RS485 is bidirectional does it mean it is fullduplex? - Page 3

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Re: RS485 is bidirectional does it mean it is fullduplex?
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That is *not* true in the situation being discussed, where there
is a cable long enough to have the two ends connected to separate
ground systems and equipment supplied by separate power systems.

See the other article I posted explaining it in detail.  In
short, grounding at one end is an expedient design that works
for *short* cables within a single building.

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That is not correct.  As noted above, see the other article, as
I'm not going to repeat it all in multiple posts.

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Telecommunications engineers, working with longer cable runs,
need to know even more... :-)

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

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

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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



Re: RS485 is bidirectional does it mean it is fullduplex?
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I keep saying it because it is absolutely true.

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In fact, that discussion is a subset, not at all similar to long
comm cables.  It helps to have a good understanding about what
causes a "ground loop" and what causes noise induction in long
cables.

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...


Do you understand what that is saying?  It's not exactly a good
example!

In fact, it's a bit of mumbo jumbo and clearly the person who
last edited it has no idea what a ground loop actually is!  But
one little part almost got the point: "induces an electric
potential across the ends of the grounding line".  That part is
right, but it is *not* from "alternating current on the lines in
the cable", and "induces" is the wrong term too.

Here's a ground loop:

       Signal Source

             o
             |
           Rload
             |
             +---------> connection =======//======= <------+
             |           to cable                           |
           Rwire         shield                             |
             |                                              |
             |                                              |
           -----  Earth Ground                            -----
            ---                                            ---
             -                                              -


Okay, there are three sources of current that affect voltages
across the two resistors.  The "signal" is listed as "Signal
Source".  Assume that is a current limited source, just to make
this more obvious.  There is induced current in the cable
shield, and there is current from the ground potential
difference between the two Earth Grounds.

All currents contribute to the voltage across Rwire.  The
voltage across Rload is affected by the voltage across Rwire,
and thus the voltage drop across Rwire and across Rload.

Current through the shield affecting the voltage across Rload is
noise.  If that is significant, it is commonly said to be a
"ground loop".  And obviously lifting the ground from the right
side will stop the current flow, and thus "cure" the problem.

However, so will reducing the resistance of Rwire, the common
ground wire.  Hence you have two choices.  One is very easy and
has no detrimental effects for *short* cables inside a building
that has both ends on the same AC power distribution and a
common ground or very low potential difference between two earth
grounds.

Long runs of comm cable do not fit that description, and
therefore use the technique allowing only *very low impedance*
common ground connections.  (The only common part would have to
be very large, low impedance, cable that is preferably short.)

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It's simply good engineering, instead of magic.

(Where "magic" is defined as stuff that isn't understood
understand.)

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I mentioned not having a great deal of experience with
lightening.  I should probably qualify that.  I have only
a few years of experience with lightening, as opposed to
decades with the rest of this.

What you say above isn't true.

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If you have no expertise in this topic, please *do not* pontificate.

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Wrong.


Wrong.


Wrong.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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You'll forgive me for laughing out loud here.

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
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Figured I should clarify that.

Floyd, I'm really trying hard (now) to follow you. I've taken every diagram
you've drawn, viewed it with a fixed-width font, and tried hard to
understand your point. (Perhaps I'm misunderstanding something.) I've gone
back and re-read all your posts. I'm utterly convinced you're sincere,
you're passionate, and you know your stuff. I'm equally convinced that said
stuff does not include RS-422/485, or medium-haul datacomms in general. I
really believe you're missing the point (common-mode).

I'm also enjoying this thread. Keep 'em coming ;).

Steve
http://www.fivetrees.com



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


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IMHO, This is related to the difference between mechanical
and electronic ground. In the systems we made years ago,
the electronic ground was isolated from the mechanical
ground. A jumper was provided that enabled the customer
to tie both grounds if he wished. He was responsible
for the earthing of the system.

You will find perhaps the following useful:
(http://www.spheresystems.com.au/RS485.html )

Ground and Earth Connections
The grounding and earthing connections in RS485 provide two
separate functions.

The second is related to safety and the first to establish a
reference voltage

Safety
The RS485 *cable screen* must be bonded to the protective earth
system of a building at *one point* only.

The cable screen must be electrically continuous throughout the
entire cable run

Voltage Reference

The screen of the RS485 cable establishes a ground reference
voltage for the RS485 signal conductors. For this reason the
cable shield must be connected to the *ground reference* for
*each node* on the network.

