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

Grant Edwards <grante@xxxxxxxx> wrote:
>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.

Frame ground is "chassis ground".

For example, Pin 1 on the 25 pin RS-232 connector is variously
labeled as "Chassis", "Protective", "Shield", or "Frame" ground.

Pin 7 is "Signal 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?

Well, the opposite side of that would be "somehow guarantee use
of receivers that can handle the existing common mode voltage
excursions". (Note that I am specifically not limiting that to
DC.)

The point is not that there is no DC connection to the outside
world, but that is has to be done *correctly*. And that is not
accomplished via a single ended one wire loop added to the
required 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'm not talking about DC. I'm talking about how to reduce
*noise* in a communications cable. Very few such cables
operate in an environment where there is no significant power
line influence, not to mention other noise sources.

If the ground system is properly designed, the noise in the
cable is reduced. If not done right, it can be substantially
increased. And it can exceed ground potential difference by
several times, too. There is no point in reducing the DC
ground potential from 10V to 0V, and in the process acquiring
20 VAC in the process.

>> 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.

The RS-485 signals are carried on a cable. Any influence on the
output which is not the input signal, is noise. It is
impossible to avoid (particularly 60 Hz power influence). One
reason RS-485 was only specified for 4000 feet is because it
isn't very immune to noise.

DC common mode offset is just another noise...

>>>> 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).

It was originally spec'd at 4000 feet. Better line driver
technology has extended that.

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

If he had significant experience with comm cables, which is most
likely, then your cable had a properly grounded shield and had
surge protection installed at both ends.

>> 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.

The RS-485 at each end is connected to the same ground that
the cable is connected to. But there should *not* be a
cable pair dedicated to connecting the two.

>> 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???

Ground that is not a signal path. E.g., common mode ground.

>> 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.

It's necessary to grasp the difference in what "ground" is,
and I'm not really aware of what your exposure to it is. I was
assuming that since you wanted to talk about RS-485 at the
hardware level that you'd probably been exposed to all of this,
but wouldn't necessarily have remembered it or found any of it
significant. In that case, simply using the vocabulary correctly
will enforce a proper set of definitions on the discussion.

But if you aren't into electricity for the sake of electricity...
yeah, this starts getting to sound like word soup! Sorry about
that.

>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.

What you are doing will result in equalizing the common mode DC
offset from different grounds. It is *not* the best way to do
it, simply because it can (not necessarily, but *can*) cause
just as many problems as it solves. Done properly, you don't
have trouble with 1) common mode offset, 2) induced AC and other
transient, or 3) lightening surges. But any of those can be
handled in other ways... which increase the potential for trouble
with one of the others. The shorter the cable run, and the fewer
hazards it is exposed to, the fewer problems. Hence it can easily
be done in ways that are not the best, and yet work very well for
years. But that doesn't mean those methods are "correct".

>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.

They should be. But you've got two different circuits you are
talking about too. One on each side of the optical isolation.
On one side the common mode range is narrow, and on the other is
is very high. The isolators are used over the cable, so ground
potential offset is not a problem (because the offset voltage
will never approach the common mode limit for the optical
isolators). On the other side, they are *all* connected to a
common ground, if they are properly engineered.

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

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