The third wire is a must for outdoor communications.
It has only the purpose of ESD module/bus protection. For indoor it is
not necessary.
There is a limit for maximum ground potential on receiver. Below that
value the common mode bus swing allows a correct communication. With
other words, a two wire bus will work also OK.

On 1/7/08, wouter van ooijen <wouter@voti.nl> wrote:
> We had a discussion on a Dutch forum about profibus (the RS485-based
> version) being 2 or 3 wires, that is: does it need a
> ground/reference/earth wire or not. I was convinced that it needs the 3d
> wire, but reading some documentation leaves me in doubt.
>
> Does anyone know the definitive answer? Can profibus couple devices
> without a 3d wire (either in the cable or implicit in the grounding of
> the various nodes)? If so, how does it cope with ground potential
> differences in relation to the input range of the receiver chips?
>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: www.voti.nl/hvu
>
>
>
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