Thats my worse problem, cause I really know very little about modbus. My previous setup was a copy from the OpenHAB1/2 Nilan heatpump. And I really struggle to understand how things works…
This new modbus did not make things any better for my understanding 
I can see from you help, I was actually on the right track regarding setting up the USB stick… What I was missing and doing wrong was the commas… And second, I was using full path to the device ie /dev/ttyUSB-RS486…
An example in the doc´s would be a great help to others, I would say.