state: 1
22:32:29.670 D: “Connecting…” busy: false state: 1
22:32:29.673 D: O: “G0\n”
22:32:29.927 D: “Connecting…” busy: false state: 1
22:32:29.931 D: O: “G0\n”
22:32:30.186 D: “Connecting…” busy: false state: 1
22:32:30.190 D: O: “G0\n”
22:32:30.450 D: “Connecting…” busy: false state: 1
22:32:30.453 D: O: “G0\n”
22:32:30.710 D: “Connecting…” busy: false state: 1
22:32:30.714 D: O: “G0\n”
22:32:30.971 D: “Connecting…” busy: false state: 1
22:32:30.975 D: O: “G0\n”
22:32:31.231 D: “Connecting…” busy: false state: 1
22:32:31.238 D: O: “G0\n”
22:32:31.497 D: “Connecting…” busy: false state: 1
22:32:31.505 D: O: “G0\n”
22:32:31.759 D: “Connecting…” busy: false state: 1
22:32:31.766 D: O: “G0\n”
22:32:32.023 D: “Connecting…” busy: false state: 1
22:32:32.027 D: O: “G0\n”
22:32:32.288 D: “Connecting…” busy: false state: 1

I have seen some similar issues to this trying to communicate with my laser engraver because of other software I have that grabs it first…

Cura (and probably other) 3D printing software grabs them if it sees them.
Q GroundControl drone control program
Mission Planner drone control software.
Arduino development library

If any of those are running then its just a crap shoot as to who gets the port.

I’ve found that if I disconnect LB from the device (switch the COM drop-down back to (Choose)) Then open Repetier-Host (or Cura, etc) Connect to the laser, disconnect, connect in LB and you are now able to control like normal.

LightBurn Please Fix!


I just set up a Mega2560/RAMPS combo with Marlin and ran into issues that I did not recognize at the time may be the same as what you are having… I spent 3 days debugging…


I found that if I issue an old school DOS MODE command for the port your engraver is on and enable DTR all works.

Mine was on COM7

Open a CMD prompt in Windows and type:



I think this is a DTR signal issue, only on Arduino-based boards, that has been fixed already for the next release.

1 Like

That was a good find!
enabling DTR brought mine back to life too.

1 Like

Further testing shows that my workaround of manually setting DTR=on only works if the Device Setting in Lightburn does NOT have the Enable DTR switch on… still doing more testing. (using version 9.20 presently… work is being done to fix this in a future version of LightBurn!!!)

downloaded the trial version of LB 9.20 a few days ago. Using a Anycubic I3 Mega S 3D printer outfitted with a 20W laser controlled thru the FAN0 port. The anycubic is a trigolilla 3 board running Marlin firmware. Connection is USB. When I connect LB to my printer - it can recognize it as a GRBL-M3 (1.1e or earlier). However I can get any movement of the laser or table - - I do see string of LB sending G0 commands to the printer. Baudrate confirmed @ 250,000.
Any suggest

I have a few suggestions… My guess is the Anycubic I3 is using Marlin firmware, and that is also indicated by the 250000 baud which Marlin defaults to. So in that case I would suggest deleting the device in LB (LightBurn) and create it manually as a Marlin device, in Device Settings in LB make sure the Enable DTR switch is OFF then use a Windows CMD prompt to issue the following command
where COMxx is the COM port your device it connected to.

This MODE command will have to be issued EVERY time you USB connected device is unplugged or powered off and on!!!

That should get LB communicating with the device for now until a more elegant fix comes from LB developers.

P.S. Lightburn does not correctly auto detect Marlin devices

Thanks for the prompt reply Terry = = appreciate your suggestions.
I deleted the profiles and manually created a new Marlin profile. Confirmed that baud set at 250,000 and DTR switch is OFF.
I’ll try this with the suggested command prompt to see what happens - Ill follow up and let you know.

btw…the Marlin Firmware is build 1.19 not 2.0.
Don’t know if this makes a diff

[quote=“quid_non, post:8, topic:30743”]
it can recognize it as a GRBL-M3 (1.1e or earlier).[/quote]

You said it recognizes it as GRBL-M3. If you’re running Marlin, you need to configure it manually as a Marlin device, as it says in the docs: Find My Laser - LightBurn Software Documentation

Thanks Oz and Terry - problem solved!
Deleting old profiles and creating a new Marlin one done.
LB communication established to the Anycubic I3 Mega S via Terry’s suggestion:
enter at Windows command prompt:

It is now working great - - made my first burn with the NEJE 20W blue laser!
Still a few wonky error G codes coming up in the LB console - but they are not affecting performance so far.

Wow this is a great software build and extremely versatile - just getting started on the learning curve but was able to burn an image and test with little effort.

Thanks for all the help - I’ll get a license!

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.