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

On 2005-06-16, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:

>>> 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.
>
> It's a frame ground.

Sorry, never heard that phrase before. I assuem "frame" and
"chassis" were the same. In the installations I've dealt with
the RS-485 common is certainly not chassis ground on either end.

> A signal ground would be a return path to ground for the
> signal. RS-232 has such a signal, and is "single ended",
> hence all signal lines share the same common ground return
> line.
>
> Differential ciruits are "balanced", the signal is between the
> two wires of a pair. Neither of them is at ground, so neither
> is called a "signal ground". The signal does not depend on
> any relationship to ground.
>
> If course, the receiver typically cannot tolerate a common mode
> voltage greater than some specified voltage. That that is not
> a signal voltage in any way. It just biases the devices out of
> their useful dynamic range.

And you've got to somehow guarantee that the recievers common
mode DC voltage is within spec. If the only DC connections to
the outside world are the A/B signal lines, how is that
accomplished?

> Optical isolators are are nice because they have a
> significantly higher "useful dynamic range". Otherwise, the
> signal on the cable is still the same.



>>> And any variation of current seen will be strictly noise.
>>
>>What current?
>
> Induced current into that ground wire accompanying the signal
> pairs. That's why you don't want it to be one small wire if
> it in the same bundle as the signal pairs.
>
>>> 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.

Are you talking about "inducing" a DC voltage?

>>I really don't understand what you're talking about. The
>
> It is a bit complex. Many people who work with cables don't
> understand it very well. But any one who sits down and
> studies it a bit can understand it. It isn't even rocket
> science! ;-)
>
>>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.
>
> Yes. You also have to be very careful about the currents
> induced into said ground connection. Do it the wrong way, and
> it adds noise to the signal pairs; do the right way and it
> will help cancel noise induced from the same source into those
> signal pairs.

What "noise"? I'm talking about controlling common-mode DC
level difference between the RS-485 transmitter and receiver.

>>> What kind of distances have you tried that with?
>>
>>A couple kilometers.
>
> That's a pretty good run for RS-485.

I think the spec is 10km for decent twisted pair and low baud
rates (<1M).

>>> 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.
>
> What kind of cable was this? Cable you installed, or telco
> cable?

Cable an electrician installed.

>>> 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.
>
> The cable still has to be grounded at both ends.

The shield may be grounded at one end or the other, but the
RS-485 common is not.

> That connects
> your two frame grounds together too.
>
>>> 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.
>
> It *should* be connected to earth ground at both ends. And that
> ground point *should* be a single point where *all* frame
> grounds for the entire building go.

What are "frame grounds" and what do they have to do with the
RS-485 bus???

> Typically equipment bays in a single row are strapped together,
> though sometimes individual racks will have separate grounds.
> There should be a single cable from each row (or each rack if
> some racks are isolated) to a common grounding point on each
> floor of a building.

You keep talking about "frame grounds" and earth and stuff.

The RS-485 systems I'm talking about are all optically isolated
from frame, chassis, and earth. If you don't connect the RS-485
commons together with the cable, then you end up with
common-mode voltages out of spec. Study all you want, that's
what happens in practice.

> Each comm cable entrance would be considered just like an
> individually isolated rack, and would have its own ground
> cable going to the grounding point for that floor. (What this
> says is that the cable is *not* connected to a rack. That a
> rack in one row is *not* connected to a rack in a different
> row. That no two rows share a single cable going to the
> ground point. But often racks in one row share a ground
> cable, and often equipments mounted in one rack share ground
> wires.)
>
> Both ends of the cable should be grounded in that fashion.

I don't care what you do with the cable shield, and frame
grounds and chassis grounds, but they aren't connected to
RS-485 common.

--
Grant Edwards grante Yow! Someone is DROOLING
at on my collar!!
visi.com
.

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