At 11:16 AM 6/3/99 -0600, you wrote:
>Modulate the DC maybe....FSK like ?
>
>Suppose I have a situation where I want to send power and control
>signals to a device. Suppose too that I want to do this all with
>2 wires (power and return). Is it at all feasible to send the data
>on top of the power line, and have it separated at the destination?

There are a number of companies doing this on top of 4-20 mA control and
signal loops.  There is even a standard to cover the protocol--uses
standard FSK signals on a high frequency carrier.  Simple capacitive
coupling at either end, and the frequency is high enough to not interfere
with the "DC" level process signals.

Check out the magazine "control engineering"--I think they have a web page
too.  The name "HART" comes to mind for the protocol, but I wouldn't swear
to it.


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William K. Borsum, P.E.
OEM Dataloggers and Instrumentation Systems
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