Core 2 and COMMU
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 @felmue Yes, thanks that was it :-) 
 So basically you compile & upload your sketch, you wait until the "...." appears, you press the button and then the upload works.
 Thanks again.
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 Hello @ChrisZang glad to hear you got it working. @TLS : thank you for figuring out the issue and letting us know. Thanks 
 Felix
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 @chriszang and one more thing that I just learned: 
 The GPIO for the CS Pin for the mcp2515 is 27 (not 12 as the documentation says).
 If you want to initialize the mcp2515 in the COMMU do this:MCP2515 CAN0(27); // (mcp2515 library) 
 or
 MCP_CAN CAN0(27); // (mcp_can library)
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 @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.
