Raspberry Pi 4 released!

[MatsA]

Hi ! in the empty box, in SignalK, to the right You must write /dev/ttyUSB0  The drop down list just shows You the available values. Not Cristal clear ;-)

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Hi, Not clear I agree :
In my /dev directory, there is no ttyUSB0 file ! Only  ttyACM0 and TTYAMA0 ! 
Forcing ttyUSB0  in the connection SK form results in SK error.
Eventually, I could get OP to work with ttyACM0, which was proposed in the form list ...

Serial devices appears with different names on different RPis. Therefore You should pick the right device, from the list, that matches You installation.
Additional hints here https://github.com/SignalK/signalk-serve...eal-inputs