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RS485 is bidirectional does it mean it is fullduplex?

Started by Swizi June 15, 2005
On Fri, 17 Jun 2005 09:32:32 -0800, floyd@barrow.com (Floyd L.
Davidson) wrote:

>>Before connecting the data cable, there would be a 5 V ground >>potential difference between the left and right side equipment. When >>the cable shields are connected, a part of the up to 25 A flowing >>through Rload will be diverted through the data cable and into the >>right hand equipment ground. > >But lets not suggest that it would be a significant part of that >25 A. We are *not* describing ground *faults*, but ground >*loops*! But, in a typical arrangement, it could certainly have >enough effect to cause one heck of a 60 Hz hum in the "signal" >as described above.
You seem to have a very limited view of various grounding practices used in the world. The TN-S system with separate N and PE conductors is not the only system used. While it is true that in a TN-S system, large currents will flow in the signal cable shields only in ground fault situations, in the TN-C system with a common PEN conductor, large currents can flow in the data cable shield in normal conditions. Based on the numerous posts, it appears that you are completely ignorant of this wiring practice used in many parts of the world. Even the US wiring system appears to be some kind of TN-C-S with separate neutral and ground within the house, but a common PEN conductor to the distribution transformer. This system also suffers from large data cable shield currents when connecting two buildings, if both ends are grounded. Paul
"Lanarcam" <lanarcam1@yahoo.fr> wrote:
>Steve at fivetrees wrote: >You will find perhaps the following useful: >(http://www.spheresystems.com.au/RS485.html) > >Ground and Earth Connections >The grounding and earthing connections in RS485 provide two >separate functions. > >The second is related to safety and the first to establish a >reference voltage > >Safety >The RS485 *cable screen* must be bonded to the protective earth >system of a building at *one point* only. > >The cable screen must be electrically continuous throughout the >entire cable run > >Voltage Reference > >The screen of the RS485 cable establishes a ground reference >voltage for the RS485 signal conductors. For this reason the >cable shield must be connected to the *ground reference* for >*each node* on the network. > >It is not acceptable practice to tie the node ground reference >to the building protective earth as this will introduce >electrical noise into the system and may lead to equipment >damage in the event of electrical fault currents
For Steve's benefit, you should have included one more section: Installation throughout Multiple Buildings There are two separate installation procedures depending on the type of electrical earthing system. MEN System at one building In this case the installation may be made as though it exists in a single building. The cable shield will be continuous throughout the installation. MEN System in each building In this case there must be electrical isolation between buildings with different MEN systems. Each building is wired as a separate and complete installation with the cable shield tied to the building protective earth at one point. That is in *each* building. -- 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 09:32:32 -0800, floyd@barrow.com (Floyd L. >Davidson) wrote: > >>>Before connecting the data cable, there would be a 5 V ground >>>potential difference between the left and right side equipment. When >>>the cable shields are connected, a part of the up to 25 A flowing >>>through Rload will be diverted through the data cable and into the >>>right hand equipment ground. >> >>But lets not suggest that it would be a significant part of that >>25 A. We are *not* describing ground *faults*, but ground >>*loops*! But, in a typical arrangement, it could certainly have >>enough effect to cause one heck of a 60 Hz hum in the "signal" >>as described above. > >You seem to have a very limited view of various grounding practices >used in the world. The TN-S system with separate N and PE conductors >is not the only system used.
I'm certainly *not* a power engineer.
>While it is true that in a TN-S system, large currents will flow in >the signal cable shields only in ground fault situations, in the TN-C >system with a common PEN conductor, large currents can flow in the >data cable shield in normal conditions. Based on the numerous posts, >it appears that you are completely ignorant of this wiring practice >used in many parts of the world. > >Even the US wiring system appears to be some kind of TN-C-S with >separate neutral and ground within the house, but a common PEN >conductor to the distribution transformer. This system also suffers >from large data cable shield currents when connecting two buildings, >if both ends are grounded.
You don't seem to be a comm engineer. As I've noted many times, telco comm cables are grounded at both ends. -- 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: >> "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. > >I repeat: please see the RS-422/485 specs.
Please quote where those specs discuss grounding of comm cables, or explain what ground loops are, or in support any of the other invalid pronouncements you've been prone to making. -- 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: >>"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message >>news:d8sihj$ffk$1$830fa7a5@news.demon.co.uk... >>> >>> By way of re-inforcing the point here, consider that the screen is only >>> meant to act as the notional extension of a metal enclosure out along >>> the wires. It should not carry any current at all (except maybe for the >>> tiniest >>> leakage current capacitively coupled from the signal wires - and even >>> that should be miniscule). Dealing properly with the screens is a safety >>> issue as well as a circuit protection and noise reduction issue. >> >>Absolutely. Connecting to local ground at both ends of the run is a recipe >>for HUGE ground currents. I've seen (very) melted ground wires. It is >>indeed a safety hazard. > > Bullshit.
When you deal with the MEGA-AMP power world you will get to see how much of a safety hazard connecting the screens at both ends is. Consider that, under certain circumstances there may exist 75 to 80 Volts between the two earths and that the current source is huge by comparison to the screen current carrying capacity. If we assume that the run of cable has a screen resistance of about 1.5 Ohms (a reasonable figure for quite a 150m run of decent signal cable) then 80V * 80V / 1.5 Ohms gives you a power dissipation of 4.267kW along the length of the screen. So, you had better limit the current flow in the screen in some manner or you fry the signal cable.
>>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. -- ******************************************************************** 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 Keinanen wrote:

