[uncle_bob]

I supose you are running the RX software in Davis VP2 emulation mode, right?

I suppose so I've selected Vantage Pro2/Vue in the Station settings.

Now it's time to talk about the VP1 switch. The VP1 switch is there exactly to prevents the auto-reset (this is a Arduino Nano feature, it auto-reset every time serial port is initialized) when Cumulus starts.
VP1 must be off, only when you install or upgrade RX software. After that, put VP1 switch on, wait at least 3 minutes to give time for the receiver get data from external sensors, and start Cumulus.
Let the RX unit working and forget... Now you can start and stop Cumulus every time is necessary.

Dam, my quoting didn't work, hope you can read that right.
Great, I'll add that jumper in pronto.

Value is always the same? I will check this.

No, it seems to change.

The System batterie voltage reading shows anything?
I ask because the two are sent on the same datapacket.

The voltage shows now that I've got a jumper between the battery and 12V pins

I've nearly finished the breadboard layout in Fritzing too.