"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:>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.That is virtually the case *everywhere*>>>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.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*. 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.>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.Clearly you are not very well grounded (that was hard to resist, so I didn't) in cables and shielding. You continue to argue that things which are standard practice would cause all sorts of wild results. They don't.>> 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.)>> 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 suchWonderful. I'm impressed. I'm only slightly older and more experienced that.>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."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 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.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. And you keep posting these off the wall articles! Do a reality check on what you are claiming. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
RS485 is bidirectional does it mean it is fullduplex?
Started by ●June 15, 2005
Reply by ●June 17, 20052005-06-17
Reply by ●June 17, 20052005-06-17
"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?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) floyd@barrow.com
Reply by ●June 17, 20052005-06-17
"Floyd L. Davidson" <floyd@barrow.com> wrote in message news:87aclo8nad.fld@barrow.com...> "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? > > 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."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
Reply by ●June 17, 20052005-06-17
On Sat, 18 Jun 2005 01:26:23 +0100, the renowned "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? > >Steve >http://www.fivetrees.comGalvanic isolation. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●June 17, 20052005-06-17
On 2005-06-18, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>>So how do you reconcile even a 20V ground potential difference with the >>+/-7V common-mode maximum of RS-485? > > Galvanic isolation.Well, this is where I walked into this movie, so I think I'll go home now... -- Grant Edwards grante Yow! Now I can join WEIGHT at WATCHERS! visi.com
Reply by ●June 17, 20052005-06-17
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message news:06v6b1hg3iujasqf5jvsn4s9luf05ujhsc@4ax.com...> On Sat, 18 Jun 2005 01:26:23 +0100, the renowned "Steve at fivetrees" > <steve@NOSPAMTAfivetrees.com> wrote: > >>So how do you reconcile even a 20V ground potential difference with the >>+/-7V common-mode maximum of RS-485? > > Galvanic isolation.My point exactly - thank you ;). Steve http://www.fivetrees.com
Reply by ●June 17, 20052005-06-17
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:>"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...>Ah. I did wonder if Floyd was talking about armour.Paul is seriously confused.>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.It can certainly be done that way... if you are talking about relatively short cable distances. Typically, as I've mentioned before, that type of cross connection between equipment is acceptable between equipment in a single rack or between racks in a bay if they all use a common ground. However, that is a very stringent set of limits, which might be relaxed significantly in many instances without an harm. In an environment where there is a great deal of potential for interference, that is what should be used.>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.If you want to talk armor, let me explain about submarine fiber cables... :-) -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Reply by ●June 17, 20052005-06-17
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:> >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.You are *grossly* confused, and don't know what the names are.>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.No, telephone cables have never used lead for armor. (Lead for armor??? You are grasping at straws! Some armor that would be...) Lead *shielding* was used for a number of years, and it would also be correct to call it "lead sheathing" too. But that hasn't been used for several decades now, and we'd probably have a very hard time finding an example of it. Today, cable has an aluminum foil shield. That is *not* armor. You are simply *grossly* confused about cable! The individual pairs are not shielded. The entire cable is! Armor is something entirely different, though it does become part of the shield, and yes it will be grounded, or not, in exactly the same way that I have described for shielding. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Reply by ●June 17, 20052005-06-17
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:>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.MEGA-Amps... ?>As I asked in another, are you speaking of armouring or shielding?Are you still confused?>> 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.Yes it is. But we were talking about 50/60Hz power, not about 30 MHz RF, which had not been mentioned until now. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Reply by ●June 17, 20052005-06-17
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:>Floyd L. Davidson wrote: > >> This message is a courtesy copy of a Usenet article posted to: >> comp.arch.embeddedBTW, Paul... I'm not sure if you really want these email duplicates or not. They are generated automatically because you have a Reply-To: header with your email address in it in every message. If that is intentional, it's certainly okay.>> -------------------------------------------------------------- >> >> "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.Sounds like what used to be called ABAM. I don't know what it is called now.>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.Fascinating! Cable is an interesting technology. A typical telephone cable has color coded pairs in color coded bundles. Each pair is twisted, and within a bundle each pair has a slightly different twist to prevent any two pairs from getting too cozy. The pairs in the bundle are swirled. The bundles are swirled in the sheath. These come in various sizes from 1 pair to at least 1200 pair cables. Around the entire swirl of bundles is an aluminum foil wrapper, with a bare tracer wire. The cable may also be filled with air, or may have a dielectric substance (silicon??) that is water resistant. Next would be an insulating sheath or sheaths. And perhaps armor and more sheathing. Armor may be as simple as what you described, or might be several layers of different types of steel banding. (The fiber optic cable used to bring a submarine cable on shore is about six inches in diameter, compared to the unarmored portion that is only about half and inch in diameter.) The material used for the insulating sheath varies depending on what and where the cable will be used. I'm not really familiar with exactly what differences there are, other than for example I'm aware that aerial cable is spliced on the pole and the cable coming down the pole to a junction box is an entirely different type of cable. Certainly there are difference for aerial cable and for buried cable, and probably several other distinctions. Of course, that describes the cable hanging on a pole outside. There are all varieties of different cables used for different purposes in different places, as you would expect. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
