RS485 is bidirectional does it mean it is fullduplex?

Jim Stewart <jstewart@jkmicro.com> wrote:
>Floyd L. Davidson wrote:
>
>> Reality check time: telephone cables all across the country and
>> around the world are grounded every 3000 or 6000 feet.
>>
>
>Not in the usage of the term "grounded" that
>I'm familiar with.  Perhaps you could give us
>a little more details.

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.

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

Floyd L. Davidson wrote:

> "Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>>Equally, everything I've said about double-grounding is also factual, and
>>demonstrable. I'm not aware of any "specs" as such on this issue, but it
>>is hard to argue with a melted signal cable.
> 
> So you declare yourself correct, eh?  No discussion, no facts, nothing
> but pontification.
> 
> If it is demonstrable, why don't you!
> 
> --
> Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson>
> Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

See

<http://www.svconline.com/news/avinstall_implementing_electronicsystems_wiringsignal/>

There is currently some discussion on AES48.

<http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>

This document is on Industrial Ethernet.

http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/Belden.pdf

This one is a facilities departments standard.

<http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120 
Cable Wire and Connectors 600V.doc>

So, there is plenty of material there that states that the signal cable 
shields shall only be connected at one end. Like I have stated before, each 
situation should be carefully considered, calculations performed, safety 
assessments performed (yes, even for the simple audio lead between intercom 
units) and the complet circumstances fuly understood before you go in with 
a half baked notion of what is right. Circumstances can change things quite 
drastically. You just need to understand the underlying physics of the 
situation a bit.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

Floyd L. Davidson wrote:


> 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.

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.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

Floyd L. Davidson wrote:

> This message is a courtesy copy of a Usenet article posted to:
>   comp.arch.embedded
> --------------------------------------------------------------
> 
> "Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>>Floyd L. Davidson wrote:
>>
>>> However, if the cable is a long run, and particularly if there
>>> is exposure to power lines, if the ground potential is different
>>> at the two ends, or if there are any other sources of induced
>>> noise in the cable, this arrangement has the best effect:
>>> 
>>>      +-------+                                    +-------+
>>>      |       | >--------- tx wire/pair ---------> |       |
>>>      | EQUIP | <--------- rx wire/pair ---------< | EQUIP |
>>>      |       |  ========= cable shield =========  |       |
>>>      +-------+  |                              |  +-------+
>>>          |      |                              |      |
>>>          |      |                              |      |
>>>          o------+                              +------o
>>>          |                                            |
>>>        -----  Earth                                 -----  Earth
>>>         ---   Ground                                 ---   Ground
>>
>>If you really do need to connect at both ends then you may need to
>>consider inserting some impedance in the screen connections at both ends
>>(usually a capacitor and resistor in parallel). The resistor is large
>>enough to prevent high current flows but needs to be small enough to
>>provide an effective electrostatic drain. The capacitor provides a low
>>impedance at higher frequencies.
> 
> 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.

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.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

Floyd L. Davidson wrote:

> And since we are *not* dealing with a power distribution system,
> your point seems to be less than appropriate.

It is more pertinent than you realise. The equipment I deal with 
incorporates a range of measurement and control systems that includes 
nano-volt signals, RS485 signals  and 4-20mA current loop signals passed 
over 150m of multi-core shielded twisted pair cable to the instrument 
racks. The environment that this lives with is extremely noisy, features 
high magnetic fields, extremes of temperature range and earth potentials 
that can leap well off the normal reference level (and it is not the 
noisiest environment I have worked with). Some of the communications 
signals run in quieter areas and span about 50 different buildings on the 
site. Each building has its own sub-station and the site has a SuperGrid 
power feed (it uses short bursts of extremely high power at regular 
intervals). So, as you can imagine, as a Systems Engineer with 
responsibility for just some of the equipment on this site, I have to know 
how the site earthing scheme works and what risks are involved. So in all 
respects I am quite on topic for this thread and relevent with the advice I 
have provided.

>>>>See my other post re isolated comms interfaces.
>>> 
>>> Why not do some real research, and find out how it is actually
>>> done.
>>
>>Many of us have and employ this sort of consideration in our daily
>>practice.
> 
> Reality check time: telephone cables all across the country and
> around the world are grounded every 3000 or 6000 feet.

