RS485 is bidirectional does it mean it is fullduplex?

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

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?

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.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

This message is a courtesy copy of a Usenet article posted to: comp.arch.embedded

--------------------------------------------------------------

"Paul E. Bennett" wrote:

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.

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.)

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

--
Floyd L. Davidson           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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

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

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.)

You are confused...

--
Floyd L. Davidson           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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.

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

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 ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************

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 ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************

......

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

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.

...

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.

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
    PC Services
              GNU H8 & mailing list info
             For those web sites you hate

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.

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 ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************

That is virtually the case *everywhere*

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.

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.

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.)

Wonderful. I'm impressed. I'm only slightly older and more experienced that.

"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.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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.

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

Steve

Galvanic 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

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

My point exactly - thank you ;).

Steve

...

Paul is seriously confused.

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.

If you want to talk armor, let me explain about submarine fiber cables...

:-)

--
Floyd L. Davidson           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

You are *grossly* confused, and don't know what the names are.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

MEGA-Amps... ?

Are you still confused?

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

BTW, 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.

Sounds like what used to be called ABAM. I don't know what it is called now.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

If you are using it in inappropriate environments, don't be complaining about how difficult it is to deal with. "Routine" doesn't make it good engineering practice.

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.

We've gone over *several* of them. If you can't remember, try Google.

It's a protocol.

A standard agreed to by a group, and then more or less adhered to by others, is definitely a protocol.

Or would you prefer it to be made into a law?

--
Floyd L. Davidson           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com