Connecting 2 Uno boards without common ground

First of all, is it possible to connect 2 Uno boards for simple command transmission between them without using the same wire? (In fact I only got 2 wires to do that and I need both way comms)

Secondly, if it's posible, how to do that?

I've tried using this (220ohm resistor, both boards are connected to separate power sources), but it provides mostly noise on digitalOutput() -> digitalInput().

I am quite noobish in electronics, so I don't really get the logic behind it.

Thanks in advance!

You need to connect the GND pins of both Arduinos together, so that you complete a circuit.

The way you have the Arduino on the left connected is analogous to connecting a single wire to a light bulb and expecting it to light up.

Okay, so it means arduino pin set to input mode doesn't provide closed circuit?
Edit: Anyway, thanks. It just poped to my head that I just need to take a look at arduino schematic to understand how pins work.

Never connect 2 output pins together without at least a 220 Ohm resistor between, if one is HIGH and the other LOW you might destroy one or both. Google "1 wire communications".

Hakul0:
Okay, so it means arduino pin set to input mode doesn't provide closed circuit?

No it doesn't, how can it provide a path back to the first Arduino?

Hakul0:
Edit: Anyway, thanks. It just poped to my head that I just need to take a look at arduino schematic to understand how pins work.

I doubt that we be of any help at all. We are talking basic circuits here, go and look at basic flash light bulbs in seriese and parallel.