How to power 2 connected Arduinos separately?

I have 2 arduinos connected using I2C on the analog 4+5. I would like to have them powered separately. However if I just plug in 2 AC adapters, the communication between both devices stops working. The two adapters are different brands, but they have the same voltage rating and polarity on the casing.

I have a feeling I'm dealing with a very basic electrical engineering principle here, but I don't get it :-?

I have a feeling I'm dealing with a very basic electrical engineering principle here, but I don't get it

The most common problem with this kind of hook-up is to forget that there must be a ground wire connecting the two boards.

Lefty

Cheers! That's it :sunglasses: