You have to remember that "voltage" is a short name for "potential
difference" (sorry if this is not the usual english name, but it is the
closest translation from my native language), so you must have two
different electric potentials to have a voltage.

It is analog to altitude: an altitude difference makes a potential
energy in mechanics, you always have to say that your altitude relates
to a known level (sea level, for example or ground potential in
electricity).

Hope this helps.
Francisco

Ben Jackson wrote:

>I'd like to have a serial (one way would be okay, two way better) interface
>between two PICs with different power supplies.  One would probably be
>on a regulator powered by a wall wart and the other would be battery
>powered.  The battery powered device will be on a rotating platform,
>so I'd like to connect just through the metal axle it's rotating on.
>
>I'm a software guy, not an EE guy, so if this idea is stupid for some
>obvious reason, please enlighten me.  Here's my circuit (one half, one
>for each side):
>
>                 VCC
>                  +
>    TX    R1_     |
>    o----|___|----|-----.
>          100     |     |
>                  |     |  C1
>                   \|   |  ||  WIRE
>                Q1  |---o--||---o
>                   <|      ||
>    RX            |       1uF
>    o-------------o
>                  |
>                 .-.
>              1k | |R2
>                 | |
>                 '-'
>                  |
>                 ===
>                 GND
>
>Q1 acts as an envelope detector and provides a logic level signal to RX.
>TX is high-Z when receiving.  When transmitting, RX sees what got sent
>and the 'slave' device can release the line if it sees an error.
>
>TX needs to be driven without a DC component, which can probably be done
>by manchester encoding bytes before sending them via the UART (start/stop
>are already balanced).  It might be necessary after a quiescent period
>to drive 0xAA (or 0x55) to frame the message and 'balance' the line.
>
>--
>Ben Jackson
><ben@ben.com>
>http://www.ben.com/
>
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>.
>
>
>

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