Unable to connect to Prusa Mk3S with Raspberry PI

My repetier-server setup with my Prusa has been running perfectly for months however, recently, has failed. 

I have reinstalled a fresh Raspberry image (Buster) 1.3.0 v28 and also reinstalled repetier server for windows 1.3.0.

It will not allow me to setup a connection to my Mk3S+ MMU2S printer.. I am only given Com 3 and Com 4 as options in the printer configuration. If I connect laptop to the Prusa directly by USB, it will report Com 6. When I remove USB, only options are Com 3 and Com 4, neither of which will allow a connection.

I have spent hours trying to resolve but without success. Windows shows that I have serial and com ports installed.

Can someone please help. Like I said above, all was working perfectly for months. I don't recall what may have changed with my setup to cause the current issues.


  • So buster image is irrelevant here as your problem is windows, right?

    You need the com port that appears when you plug in usb to prusa. So on windows server you need Com 6 to connect, select Marlin as firmware and continue. Prusa as a special "problem" that the firmware is not powered by usb while port chip is, so you must enable prusa main power to allow server to detect the printer. Also after wizard is through go to printer configuration->Connection and select "Port is visible even if not running" or similar. Otherwise you see constantly server trying to connect as long as usb is connected. Was especially added for this case.
  • Thanks for your reply.

    Main power is connected to the Prusa. I can only get it to connect on Com 6 when the PC is connected to the printer via USB cable. If I set the printer up and then remove the USB, connection fails.

    "Port is visible" setting is enabled.

    Without USB connection to the PC, the only port options I am given are com 3 and com 4, which according to windows are "standard serial over bluetooth links".
    When the prusa is connected via USB cable to the PC, an additional port appears "Original Prusa i3 Mk3 (Com 6)"

    I had no problems at all when I initially set this up over 12 mths ago....just can't work out why I now have these issues.
  • > If I set the printer up and then remove the USB, connection fails.

    Without usb cable you can not communicate with printer, that is normal! 

    Is this where the pi comes into play? You can connect it only to pi or windows, not both. And it will only operator on the server where oyu have the physical connection If you connect it to pi you have to enter on your laptop the ip of your pi as url like
    if your windows supports zeroconf.

    and you see the printer connected to pi. Same if you want to see it on your mobile or other computer.
  • I can connect to the PI from the laptop by using the IP address - this brings up repetier Server Pro - Aurora 54.

    Sorry, but I am not an expert on these issues so hopefully mt information assists with troubleshooting.

    One thing I have noticed that I am currently investigating, is that when I try and reflash the printer firmware through PrusaSlicer (just to make sure the version of firmware I have was not corrupted) the flashing fails with the message unable to open COM port. I am assuming this is related to my issue.

    Error log:

                    Using Port                    : COM6
                    Using Programmer              : wiring
                    Overriding Baud Rate          : 115200
    avrdude-slic3r: ser_open(): can't open device "\\.\COM6": Access is denied.

    avrdude-slic3r: Could not open port: COM6

    avrdude-slic3r done.  Thank you.
  • If server is connected using port 6 you can not flash with prusa slicer. Only one app can be connected at a time. Deactivate printer in server on windows installation to do so.

    It is still unclear to me if you want to connect prusa to pi or windows. You only need the server configured for prusa on the server you connect it to. If configured on both only one will work of course. On windows ports are named COMxxx on linux they have different names and you best use /dev/serial/by-id/... if I remember right it contains even prusa in name there.
  • I did another clean install of windows with a new Raspberry image and everything is working well now.

    I don't know what had happened but did note some strange issues with windows recently, so I'd suggest something must have been corrupted which was preventing me getting the windows to Repetier connection working.

    Thanks for your help.
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