RS485 is bidirectional does it mean it is fullduplex?

"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

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

And they need not worry that it will, either.  What's your point?

>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).

Yes, and you have a *very* unusual environment too.  Are you suggesting
that everyone else engineer their equipment to match something they will
*never* encounter?

>> 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.

I'm just overjoyed that you are aware of that.  However, in many instances
it is simply *not* enough.  That is why virtually *all* telephone cables
are installed as I've described, with a good earth ground at *every*
point were sections of cable are spliced.  That is commonly either at
3000 or at 6000 feet.

Now, you can cite all the less than authoritative sources on the Internet
that you like, and concoct all the unusual circumstances you'd like too,
the *facts* are not going to change, and the standard practice is exactly
what you say can't be done.

>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.

If you want a good book, try "Telecommunication System
Engineering", 3rd Edition, 1989, by Roger L. Freeman.

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

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:
>
>> 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 

We already realize that you have a very unusual situation in regard
to AC power.  Suggesting that everyone engineer equipment to match
that environment is not appropriate.

>> 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).

That's *bullshit*.  Years ago they used lead sheathed cables,
and for the past several decades have used foil wrapped cables.

You have *NEVER* checked, and don't have a clue what you are
talking about.

As I mentioned in another post... cross connect wire (used on a
distribution frame or between racks in a single row, all with a
common ground) is utp.  So is typical "drop wire".  All outside
plant distribution cables are shielded.  And everything between
bays with separate ground systems is shielded if it carries
anything other than voices frequency circuits, and probably is
even if that is all it has.

(Unshielded outside cable plant hasn't been commonly in use since
/open/ /wire/ systems were replaced with microwave radios starting
in the late-1940's.  It was fairly hard to find after about 1960
or so.)

>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.

Listen up and learn then.  You'll need it.

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

"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message 
news:d8vp7o$2eq$1$8300dec7@news.demon.co.uk...
>
> 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.

Ah. I did wonder if Floyd was talking about armour.

FWIW: the RS-422/485 cabling I've mostly dealt with is unshielded. The data 
signals are carried in twisted pairs. A tracer, or a spare signal cable, is 
used to provide the common-mode reference (note I avoided using the word 
"ground" here) for the remote end - if required. It isn't required for 
situations where ground can be reliably used as the common reference (i.e. 
no more than a volt or two differential under all conditions); it *is* 
required for those where it can't.

In practice, this common-mode reference provides the 0V connection for a set 
of isolated transceivers at the remote end - i.e. there is no need for a 
remote ground connection. The reference cable is not expected to carry any 
significant current, hence develop any significant voltage along its length. 
Noise immunity is achieved by the balanced differential signals, and the use 
of twisted pairs.

Floyd, that's as clear as I can be. I'm stating that as succinctly and 
clearly as I can in the hope (and indeed expectation) that we have been 
talking at cross-purposes.

Steve
http://www.fivetrees.com

Floyd L. Davidson wrote:

> Yes, and you have a *very* unusual environment too.  Are you suggesting
> that everyone else engineer their equipment to match something they will
> *never* encounter?

You will find the sort of environment I speak of is similar to most 
factories. I just get it about 3 times as bad because of the really high 
energy that we are dealing with in two parts of our site. That really is 
not that much of a margin above most of the others. Perhaps a profile of 
the group members working environments might show you how many share 
conditions similar to those I have dealt with for more than 30 years.

> Now, you can cite all the less than authoritative sources on the Internet
> that you like, and concoct all the unusual circumstances you'd like too,
> the *facts* are not going to change, and the standard practice is exactly
> what you say can't be done.

As I asked in another, are you speaking of armouring or shielding?
 
> 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.  

At 50Hz, probably 60Hz, that may be the case on a good day. However, I have 
other techniques that deal with the 50/60Hz noise issues which do not rely 
on the shield. However, we have a number of electronic motor drives, 30MHz 
RF sources (3 by 8MW) microwave systems of about 4MW and several cameras 
within the experimental zone. The shielding is effective at those 
frequencies.

