Without thinking too hard about it I have two thoughts.

1. The higher in frequency you go the easier it is to
separate out the signal.  So don't be too afraid of  higher
rates.  You might even want to create a "carrier" signal and
modulate that.   Actually you'd have to because an asynchronous
signal at rest looks too much like DC anyway.

2. You could put power on the line and do your signalling
by raising or lowering the voltage (or current) rather
than cutting it off completely.  This would leave the
line powered at all times.  Come to think of it, RS485
is differential or something which means you just see
polarity changes.  You could pull your power off
through a diode bridge, then.

Barry



At 06:55 PM 7/11/01 +1000, you wrote:
>Hi Everyone,
>
>I have an application that I'd like to use standard 3 core power extension
>leads (different connectors) for powering and sending data to many external
>units - I want to use these cables for cost and durability reasons.
>Unfortunately I can't really be too explicit about the actual application
>(it is commercial), but the units are spread over a large area (say up to
>200 metres) from the controller).
>
>I was thinking of an arrangement similar to television receiver "mast-head
>amps" where DC current is feed to the remote amp via the same line as the
>signal. Capacitors at each end are used to allow the video signal to share
>the DC line.
>
>In my case I'd like to use something like a RS485 network (at say 19200
>baud). The remote units have to be able to transmit, but I was going for
>half-duplex. The power will be about 24 volts DC.
>
>The DC power going to the unit can and will have some current pulses as
>things activate.
>
>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.
>
>Am I insane? :) Has anyone done something like this or could offer some
>advise?
>
>Is there any other way? Can I use a single wire + ground (leaving the other
>to supply power) to get reliable data rates of at least 19200 baud over
>these distances? I may be able to arrange the system into a "tree" structure
>where the nodes of the trees are "repeaters". There is one "master" on the
>network, and the remote nodes only reply to messages after they have been
>directly addressed. Perhaps a "current loop" system like MIDI?
>
>Thanks for any suggestions you can give.
>Ash.
>

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