> I was thinking of having a Ground (0v) wire then the remaining two wires
> would deliver the DC current and form the "loop" for the RS485.

There are lots of possibilities.

(1) Use a old fashioned  "Phantom" circuit. The two "live" conductors are
used with a center-tapped transformer at either end
to send or receive a differential signal with the center-tap being used for
the power feed. The earth wire is the DC return.
(The "transformer" would only need to be a ferrite ring.....The magnetic
field from the DC cancels out due to the center-tap)

(2) Use two wires to feed the power (via bridge rectifiers and cap). The
power is transmitted by a "sort-of" AC signal.
The presence of a reversal signals a "0" and the absense of a reversal
signals a "1".
Would be only uni-directional however.

(3) Use normal DC (or even AC)  to power the devices, but use simple filters
to allow an audio (or ultrasonic) signal
to carry the data.

(4) Use existing off the shelf mains-signaling modems to carry the
data........(search for X-10)

(5) I vaguely remember there is a class of self-powered CAN-bus devices.
Anybody?

........................ Zim

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