Rs485IsBidrectional

TableOfContents
Category:Sasecurity
=== RS485 is bidirectional does it mean it is fullduplex? ===
http://groups.google.com/group/comp.arch.embedded/browse_frm/thread/22511591b16678dd/4184893c4c3f9888?lnk=st&q=RS485&rnum=1#4184893c4c3f9888
Rs485Post104
==== posts ====
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.
Hence it actually is a 2-wire or 4-wire link.
> - RS-422 is also multidrop (10 max drops, IIRC).
> - RS-422 is often implemented using a pair of RS-485 devices these days,
>since the RS-485 spec is superior to the original RS-422 spec. This confuses
>things slightly, but means that RS-485 tends to be used in both 3-wire and
>5-wire configurations.
The practical problem is that the receiver is made of bipolar transistors and hence require some kind of base current. This current can be supplied through a more or (usually) less clean common ground connection or the differential signal path can be treated as bipolar current loop with the "fail safe" termination resistors supplying the small base current to the transistors. 
The specification includes the maximum ground offset voltage permissable. (I don't recall what it is, or how realistic it is for common 4000 foot runs of twisted pair cable.) But attempting to supply a separate "signal ground", for example
via a single separate cable pair, will almost certainly result in a poor frame ground connection instead! That is not a good idea over a 4000 foot loop, but it probably wouldn't make any difference at all if the cable run is relatively short. (Unless it is between two locations on separate power distributions, and one of them has a bad ground.) 
I have the same experience. I ran an RS-485 network in a factory invonment, using one twisted pair for data and two other pairs to distribute the power. Total length appr. 500 meters (1500 feet). Never ever had a problem, all boards shared the same power supply ground. When a link had to be made between two buildings, the power was split and only both datalines were
connected. At both ends, the systems we ground. Every thunderstorm blew the Tx chips. You *absolutely do* want a good ground connect at both ends.
The only think you absolutely want is some means to prevent excessive common mode voltage outside the range of the TX chips. How you achieve this, is not important. This could easily be done with the "third wire" to connect both
commons (I explicitly won't call it "ground"). 
==== Paul Keinanen ====
On Wed, 15 Jun 2005 15:40:22 -0800, f...@barrow.com (Floyd L.
Davidson) wrote:
>The specification includes the maximum ground offset voltage
>permissable. (I don't recall what it is, or how realistic it is
>for common 4000 foot runs of twisted pair cable.)
It would be unrealistic to assume that the grounding electrodes of two
separate buildings would stay within the -7.. +12 V common mode range
at all times (especially during thunderstorms), so optoisolation
should be used to keep the grounds separate. The real question is, is
the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
a fiber optic cable be used instead.
>But attempting to supply a separate "signal ground", for example
>via a single separate cable pair, will almost certainly result
>in a poor frame ground connection instead! 
You do not want a good ground connection at least not at both ends in
this situation. (Floyd L. Davidson has other views ). It is quite common to use a 100 ohm resistor between
the signal ground wire and the frame ground (PE) to limit the loop
current. Preferably, at least one end of the connection should be
floating (optoisolated) in which case the signal ground wire goes only
to the C terminal of the floating interface, but there is no
connection between the signal ground and frame ground (PE) at that
end.
=== Paul E. Bennett ===
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Floyd L. Davidson wrote:
> "Meindert Sprang" <mhspr...@NOcustomSPAMware.nl> wrote:
>>"Floyd L. Davidson" <f...@barrow.com> wrote:
>>> >It would be unrealistic to assume that the grounding electrodes of two
>>> >separate buildings would stay within the -7.. +12 V common mode range
>>> >at all times (especially during thunderstorms), so optoisolation
>>> >should be used to keep the grounds separate. The real question is, is
>>> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should
>>> >a fiber optic cable be used instead.
Having designed a comms interface for one of the grottiest, noisiest
working environments I have ever come across and ensured that it would
survive the ordeal, I can confirm that management of the energy is where
you need to focus in order to prevent such failures occurring.
>>> I don't have a lot of experience with thunderstorms, so I can't
>>> really comment on that. Otherwise, that simply is not true.
You can get similar problems with a 20MA single turn secondary running from
a primary at 1kV which, when the circuit unexpectedly disrupts, releases
very large transients back through almost all the systems. Even the most
sensitive of our inputs (<10nV) is protected for at least 8.4kV short
duration transients. We only ever connect screens at one end (usually the
instrumentation cabinet) as, with that much energy around, the three
different earths can separate by as much as 75V for a short time. It is not
fair on cable screens to carry the sort of current that would imply (it
would probably cause a fire anyway).
