Hi @felmue and thanks. This helps a lot.

Hi Vielen Dank für die Antwort bin mittlerweile einen Schritt weiter. Aber sobald ich den Button drücke kommt jetzt zuerst ein A und danach tauchen lauter 255 auf und der Drucker druckt lauter Chinesische Zeichen.

@chriszang and another update. So I am happily working on my CAN receiver sketch and just noticed that I forgot to press the button that grounds INT-output of the mc2515 but the upload still worked. Gave it another try (w/o pressing the button) and it's still working - weird. Then I swapped this COMMU module for my 2nd COMMU Module and here the upload doesn't work. My explanation is that the grounding of the mcp2515 INT-output basically created a short circuit in the transistor that controls this pin and eventually burned it out. Not a big problem for me as a) I am not using interrupts in my sketch b) I have a 2nd COMMU unit I just wanted to warn you guys that grounding the INT output has unintended consequences.

Hi FunkyFab interesting... Thanks for sharing. Glad you got it working again. Cheers Felix