Michael Watterson wrote:
> Ruben J=F6nsson wrote:
>   =

>>> We make DMX LED controllers - the DMX front-end is a 65lbc179 that =

>>> feeds a fast opto-isolator.  The front end is powered with an =

>>> isolated DC-DC converter.
>>>
>>> The 'lbc179 ** REQUIRES ** that you constrain its input signal to =

>>> within its common-mode range.  The easiest way to do that is to =

>>> require that the installer uses shielded cable to transport the DMX =

>>> signal, and to connect the drain wire at both ends.
>>>
>>> We don't have ground-loop problems because the DMX front-end is =

>>> completely isolated from the rest of our circuitry.
>>>
>>> dwayne   =

>>>       =

>> So the shield is not connected to chassie/earth?
>>  =

>>     =

> It would obliviate the point of isolated DMX if it was.
>
> This is the point I'm making. In fact if you are going between buildings =

> it might even be illegal in some cases to interconnect the earths.
>   =


But that's the point we're making.  You're not interconnecting the =

earths. You're connecting the common reference.  That reference may or =

may not be mains earth.  There's no requirement for it to be earth - it =

just needs to be a common reference.

In the case of isolated RS485, do you have a common reference (often the =

shield)? Yes!  Is it mains earth? Not if both ends are truly isolated.

As long as the I/O of only one device is earthed and the rest are =

isolated, there's no problem. Ideally they are all isolated from mains =

earth so that a single fault (common contacting earth) will have no ill =

effect.

So, we're all agreed that differential signals (RS485 for example) =

travelling between places of differing earth potentials should be =

isolated.  That isolation doesn't negate the need for the common =

reference conductor, it just means the I/O sections are floated with =

respect to earth.
David...

-- =

___________________________________________
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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