>>I have the same experience. I ran an RS-485 network in a factory
>>invonment, using one twisted pair for data and two other pairs to
>>distribute the power. Total length appr. 500 meters (1500 feet). Never
>>ever had a problem, all boards shared the same power supply ground. When a
>>link had to be made between two buildings, the power was split and only
>>both datalines were connected. At both ends, the systems we ground. Every
>>thunderstorm blew the Tx chips.
You probably need to look at including more serial resistance and heavier
diode clamping at the intgerface connections. Also, control the impedance
of the connection of your power supply 0V to the chassis within each
module. Really think about the path that the surge/transient energy is
going to take.
> The obvious conclusion to be drawn is that it was not designed
> properly, almost certainly due to a lack (then and now too
> perhaps) of understanding about what caused the damage.
I'd concur with that conclusion.
> That has virtually *nothing* to do with signal ground, frame
> grounds, power distribution, or for that matter common mode
> voltages.
> It has to do with induced voltages from static discharge
> (lightening) and protection against such surges. You mentioned
> nothing about that, and I assume there was none provided.
Actually, you have to consider both the grounding issues and the
static/transient discharge issues in concert with each other. Some things
that may be very good for one are absolutely awful for the other. If you
need to minimise noise pick-up as well then you have quite a bit of work to
do. When I tell you to manage the energy, it is all of the energy you need
to consider (which includes your desired signal as well. PSpice can be a
tremendously good tool for sorting these issues out.
>>> You *absolutely do* want a good ground connect at both ends.
No, just the one end and preferrably the instrument rack/master end of the
cable. In the rare circumstance that you cannot isolate the screen at the
remote end put a break in the screen at a convenient point close to the
remote end and make sure it stays broken.
- Hide quoted text -
- Show quoted text -
>>The only think you absolutely want is some means to prevent excessive
>>common mode voltage outside the range of the TX chips. How you achieve
>>this, is not important. This could easily be done with the "third wire" to
>>connect both commons (I explicitly won't call it "ground").
> It *is* important. And it isn't, as your example above
> demonstrates, just common mode voltage equalization. This
> "third wire" would have done *nothing* to benefit your situation
> above.
> The point is that if frame grounds are connected properly the
> *normal* amount of noise on the cable is reduced. If done the
> wrong way the noise will be increased. In fact, *either* way
> will probably work most of the time! That leads to a lot of
> statements to the effect of "we did that, and it worked". But
> if you have a nationwide network, with millions of examples,
> those which are done wrong do show up as being where 90% of the
> maintenance costs go.
By way of re-inforcing the point here, consider that the screen is only
meant to act as the notional extension of a metal enclosure out along the
wires. It should not carry any current at all (except maybe for the tiniest
leakage current capacitively coupled from the signal wires - and even that
should be miniscule). Dealing properly with the screens is a safety issue
as well as a circuit protection and noise reduction issue.
I shall have to look up the equipment build standard that details all these
issues and post the number. All embedded systems engineers should really
know this stuff anyway.
-- 
********************************************************************
Paul E. Bennett ....................<email:/...@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/
******************************************************************** 
=== 12 ===
=== 17 ===
Floyed could be right on the copper rod thing. This is how MeshNetworking is grounded.
At every point where sections of cable is spliced, the
shield on both sides of the splice is bonded, and grounded
to an earth ground. That is generally a copper rod driven
into the ground. 
post18
You are presumably talking about the static
drain and foil shield that wraps around all
the pairs. I've seen the cables and I'll
certainly acknowledge its existance.
OTOH, when I think of a telephone cable, I
think of a pair. The pair is *not* grounded.
In fact, FCC part 68 is quite clear on it
being balanced and the balance having to be
maintained.
As to the splice points, no, not all of them
are grounded. They are grounded where the
cable loops into a building, but not necessarily
on the pole.
See Meshgrounding think it is HardWare
Post 19 When the cable is installed, at each point where a new section
(roll) is started, the cables are spliced and grounded. I think you are referring to there being a ground at the network
interface on a building, but that is not grounding a shield. It
grounds a surge protection device, not the cable. 
post20: Even if there would be a small voltage difference between the ground
potential around stations A and B, the current would still be limited
by the grounding resistance at each electrode. This resembles the
common grounding practice which used in quite few RS-422/485 systems,
in which the shield is not connected directly to the frame ground, but
instead a 100 ohm resistor is used to limit the "ground loop" current.
While your claims about grounding the telephone cable at every 1-2 km
is believable, it does not justify your claim that RS-485 cables can
be directly grounded (without current limiting resistors) at both ends
in all cases. 
post43 You keep saying this, but it's wrong. If you won't believe us, use Google to
do some searching. A few seconds of searching turned up:
 http://www.hw-server.com/rs232_signals.html
It's about RS-232, but the grounding issue under discussion is the same
(except it's worse with RS-232, since ground differences are seen as
signal). I'll quote a section:
1 Protective Ground
Name: AA
Direction: -
CCITT: 101
This pin is usually connected to the frame of one of the devices, either the
DCE or the DTE, which is properly grounded. The sole purpose of this
connection is to protect against accidental electric shock and usually this
pin should not be tied to Signal Ground.
