Re: RS485 is bidirectional does it mean it is fullduplex?

Grant Edwards <grante@xxxxxxxx> wrote:
>On 2005-06-15, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
>
>> RS-485 is a differential (balanced) system, and there is no
>> signal ground connection. The cable used might well include a
>> frame ground, but that is for noise induction cancellation, not
>> signal ground.
>
>In my experience, the third/fifth wire is required to limit the
>common mode voltage seen by the receivers. In that respect, it
>is a signal ground. IIRC, most receivers them can only tolerate
>8-12V common-mode DC. If you let the two devices float with
>respect to each other, you can get fairly high common-mode
>voltages and the recievers will stop working.

That is a frame ground, and not a signal ground. It will carry
no signal current at all.

And any variation of current seen will be strictly noise. The
trick is to get the induction into the ground wire to then, in
the cable between the ground wire and the signal pairs, cancel
the induction into the signal cables.

What kind of distances have you tried that with? I'd expect
that across the room or around the bend might be just fine (and
wouldn't be needed because the offset between the ground systems
wouldn't be high enough to be a problem). But if this went down
the road 3000-4000 feet, and you actually did get a ground
offset high enough to be a problem, using a single wire in the
same cable to equalize the ground potential should add enough
noise to your cable run to make it a real problem.

A proper ground on each would be much better. And a cable
sheath that is properly grounded at *both* ends, to the same
single point building ground that the RS-485 equipment is tied
to, would be the preferred way to make sure there wasn't too
much common mode difference.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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