> > Yes, it's possible, but it isn't RS485. RS485 is differential 
> > signalling
> > within a defined common mode voltage. That way it can operate 
> > without
> > transformers & opto-isolation being required.
> 
> I agree with you. But ...

But ..... It appears, you miss the point completely. 

It is _possible_ to convey information without a ground reference and 
ethernet and telephone are fine examples of that. 

However, the subject is RS-485. RS-485 is an interface where 
differential signalling is used and the ground reference is _required_ 
and specified. 

It's an important aspect of the spec as it makes it more robust than 
RS232, but less so than a fully isolated differential bus. 

It needs to be emphasized as it is one of the most common RS485 
misconceptions and is probably the number one reason why the 
interface chips are socketed.

> > An example of another totally unrelated system is a fibre-optic
> > link. No doubt there are others.
> Here I disagree :-)

Why ? Just as your example, it fits into the group "interfaces that aren't 
RS485"

> Not wishing to argue (especially much :-) ) but I was trying to make a
> technical point for the edification of others and i still think it's
> valid. I'm sure we don't actually disagree on the technical aspects
> once we get the background cleared up.

Your point is perfectly valid and I have no technical argument. Perhaps 
you should change the subject to the one I have suggested above.

Steve.



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