This pin should connect the chassis (shields) of the two devices, but this
connection is made only when connection of chassis grounds is safe (see
ground loops below) and it is considered optional.
Ground loops are low impedance closed electric loops composed from ground
conductors. When two grounded devices are connected together, say by a
RS-232 cable, the alternating current on the lines in the cable induces an
electric potential across the ends of the grounding line (either Protective
Ground or Signal Ground), and an electric current will flow across this line
and through the ground.
Since the loops impedance is low, this current can be quite high and easily
burn out electric components. Electrical storms could also cause a burst of
destructive current across such a loop. Therefore, connection of the
Protective Ground pin is potentially hazardous. Furthermore, not all signal
grounds are necessarily isolated from the chassis ground, and using a RS-232
interface, especially across a long distance, is unreliable and could be
hazardous. 30 meters is considered the maximum distance at which the
grounding signals can be connected safely.
<<
I repeat: what you keep proposing makes no sense. I can only presume that
you're confusing this issue with something else entirely.
Consider what happens when lightning strikes one location - local ground
potentials go *nuts*. Your (apparent) connection between two local ground
systems will try to connect them together, and it will lose. Bigtime.
You've mentioned telephone system cabling: I'm no expert, but I had thought
telephone cables were current-driven loops, with the power supplied by
a -48V supply and a local ground *at one end*. Perhaps this is only the
subscriber loop. For *very* long trunk cable runs, I have seen what you
describe - shield connected to local ground via a dirty great grounding
pole - but I had understood that the series impedance of the shield was
quite high, which is what saved it from becoming a very long and expensive
fuse. This is not what we're discussing here (the need for all RS-485
signals to be within a defined common-mode range).
Steve
http://www.fivetrees.com 
=== lanarcam ===
http://www.spheresystems.com.au/RS485.html
=== 55 Ground only on one side or include resistor ===
=== 72 Must ground both ends for dangerous volts ===
=== 77 Common mode voltage floating ===
> 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.
post78 :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. 
A well grounded cable sheath, at both ends, almost certainly
will correct the problem. Which is to say, I've never seen it
fail, but have seen instances where it was not as good as we'd
have liked to see. Invariably that has to do with inability to
get a good ground connection. But it *is* good enough for
RS-485, as long as the ground systems for equipment on both ends
are in fact connected to the ground system the cable is attached
to. Multiple grounds won't do, even if they are relatively
good. The cable and the equipment both must be tied separately
to a single building ground. 
=== Groundwire picks up noise ===
Thought I'd throw my 2 cents in. I used to design systems like this back in
the 80's and you should be aware that there is always the potential that you
could cross power grids. I worked on a project where the same company had 2
buildings across the street from each other and they were on seperate power
grids. The grid different was not in volts, but in 10's and 100's of volts.
It was something we never expected but it's real, and your ground wire won't
protect against this.
dbrown
"Floyd L. Davidson" <f...@barrow.com> wrote in message
news:87u0jzjav2.fld@barrow.com...
- Hide quoted text -
- Show quoted text -
> Grant Edwards <gra...@visi.com> wrote:
>>On 2005-06-15, Floyd L. Davidson <f...@barrow.com> 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.
=== 81 ===
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. 
=== 82 Signal ground and differential signals ===
It's a frame ground. 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.
Optical isolators are are nice because they have a significantly
higher "useful dynamic range". Otherwise, the signal on the
cable is still the same. 
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.
The cable still has to be grounded at both ends. That connects
your two frame grounds together too. 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.
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. 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. 
=== 83 ===
I read this to mean that if the common modes are not grounded together the common mode volts will 
exceed +-7volts blowing the chips.
The shield may be grounded at one end or the other, but the
RS-485 common is not. Grant Edwards means by this that the optically isolated circuits should be connected to common on both ends of the RS485 circuits.
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. 
=== 84 ===
 think RS485 problem is 2 :
 *) Noise over long distance cable and
 *) Faulty data because of different node voltage reference
To prevent noise over long distance cable, we can earth one side of the
cable shield (refer to Ott, Henry, Noise Reduction Techiques in
Electronic Systems).
To prevent faulty data because of different node voltage, we can use
common line. But since RS485 is a differential mode protocol, we can
use either A or B line as our common line.
However, if we connect using this fashion, when the master is in the
idle mode, there will be floating voltage between A or B line (since
nobody is driving the bus). Therefore, we connect pull-up/pull-down
resistors in the A and B line (to give at least definite voltage level
when nobody's driving the bus).