Last time I checked around here the telephone company cables were 
unshielded twisted pairs (but that was quite some while ago).

The other types of cable we use on site include unshielded and shielded 
power cables, heliax coaxial cables and triaxial cables. I have to know how 
to deal with each and every type.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>
>See
>
><http://www.svconline.com/news/avinstall_implementing_electronicsystems_wiringsignal/>

This one starts out pretty good, when discussing theory, and fails
terribly when discussing implementations.  If nothing else, the
discussion of jack fields should be enough to indicate the confusion.

Regardless, nothing there contradicts what I've said.  The entire
discussion is about cables within a single structure, ground system
and power distribution.  It also appears to be discussing single
pair cables for the most part.

>There is currently some discussion on AES48.
>
><http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>

    IEC 61000-5-2, 6.4.2, points out, "Shields of
    cables are bonded to the earthing network at one or
    two extremities depending on the signals being
    transmitted and on possible electromagnetic
    interference sources."

However, despite contradicting *you*, the context is not
complete enough to actually know *what* they are referring to.
And casual discussion, much like those on this newsgroup, are
not authoritative sources.

>This document is on Industrial Ethernet.
>
>http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/Belden.pdf

This one contradicts your first cite above in regard to the
effects of a ground loop.  Shame on Belden.  It is also talking
only about short distance ethernet.  It would make no sense at
all to ground both ends of an ethernet cable.  Or, more correctly,
it would make no sense to attempt using ethernet over a cable where
grounding at both ends was useful.

>This one is a facilities departments standard.
>
><http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120
>Cable Wire and Connectors 600V.doc>

This is also talking only about relatively short runs of
"control cable" all within a building, all on one ground system,
and all where both ends are on the same power distribution.
Grounding only one end of a cable, in those circumstances is
expedient.

>So, there is plenty of material there that states that the signal cable
>shields shall only be connected at one end.

No, that is *NOT* what your sources above indicate.  They do
indicate that is some circumstances (which I've already pointed
out in previous articles), the easiest means of avoiding trouble
with ground loops is simply to ground at only one end.  That is
not a "shall only" situation at all, nor is that necessarily
the best solution.

>... You just need to understand the underlying physics of the
>situation a bit.

And we have yet to see *any* indication that you do.

Here, try reading these...

Here's a blurb on equipment to test the ground and shield
continuity at comm cable splices (where you are saying grounds
don't exist).

  http://www.tequipment.net/WilcomTeleTransT124.asp

Here are a couple of specifications for terminating cables
to building ground systems:

  http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
  http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuidelines/16743%20-%20Telecom%20Risers,%20Closets%20and%20Entries.htm

Here's short URL for that last one,

  http://tinyurl.com/b9gu2

Here's an even more authoritative treatment of grounding,

  http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc

All of these describe the grounding at *both* ends of a cable.

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

Floyd L. Davidson wrote:

> Telephone cable, the stuff on the poles or buried in a trench,
> that goes between buildings and on down the road, has a shield.
> (Heck, some cables have six inches of armor plated steel around
> them!)
> 
> Cross connect cables between bays does, and between racks does
> not.
> 
> Drop wire doesn't.  Frame wire doesn't.

I'll believe that it may be the case where you are.
 
>>I am certainly not in telephony.
> 
> Then you should not propose pseudo-authoritative answers to
> questions relating to that field.

Except that we are talking about RS422/RS485 data communications systems 
here and not telephony. This is an area that, like Steve and a few others 
here, have a fair bit of experience in. I know that some of us are in very 
heavily noise ridden environments like myself. I also appreciate that some 
of the other contributors to this thread are in no worse than a normal 
office environment. So, when it comes to talk of connecting shields I think 
we are all well aware of the situations where to connect both ends leads to 
problems.
 
>>> If they had a melted cable, it wasn't simply the difference in
>>> ground potential.  (Telecom cables generally are spliced and
>>> grounded every 6000 feet, max.)
>>
>>I know that on navy ships and submarines they connect the outer screen to
>>the feedthrough grounds as they penetrate each bulkhead. There are special
>>feedthrough glands made for this purpose. Then, they also go through a lot
>>of effort to ensure that the electrical ground really is as close to ship
>>ground as possible (usually less than 250mV). A ship or submarine can be
>>considered akin to one building with own power source, however, the cable
>>runs are quite long.
> 
> The longest run on an aircraft carrier, or a super tanker, is
> short.  More than 3 miles is considered "long" in general,
> though specifically that may not always be the case.

