Michael Watterson wrote:
> You can easily design an R485 interface that doesn't need a ground wire. 
> In fact adding a ground wire connected at both ends could result in 
> exceeding common mode limits in certain circumstances. If the common 
> mode voltage is high then you will have a problem of "ground " loop 
> current. Designing a better interface is a better solution.
>   

Well, you seem to flying in the face of recommended and accepted 
practice then.

As has been said previously, non isolated (from mains earth) RS485 
interfaces can drag ground currents through the shield.  A lot of the 
time, this is low enough that the connection suffers no ill effects.

If you use RS485 as a 2 wire medium, the reference will usually be the 
mains earth.  Yes, you have two data wires, but that mains earth is 
still providing the reference. (3rd wire)

I'm ready to be convinced if you can show a solution for RS485 that uses 
only 2 wires, and no reference.  Mind you, you don't get any points for 
presenting a solution that uses an LED as the receiver!
David...

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David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
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