Re: RS485 is bidirectional does it mean it is fullduplex?
- From: Grant Edwards <grante@xxxxxxxx>
- Date: Thu, 16 Jun 2005 04:15:32 -0000
On 2005-06-16, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
> 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.
It doesn't carry any signal current, but it is the ground to
which the receiver's input signal range specs are references.
It's the ground that defines what "0V" is for the signal
inputs. I call that the signal ground.
> And any variation of current seen will be strictly noise.
What current?
> 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.
I really don't understand what you're talking about. The
differential receiver inputs can deal with only a few volts of
common mode DC voltage. You have to use a ground that's common
between the transmitters and receivers to make sure that the
common-mode DC voltage seen by the receivers is within spec.
> What kind of distances have you tried that with?
A couple kilometers.
> 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.
It didn't seem to.
> A proper ground on each would be much better.
Not allowed for safety reasons. The RS-485 transceivers at
both ends are optically isolated from earth.
> 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.
Nope. The cable sheild is earth ground at one end or the other
and can't be electrically connected to the RS-485 signal or
"ground" signals.
--
Grant Edwards grante Yow! A shapely CATHOLIC
at SCHOOLGIRL is FIDGETING
visi.com inside my costume...
.
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