RS485 is bidirectional does it mean it is fullduplex?

Paul Keinanen <keinanen@sci.fi> wrote:
>On Fri, 17 Jun 2005 19:34:35 -0800, floyd@barrow.com (Floyd L.
>Davidson) wrote:
>
>>I can't say that what you are doing is right or wrong, but if
>>you are dealing with the 80 volt common mode differences that
>>you and Paul claim, maybe somebody needs to review what you are
>>doing.
>
>While my namesake had an example about 80 volt common mode difference,
>I demonstrated in an other post, how you momentarily can have a 115 V
>(for 230 V mains) or 55 V (for 110 V mains) common mode difference in
>various ground fault or short circuit situations.

Non-sequitur.  *Nothing* can be expected to work right if you
power feed is miswired.

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

Paul Keinanen <keinanen@sci.fi> wrote:
>On Fri, 17 Jun 2005 13:34:14 -0800, floyd@barrow.com (Floyd L.
>Davidson) wrote:
>
>>>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.
>
>When making more or less general purpose systems to be installed into
>customer premises by people with very varying level of experience, you
>have to make sure that they can get the system to work on their own.
>The product must be designed properly and the installation instruction
>be clear and simple that works in all situation. It is not cost
>effective to travel to the other side of the world to solve
>"mysterious" communication failures or burnt transceivers.
>
>It would be too much to expect a local instrumentation technician to
>be able to figure out if the customer wiring system is

Paul, this is not logical.  Grounding of comm cable shields has
nothing to do with the type of power distribution.

>- TN-S, in which case the grounding at both ends could be justified
>- TN-C, in which case only a single point grounding should be used
>- TN-C-S, in which case you would have to check that all current and
>possibly later added devices are in the _same_ TN-S branch now and
>that this will remain in the future
>- IT (floating mains), in which case there should not be too much
>problems, even if grounded at all places
>- TT (N grounded at transformer only, PE separately in each building)
>which might have some consequences in fault and thunderstorm
>conditions
>
>Even if the technician would be knowledgable about these issues,
>getting reliable and up to date information about the wiring in an
>existing building can be a problem, especially if the wiring has been
>done during different periods.
>
>In order to keep the instructions clear I prefer to recommend methods
>that work well in most cases and firmly grounding the shield at both
>end is certainly _not_  one of those methods, even if it does not
>cause harm in certain special cases.
>
>Paul

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

Paul Keinanen <keinanen@sci.fi> wrote:

  [many lines of imaginative fiction deleted]

>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.

Ah, we are making progress.  You now admit that yes in fact that
*is* standard engineering practice.

When you get a better grip on it, you understanding of how to
engineer RS-485 systems will improve too.

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

Paul Keinanen <keinanen@sci.fi> wrote:
>On Sat, 18 Jun 2005 01:21:29 +0100, "Steve at fivetrees"
><steve@NOSPAMTAfivetrees.com> wrote:
>
>>"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?
>
>If we want the devices to survive after a ground fault (without any

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)                         floyd@barrow.com

On Sat, 18 Jun 2005 18:37:53 +0200, Anton Erasmus
<nobody@spam.prevent.net> wrote:


>Do you perhaps have a URL that describes the above wiring systems in
>more detail ?

You can get quite a lot of usable hits with a google search

TN-C TN-S TN-C-S

The British IEE tutorial at
http://www.iee.org/oncomms/pn/betnet/Geoff_Cronshaw.pdf 
contains some nice pictures and other info.

Wikipedia at http://en.wikipedia.org/wiki/TN-C-S and links from there
to other grounding systems will discuss the pros and cons for the
various conventions.

http://www.epanorama.net/documents/groundloop/electrical_wiring.html
contains information about various wiring systems in different
countries and about avoiding ground loops.

Paul
 

"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87is0b5ze0.fld@barrow.com...
> Paul Keinanen <keinanen@sci.fi> wrote:
>
>  [many lines of imaginative fiction deleted]
>
>>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.
>
> Ah, we are making progress.  You now admit that yes in fact that
> *is* standard engineering practice.
>
> When you get a better grip on it, you understanding of how to
> engineer RS-485 systems will improve too.

Floyd, you're being unnecessarily insulting. It's a little unseemly.

You have demonstrated time and time again that your knowledge of RS-485 is 
sketchy at best. You've taken us on a long-winded trip along armour 
grounding in telephony, which, while entertaining in a way, really has 
nothing to do with the point under discussion (RS-485 common-mode voltage 
range).

Please go away and actually *read* the RS-485 spec [1]. Then perhaps we can 
have a grown-up discussion without resorting to petty insults.

[1] I can provide a synopsis if needed.

Steve
http://www.fivetrees.com 

Floyd L. Davidson wrote:

>>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.
> 
> Telecommunications is not a narrow field.  It includes data
> communications, and specifically RS422/RS485.  Both of you seem
> to think that cables never extend outside of a building.  But
> those protocols are spec'd for 4000 feet, and that puts it right
> into *my* experience (and obviously far from yours).
> 
> I pointed out why this idea that cable shields can *only* be
> grounded at one end is wrong, explained why and under what
> circumstances they might be engineered differently.  You two
> claimed that was wrong.  I pointed out that every outside plant
> telephone cable in the country does in fact have multiple points
> where the shield is grounded.
> 
> You've both been trying to deny that.  But it is a simple
> *fact*.