The ships themselves may not be that long but by the time you consider the 
routes that some of those cables take to get from one end to the other then 
you should not be surprised that there can be single runs that are of the 
order of 7km. Then you think about the number of bulkhead feedthroughs and 
the mind begins to boggle at the number of clamp-down points that the cable 
sheath has to be pared away, ferrules fitted and boots shrunk down.
 
>>> That is fine within one building, and is not the best practice
>>> for longer cable runs, for reasons explained in the message
>>> listed above.
>>
>>I would hope that it is obvious to all from this discussion that the issue
>>of connecting screens/shields is rather dependent on the circumstances
>>that
> 
> You've now mentioned ships and "MEGA-Amp equipment".  There are
> many special cases, probably none of which lead to any
> enlightenment except when they are explicitly discussed as
> extremes and flagged as unusual.

In the 30 years of my career it has, to me, been the norm to work with such 
environments. My main field of Systems Engineering is Energy and 
Transportation. That includes Oil-rigs, Mines, Power Stations (Coal, Oil 
and Nuclear), Railway Rolling Stock and Trackside Equipment. I have also 
done some work in the Banking Sector and the Medical Devices Sector. I have 
to appreciate everything from simple mechanics and low level signal 
equipment through to high energy systems. I have also to deal with vacuum 
and cryogenic equipment. It has certainly been an interesting career.
 
> I could also mention a few odd places...  the Faraday enclosure
> for radio and carrier equipment at a Coast Guard Loran station.
> Anything near a large power generating facility.  The "tempest"
> shielding into a room where the USAF was doing super secret
> seismic analysis (to detect nuclear testing in Siberia).  Or the
> operation of electronic in proximity to DEWLINE or Long Range
> Radar sites.  For that matter a Toll Center is one of the
> harshest electrical noise environments around.

I am also aware of some of those systems and the issues invvolved.
 
> These are all wonderful stories and any time you want to swap
> tales, we can.  Using them to try to explain grounding on comm
> cables won't help anyone understand what is involved though.

I explained in another reply featuring the diagrams you provided. I am sure 
that if you look at the maths of the situations you might get to see the 
consequences that most of us here are trying to prevent.
 
>>the equipment is going into. The screen connection impedance idea that I
>>mentioned in my earlier post would probably work for the widest range of
>>different situations but I would still countenance understanding the
>>issues that surround each individual instalation. It can save a huge
>>amount of grief later.
> 
> Understanding the basics comes first.  Trying to extrapolate
> these extreme situations to be representative of the basic
> theory is exactly what causes so much confusion.

I maintain that you do the math first then you may see how much and what 
effect the various schemes will have. I know that the power levels and 
situations that I am dealing with are heavier than most others here but 
most of them will also have to understand the fundamental principles 
that I have described. 

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>Floyd L. Davidson wrote:
>
>> 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.
>
>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).

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...

>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.

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

>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.

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)                         floyd@barrow.com

Floyd L. Davidson wrote:

>   http://www.tequipment.net/WilcomTeleTransT124.asp
> 
> Here are a couple of specifications for terminating cables
> to building ground systems:
> 
>   http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
>   
http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuidelines/16743%20-%20Telecom%20Risers,%20Closets%20and%20Entries.htm
> 
> Here's short URL for that last one,
> 
>   http://tinyurl.com/b9gu2
> 
> Here's an even more authoritative treatment of grounding,
> 
>   http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc
> 
> All of these describe the grounding at *both* ends of a cable.

Just to be clear is what you are refering to as the shield (the bit you 
keep insisting must be grounded at both ends and every 3000ft) actually 
known to the rest of us as the armouring. If that is the case then it is no 
wonder that we seem to have been talking at cross purposes. Yes, armouring 
on cables does have to be grounded at both ends. The armour is normally a 
very substantial close steel wire wrap whose purpose is mainly mechanical 
protection. However, the rest of us, I believe, were speaking of the shield 
connection which is a much finer enclosing conductor and which we are all 
maintaing should only be connected at one end unless special arrangements 
are made.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@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/
********************************************************************

"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:873brga5fa.fld@barrow.com...
> 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...

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