> I have a client who is wants to do a cheap..ok...inexpensive 
> communications between two PICs that the only connection between them is a 
> 12VAC set of wires.  They only need to transmit a few bytes now and then, 
> and RF
> is too expensive, and may not have range.  The 12VAC also is the supply 
> for the end device.

> The solutions I see are either imposing the data on the AC much like 
> LONworks or X10 does, or actually kill the AC and then transmit the data 
> over the pair.

What bit rate is acceptable?
What is the AC used to drive?
What voltage?
What frequency?
What power level?

Tone near zero crossing will probably not undergo as much loading as 
elsewhere if AC line is rectifed to DC for power.

If you can add filters then tone + AC can coexist easily. At a high enough 
frequency the tone filter can be small & cheap.

If AC frequency bit rate is OK then a bit per half cycle may be easy enough 
by phase shifting the zero crossing point - eg adding or subtracting voltage 
so zero crossing time is shifted. This is arguably equivalent to adding DC 
as was suggested - ie you just add a small modulated low frequency signal to 
the AC in phase with the AC. Mean DC level may have to be zero depending on 
the AC load.


        Russell McMahon

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