I wanted to connect my Multi Plus to a raspberry pi I use for minitoring temperature and controlling fans etc, but despite examining the protocol doc and experimenting for hours I couldn't get it to work. It just kept sending version frames.
To make a long story short: the MK3 interface came with a patch cable to go from it to the Multi Plus, which I used. However on closer examination I see that it only maps 4 pins! So I replaced it with an ethernet patch cable and voila! it started to work. And the pc based VictronConnect and VC Configure programs could connect as well.
I remember that I originally configured this unit on my workbench where I must not have used that cable.
So beware the useless patch cable delivered with your MK3 adapter!