Am 07.11.2015 um 10:26 schrieb Colin Law:
>>
>> When you connect the ground line to data-, you have a different current
>> on the common ground/data- line because the ground line also carries the
>> supply current. So there's no way the receiver can successfully
>> eliminate line noise. Instead, it picks up supply current noise. It just
>> doesn't work.
>
> Does that argument hold in the case of parasitic power, where the data
> and common lines do carry the same current?
>
Your argument is good as you have two data wires and no ground
connection in that case. The same when the slave circuits are powered by
local batteries.
*BUT* this only works when the host ground is *not connected* to earth,
not even by a noise filter. As soon it is, your circuit isn't symetrical
anymore as one of your two data wires (the "ground") is tighter coupled
to earth than the other, and subsequently picks up noise more eagerly.
Which destroys the loop's cancellation effect.
You can try to minimize that effect by mounting your twisted pair
(again) on a pole and avoid shielding at any cost.
Kind regards
Jan
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