Russell McMahon wrote:
> I disagree. If you present me with a wire pair with differential
> signalling on it it is possible to extract the digital signal with no
> other reference. There can be some issues over "DC restoration"  but
> at every moment the relative polarity of the wires, and thus the
> intended signalling state, is clearly measurable. (Crossovers of
> course occur between symbols.)

Yes, but this doesn't work so well for transmitting RS485 since the common
mode range of a complient receiver need not be that large.

> An example of such a system is a telephone where the speech signal
> (here analog) is not essentially ground referenced.

And 10base-T ethernet where the both ends are transformer coupled and the
common mode voltage of the twisted pair is irrelevant over a wide range.


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