It is not acceptable practice to tie the node ground reference
to the building protective earth as this will introduce
electrical noise into the system and may lead to equipment
damage in the event of electrical fault currents


Re: RS485 is bidirectional does it mean it is fullduplex?
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For Steve's benefit, you should have included one more section:

  Installation throughout Multiple Buildings

  There are two separate installation procedures depending on
  the type of electrical earthing system.

    MEN System at one building

      In this case the installation may be made as though it
      exists in a single building. The cable shield will be
      continuous throughout the installation.

    MEN System in each building

      In this case there must be electrical isolation between
      buildings with different MEN systems. Each building is
      wired as a separate and complete installation with the
      cable shield tied to the building protective earth at one
      point.

That is in *each* building.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
On Thu, 16 Jun 2005 18:49:55 -0800, snipped-for-privacy@barrow.com (Floyd L.

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In the TN-C electric distribution system, the "Signal source" is the
mains phase voltage (100-240 V depending on country), Rload perhaps
10-1000 ohms, Rwire perhaps 0.2 ohm. An AC current up to 25 A could
flow though Rload and through Rwire. The voltage drop across Rwire
would be up to 5 V.

Before connecting the data cable, there would be a 5 V ground
potential difference between the left and right side equipment. When
the cable shields are connected, a part of the up to 25 A flowing
through Rload will be diverted through the data cable and into the
right hand equipment ground.

The magnitude of this "ground loop" current depends on the ratio of
the Rwire compared to the combined resistance of the cable shield and
the right hand side equipment grounding resistance. Especially, if the
right hand side equipment is close to the main distribution panel, the
total path resistance is dictated by the shield resistance. If the
alternate path total resistance is as low as Rwire, more than 10 A
could flow in the cable shield, which could even cause a fire hazard.

Paul
 

Re: RS485 is bidirectional does it mean it is fullduplex?
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You should also realize that 'signal source' above includes a
myriad of possibilities, including lightning, pools of acid forming
batteries, whatever.  We can never really get at a true ground
except through some sort of impedance.  Luckily most (but not all)
of our controllable signal sources are imposed between the marked
signal point and the junction of Rwire and Rload.

Also the effects of Rwire, and the IR drop across it, can be
present at either the near or far end, or both.  Those R things are
not necessarily resistive, nor passive.

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: RS485 is bidirectional does it mean it is fullduplex?
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No.

"Signal Source" means whatever your *desired* signal is.  Not
the AC power distribution... simply because how much that is
perterbed is of no significance.

Across Rload...  is your Hifi Amp, your telephone set, the PA
system, computer data, gas tank level sensor output, whatever...

However, we could add that and many other sources of voltages.
It makes the diagram and the discussion more complex, but the
one particular example you chose is an excellent one because
it is commonly seen and often has significant effect.

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But lets not suggest that it would be a significant part of that
25 A.  We are *not* describing ground *faults*, but ground
*loops*!  But, in a typical arrangement, it could certainly have
enough effect to cause one heck of a 60 Hz hum in the "signal"
as described above.

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We need to make the distinction that this is a ground system fault,
not an inherent characteristic of the indicated connections.  And
it can happen whether the arrangement is properly designed or not.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
On Fri, 17 Jun 2005 09:32:32 -0800, snipped-for-privacy@barrow.com (Floyd L.

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You seem to have a very limited view of various grounding practices
used in the world. The TN-S system with separate N and PE conductors
is not the only system used.

While it is true that in a TN-S system, large currents will flow in
the signal cable shields only in ground fault situations, in the TN-C
system with a common PEN conductor, large currents can flow in the
data cable shield in normal conditions. Based on the numerous posts,
it appears that you are completely ignorant of this wiring practice
used in many parts of the world.

Even the US wiring system appears to be some kind of TN-C-S with
separate neutral and ground within the house, but a common PEN
conductor to the distribution transformer. This system also suffers
from large data cable shield currents when connecting two buildings,
if both ends are grounded.

Paul


Re: RS485 is bidirectional does it mean it is fullduplex?
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I'm certainly *not* a power engineer.

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You don't seem to be a comm engineer.

As I've noted many times, telco comm cables are grounded at both
ends.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

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


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OK, we will start by assuming that the Earth Ground potential at the left
and right ends are exactly the same, just for starters. We will also assume
that the Rwire for the right hand end is the same as the Rwire at the left
hand end.