To make this "definite voltage level" same at the receiver /
transceiver point, we need to earth their voltage reference node at the
both side. I think everyone is correct here. Just the naming convention that makes
confusion.
=== 85 replies to 84 ===
> May I summary this ?
> I think RS485 problem is 2 :
> *) Noise over long distance cable and
> *) Faulty data because of different node voltage reference
> To prevent noise over long distance cable, we can earth one side of the
> cable shield (refer to Ott, Henry, Noise Reduction Techiques in
> Electronic Systems).
> To prevent faulty data because of different node voltage, we can use
> common line.
Yes.
> But since RS485 is a differential mode protocol, we can use
> either A or B line as our common line.
I don't see how you can use A or B as a "common" line. 
> However, if we connect using this fashion, when the master is
> in the idle mode, there will be floating voltage between A or
> B line (since nobody is driving the bus). Therefore, we
> connect pull-up/pull-down resistors in the A and B line (to
> give at least definite voltage level when nobody's driving the
> bus).
What happens when nobody is driving the bus is a (mostly)
different issue. Usually solved by pulling one line to the
reference/common node and the other to 5V (with respect to the
reference node).
The reference node connection between the two ends is required
to keep the A/B signal values being output by the transmitter
within the common-mode voltage range spec for the receiver.
> To make this "definite voltage level" same at the receiver /
> transceiver point, we need to earth their voltage reference
> node at the both side.
You don't need to earth either one, as long as the
transmitter/receiver reference nodes at the two ends are tied
together.
=== 86 Ground loops reply to 85,84 ===
=== 87 Impedance at both ends needs resistor/cap in parallel ===
post88: No, the whole idea is that you *want* that current to flow. In particular it is the 60 Hz power line induced current that makes up most of the current flow. Keep in mind that the whole idea is to allow the current flow to generate an equal and opposite induction into the signal pairs. 
post89 Paul disagrees: I don't think anyone wants 53A being carried by the shield of a signal
cable. I know I certainly wouldn't like to see that happen. This is the
sort of thing that we have been trying to get you to see as a real risk for
some of the systems we are dealing with. I have even seen scope leads fry
due to someone not respecting the earthing scheme in place (on a high power
motor drive).
> Keep in mind that the whole idea is to allow the current flow to
> generate an equal and opposite induction into the signal pairs.
If you are speaking of twisted pair screened cable (the type in very
extensive use in my workplace) then I thought that we had already agreed
that the twist in the cable cancels out most of the noise of a signal
because the noise is a common mode across the pair. The screen, in this
case, really does extend the shielding of the enclosure out to the plant. I
am certain that these principles are in many of the books on
telecommunications and electrical theory. 
post90: >The screen, in this
>case, really does extend the shielding of the enclosure out to the plant. I
>am certain that these principles are in many of the books on
>telecommunications and electrical theory.
You do realize that the "shield" effect, at 60 Hz power line
frequencies, reduces noise in a cable by about 0.04 dB? In
other words, it has no effect at all. I'd have to look up the
numbers, but it essentially has little effect at any frequency
below about 10KHz. Obviously the shield on a telephone cable is
*not* there to reduce noise simply by keeping stray
electro-magnetic fields out of the cable. 
=== 94 ===
>> 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. 
=== 96 ===
=== Post 97 ===
 | Opto | +-----+ \
 uP|-Tx--| Iso |--Tx--| 485 |--|---A-----/ /--
 |-Rx--| |--Rx--| xcvr| |
 |-dir-| |-dir--| |--|---B-----/ /-- Mirror
-++-+ | | +-++--+ | Image
 |`--Pwr--| Pwr |-IsoPwr-'| |
 `---Gnd--| Iso |-RScom---+-----|---com---/ /--
 | +------+ /
 /// +--+--------/ /--
 |
 ///
Or something pretty close to that. In some pieces of gear the
uP was floating (uP ground was RS485 common). In all cases the
RS-485 interface was floating with respect to earth/chassis
ground.
I don't remember if people were told to ground the shield at
one end or both. There were A, B, common, and chassis ground
lugs on each of our bits of gear (in addition to some other
stuff unrelated to the discussion).
> You don't need to use a cable pair to connect the common mode
> grounds on the RS485 drivers, because the shield should have
> a separate ground strap going to a building ground at each end.
> The RS485 common mode ground points should also be connected
> to those same building grounds, with a separate cable.
That's sure not the way I remember it in the process control
systems I worked with (it's been 6-7 years). The RS-485 bus was
pretty much always floating w/ respect to ground, with A, B and
a floating common wire between the two RS485 transceivers.
There may have been people that grounded the RS-485 common node
at some point, but it was expected to work if it was floating.
=== Links ===
 * http://www.hw-server.com/rs232_signals.html
 * http://www.spheresystems.com.au/RS485.html
 * RS485 , ["RS485"]
 * Rs485Post104