> On Fri, 17 Jun 2005 00:22:38 +0100, "Steve at fivetrees" > <steve@NOSPAMTAfivetrees.com> wrote: > > >>Absolutely. Connecting to local ground at both ends of the run is a recipe >>for HUGE ground currents. I've seen (very) melted ground wires. It is >>indeed a safety hazard. > > Yes, this is a severe problem when the TN-C wiring system (L1, L2, L3 > and N) in which the neutral is connected directly to each equipment > frame. In this case quite a few Amperes can flow in the cable shield. > > However, with TN-S wiring (L1, L2, L3, N and PE) with the PE connected > to each frame, grounding the data cable at both end is not so bad, > since large AC currents will flow in the cable shield only during > ground faults (Lx to PE) until the fuse is blown. > > Unfortunately pure TN-S systems are hard to find, since often the N, > PE, the grounding electrode and all metal piping is connected to the > same main grounding bar (equipotential bonding) close to the mains > cable entry. From the main grounding bar to the distribution > transformer, the system is just TN-C. > > Thus, most wiring systems claiming to be TN-S are actually TN-C-S, > thus grounding the data cable shield at both ends would not be that > bad _within_ the building, but connecting a data cable shield between > two buildings, will suffer from the same huge currents as a TN-C > system would.
Each installation does need to be examined and understood. I know that our earthing scheme is split three ways. There is building earth which is the main safety earth for the entire building. We also have an instrumentation earth which is a quite large (10mm) cable from an instrument earth busbar around the building and linked to building earth in certain specific locations. Finally we have machine earth which is associated with the MEGA-Amp equipment that we use in our experiments and which only connects to building earth at certain specific points. During experimental operations we can see differences between the machine earth and the instrument earth that are of the order of 75 to 80V. If you need to work on some of the equipment during operational periods we have to write a safe working procedure fill out a permit to work form and get it all signed by a large number of people before we are allowed near the equipment. Sometimes it can take a couple of days before you are allowed to repair some measurement equipment. -- ******************************************************************** 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:

> I've explained that *in detail*. That *is* the way telephone > cable systems are installed.
If you are speaking of purely telephone wires then most cables I have seen in that category are not screened. Just twisted pairs. However, as I do not work in telephones I will bow to yours and anyone elses knowledge of that aspect (I could probably ask my brother who works with telephones as well as pwer systems in his daily work).
>>As I've just said elsewhere, this is simply nonsense. > > And you are out of your field...
I am certainly not in telephony.
>>Consider: you're >>trying to short out a part of the mains utility distribution system, which >>may be carrying significant leakage currents over long distances, with a >>flimsy little signal wire. Ground potential differences can be >>significant, and the source impedance is very low indeed - certainly >>capable of delivering tens of amps. I've seen guys who should know better >>staring at melted cables, scratching their heads... > > 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.
>>Please see my (and others') post(s) re isolated comms interfaces. > > See the tutorial I posted: Message-ID: <87vf4egfri.fld@barrow.com> > >>Use one >>ground; isolate from the other - i.e. keep both Tx and Rx relative to one >>ground. > > 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 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. -- ******************************************************************** 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: >>>Absolutely. Connecting to local ground at both ends of the run is a recipe >>>for HUGE ground currents. I've seen (very) melted ground wires. It is >>>indeed a safety hazard. >> >> Bullshit. > >When you deal with the MEGA-AMP power world you will get to see how much of
And since we are *not* dealing with a power distribution system, your point seems to be less than appropriate.
>a safety hazard connecting the screens at both ends is. Consider that, >under certain circumstances there may exist 75 to 80 Volts between the two >earths and that the current source is huge by comparison to the screen >current carrying capacity. If we assume that the run of cable has a screen >resistance of about 1.5 Ohms (a reasonable figure for quite a 150m run of >decent signal cable) then 80V * 80V / 1.5 Ohms gives you a power >dissipation of 4.267kW along the length of the screen. So, you had better >limit the current flow in the screen in some manner or you fry the signal >cable. > >>>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. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
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.
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>Floyd L. Davidson wrote: > >> I've explained that *in detail*. That *is* the way telephone >> cable systems are installed. > >If you are speaking of purely telephone wires then most cables I have seen >in that category are not screened. Just twisted pairs. However, as I do not
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 am certainly not in telephony.
Then you should not propose pseudo-authoritative answers to questions relating to that field.
>> 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.
>> 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. 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. 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.
>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. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com