All we have seen is a lot of hand waving. I have, at least put numbers to 
the, admittedly, serious case but I think it holds for quite a number of 
situations (even if it isn't all). Would you care to put numbers to your 
scheme and state clearly the assumptions, deal with the assumptions we have 
made about different earth potentials (or put real numbers to why it does 
not happen over such long distances). Then you might get a bit more respect 
for your position. Steve and myself have stated that we seen fried cables 
because people disobeyed the rulings about screen connectivity.
 
> And how that applies to RS-422/485 should be obvious to both of
> you, but doesn't seem to be.  You've both been exposed to a
> little bit of technology using twisted pair as a transmission
> line, and have learned some rules of thumb, but not the basic
> theory behind it.  If you would cease being so stubbornly hung
> up on your simple rules of thumb, and learn something about it
> on a broader scale, I'm sure that both of you would enjoy the
> value you get from a better understanding.

We are still waiting some serious attempt to convince us otherwise.
 
> "Most of us" are reading this and getting *sick* of this exchange.
> You continue to claim that what is done as a standard practice every
> where can't be.  It still is, whether *you* can understand it or not.

I never claimed it to be standard practice everywhere but certainly is in 
situations that I have come across. I have, as I have stated, also seen the 
other side in the ship wiring side of things.
 
> All the math in the world won't help if you don't understand how
> to spot parameters that don't fit.  If you *start* with off the wall
> numbers, you math is going to give you off the wall results.

My numbers were not off the wall as I have the readings from my log book to 
back up those I used in the example. We still haven't seen you put numbers 
to your diagram so I wait.

-- 
********************************************************************
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@barrow.com (Floyd L. Davidson) wrote:
>"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>>Floyd L. Davidson wrote:
>>> 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.
>
>I don't know.  But that sounds like more of your imagination.  A
>7km run is a lot of twisting back and forth to do...  (However,
>it does happen that I *can* verify that, and will.)

I did in fact check with someone who spent a few years on an
aircraft carrier.  The above is a little too much imagination.

Aircraft carriers are up to 1,100+ feet in length.  He figured
the longest loop on the ship would be the "sound powered phone",
but can't imagine that having much more total cable than maybe
twice the length of the ship (I suggest three times, and he
shrugged).

The longest actual loop would be a looped back data circuit
between the stern located approach radars and the bow based
command center.  His flat statement was "twice the length of
the ship".  No weaving through many ups and down on the way
either, a pretty much straight shot stem to stern.

When I told him the discussion was about the possibility
of a 7 km loop on a ship, he looked at me like I was nuts.

So we talked about ground differentials, and that added another
interesting perspective.  His eyes lit up a the mention, and he
said most people have no idea how big a battery that steel hull
is, or how much voltage it generates just sitting in salt water.
As to ground differentials, "*Lots* of voltage!"  So I asked how
much, 10's or 100 Volts, or what.

He said, "Oh, I don't know exactly, maybe 30 volts!"

Of course he thought the whole discussion of RS-485 was a
useless endeavor, and just dumped any reference to it in favor
of fiber optics.

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

"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote in message
>news:87is0b5ze0.fld@barrow.com...
>> Paul Keinanen <keinanen@sci.fi> wrote:
>>
>>  [many lines of imaginative fiction deleted]
>>
>>>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.
>>
>> Ah, we are making progress.  You now admit that yes in fact that
>> *is* standard engineering practice.
>>
>> When you get a better grip on it, you understanding of how to
>> engineer RS-485 systems will improve too.
>
>Floyd, you're being unnecessarily insulting. It's a little unseemly.

What is insulting about that?  A little bit of "I told you so,
right at the start of this lengthy conversation" is not
inaccurate at this point, nor inappropriate.

What you have posted, right from the start, was *far* more
insulting!

>You have demonstrated time and time again that your knowledge of RS-485 is
>sketchy at best.

And better grounded that yours is, so what's the point of trying
again to insult me.  It won't help you understand how it works.

>You've taken us on a long-winded trip along armour
>grounding in telephony, which, while entertaining in a way, really has
>nothing to do with the point under discussion (RS-485 common-mode voltage
>range).

I was *asked* about armor, and was *not* the one who brought it
up.  Of course, you obviously still don't have any idea how this
applies to RS-485, which is *exactly* the problem!  You don't
seem to know what the difference between armor and a shield is,
and clearly don't understand the effects of a shield on twisted
pair cables.

>Please go away and actually *read* the RS-485 spec [1]. Then perhaps we can
>have a grown-up discussion without resorting to petty insults.

So stoop to posting articles that are *nothing* but insults.
You can't argue technical points, and are confused by basics, so
it's down to insults...

>[1] I can provide a synopsis if needed.

I don't expect that you could.  Not an accurate one.


Note that the others in this discussion tend to keep the
discussion technical, which you seem to try avoiding.

I won't respond to any more of your articles if they do not
contain technical discussion.

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

"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87acln5qxb.fld@barrow.com...
> Of course he thought the whole discussion of RS-485 was a
> useless endeavor, and just dumped any reference to it in favor
> of fiber optics.

I suggest that's because you've totally misunderstood what we're talking 
about - RS-485. For looooong distances, sure, use something else. For what 
it was designed for (high-integrity short- to medium-haul datacomms), it's 
the mutt's nuts.

Steve
http://www.fivetrees.com