user8004: so that there a 2 wires rather than 3 to connect between the two for purposes of serial communication.  And the same question for 3 arduino megas on a common rail and using serial connections?

user368: All voltages are ground referenced, hence the ground is the only (supply) connection required. Remote modules can be self-powered and signals will still be discriminated as long as ground is connected.

8004: <p>so that there a 2 wires rather than 3 to connect between the two for purposes of serial communication.  And the same question for 3 arduino megas on a common rail and using serial connections?</p>
 368: <p>All voltages are ground referenced, hence the ground is the only (supply) connection required. Remote modules can be self-powered and signals will still be discriminated as long as ground is connected.</p>