-- 
********************************************************************
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:
>
>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.

Armoring?  No.  It is aluminum foil, not steel.

>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.

A contradiction to *everything* you've said.  There is *no*
difference, electrically, between an aluminum foil shield and
the steel if it is armored.  (And you won't see many cables with
steel armor on them, unless you just happen to be exposed to
submarine cables.)

>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.

You are confused...

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

Floyd L. Davidson wrote:
> 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.

As much as I hate jumping into this shitstorm,
I guess I have to.

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.

Floyd L. Davidson wrote:

>>Last time I checked around here the telephone company cables were
>>unshielded twisted pairs (but that was quite some while ago).
> 
> That's *bullshit*.  Years ago they used lead sheathed cables,
> and for the past several decades have used foil wrapped cables.

I am sensing that you have all along been talking of the cable armouring 
and not the cable shields. When you refer to things by their proper names 
it gets a whole lot less confusing. Yes our telecom cables had lead 
armouring. The twisted pairs inside when I last saw one of these cables 
some 20 years back did not have shields.

-- 
********************************************************************
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/
********************************************************************

On Friday, in article <873brga5fa.fld@barrow.com>
     floyd@barrow.com "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:
>>
>>> Here's a ground loop:
>>> 
>>>        Signal Source
......

[snip diagram everyone has seen before]
....

>>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

In *your* experience, in two situations seen all over the UK, Railway Stations
with multiple platforms can often see 10-20V from the main earthing point as
they progress outwards across platforms. One near me saw a 60V in 100ft of
cabling that went 60ft by line of sight. Certain Eurostar depot called North
pole one contractor in its building phase was seeing all sorts of problems
with ground potentials in excess of 70V.

These were for data communications issues, some of it was for passenger
information systems, so not heavy machinery in itself. These figures were
quiescent conditions, not when large diesel-electric or other types of
large electric motors started up.

I have seen all sorts of issues involved in ground potentials affecting
signalling not cable sleeving, in places such as hospitals, factories,
and other establishments.

...
>>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.

The outer shield that is for protective earthing to stop the metal
outer casing becoming live by any means, including internal cable fault.
It has nothing to do with the signal levels, and if it is used for that
then you are asking for problems.

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

Learn the difference between signal reference and protective earthing.

Keeping things floating is done for many reasons, knowing that in Medical
Equipment in Europe you have to watch simple things like is the patient in
an operating considered floating or earthed? Well it depends which country
you are in, which effects signalling and protective earths how they are done
and what is isolated from what.

-- 
Paul Carpenter          | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>    PC Services
<http://www.gnuh8.org.uk/>              GNU H8 & mailing list info
<http://www.badweb.org.uk/>             For those web sites you hate

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:
>>
>>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.
> 
> Armoring?  No.  It is aluminum foil, not steel.

Perhaps then to enable te rest of us to be very clear you could describe 
the cable make-up.

My most common long-run cable is 16 twisted pairs individually 
aluminium/mylar foil screened with aluminium wire drain tracer and with an 
overall aluminium foil/mylar overall screen, Lo-Hal insulation and sheath. 
Where it is necessary for additional mechanical protection we have the same 
cable with a multi-stranded steel wire armour (0.25mm strands) and overall 
PVC sheath. We have, as I have indicated a range of other cable types for 
different purposes.
 
> A contradiction to *everything* you've said.  There is *no*
> difference, electrically, between an aluminum foil shield and
> the steel if it is armored.  (And you won't see many cables with
> steel armor on them, unless you just happen to be exposed to
> submarine cables.)

I have plenty of exposure to steel wire armoured cables. It is not unusual 
in any of the Energy or Transport sectors I usually work in. They may not 
appear as much in factory installations except as a power cable.
 

-- 
********************************************************************
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/
********************************************************************