Current flow in the cable shield will be roughly (0.5*Rwire)/Rload. If
Rload was a few kOhms and Rwire was nearer 0.01 Ohms then there is almost a
negligable current flow.

Now consider the case where the left hand ground moves up to 80V away from
the right hand ground, but that the impedance between the two ground
connections remains less than 10 Ohms (not unreasonable in some very large
buildings). I am sure that you can see why we have been advocating the
grounding at one end only rule as a safety aspectg of RS422/RS485 networks,
as well as almost any other low level signal measurement/management system.

Where you do need to connect the screen wire to both ends then you had
better include some impedance in the connection that limits dangerous
current flow and yet provides sufficient coupling to ground for the higher
frequency signals. As I have stated before, all the circumstances have to
be well understood to make the right choice.

--
********************************************************************
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Re: RS485 is bidirectional does it mean it is fullduplex?
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Actually, I believe the specification *is* 10 Ohms, and the
target is 5 or lower.  An 80 volt difference in ground potential
is...  so unusual that we can ignore it.  Lets assume it never
gets higher than 20.  Or 30, if you like.  (Everything I recall
seeing was engineered for 20 V, max.)  No doubt there *are*
unusual instances were we might well see figures outside this
range.  And if we do, we deal with them as unusual instances...

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You are again proposing unrealistic circumstances to portray the
"norm".

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No. The trick is to ground the cable every 3 or 6 thousand feet, so
there is never get anything like an 80 volt difference.

Can't you come up with something less boorish than repeating the same
thing over and over?

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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So how do you reconcile even a 20V ground potential difference with the
+/-7V common-mode maximum of RS-485?

Steve
http://www.fivetrees.com



Re: RS485 is bidirectional does it mean it is fullduplex?
On Sat, 18 Jun 2005 01:21:29 +0100, "Steve at fivetrees"

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If we want the devices to survive after a ground fault (without any
residual current breakers), it should be noted that even with a TN-S
system with separate N and PE conductors and assuming the same cross
section as the L wire, a ground fault between L and PE would cause an
identical voltage drop in both wires. With 230 V mains, the ground
fault potential would go up to 115 V before the fuse is blown. Any
equipment connected directly to the PE close to the ground fault point
would get that potential and would most likely cause some permanent
damage to the RS-485 system.

Even with the often used 100 ohm resistors between the cable shield
and frame ground would limit the current to about 1 A (or 0.5 A if all
stations use the 100 ohm resistors), which would perhaps increase the
survivability of the data communication equipment.

It should be noted that a simple short circuit (eg. at the filament of
a dying incandescent lamp) would cause in the TN-C system a high
current peek in the common PEN conductor and some equipment connected
directly to the PEN connector would jump to 115 V (at 230 V mains).

So in practice, if high reliability is a requirement, use optical
isolation or at least keep the RS-422/485 signal grounds well away
from the PE or PEN conductor potentials.

Paul
  

Re: RS485 is bidirectional does it mean it is fullduplex?
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I've seen about 3 serious ground faults.

Only one of them produced no serious damage (it was a reversed
two wire 120VAC drop in a bush village, and while it had the
potential to kill someone, there were no unusual voltages
applied to equipment).

The others all caused serious equipment damage to many things,
each costing thousands of dollars.  I doubt that many RS-485
systems have ever been engineered to survive such a condition,
and further that there would be little point in such and expense.

90% of the equipment the RS-485 was associated with would be
destroyed.

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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So how do you reconcile even a 20V ground potential difference with the
+/-7V common-mode maximum of RS-485?

Steve
http://www.fivetrees.com




Re: RS485 is bidirectional does it mean it is fullduplex?
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Any of a number of ways.

But the first consideration would be whether RS-485 is a
appropriate protocol in that situation.   The answer is
almost certainly "No."

--
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)                         snipped-for-privacy@barrow.com

Re: RS485 is bidirectional does it mean it is fullduplex?
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But that's precisely what we're dealing with, routinely. RS-485 is fine in
these situations (which, as Paul said, are far more common than you seem to
realise) - so long as one pays attention to common-mode references.

Please give examples of the "any number of ways" you mention.

(<pedant>RS-485 isn't a protocol, it's a hardware signalling
standard.</pedant>

Steve
http://www.fivetrees.com



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