network@zeus.unl.edu (10/04/89)
Currently on our campus we have thick and thin Ethernet coax lines.
I have heard that it is advisable to ground these lines.
Building to building connections are done with fiber optic bridges,
so I'm not worried about these lines getting fried.
I'm more concerned about light and generator problems.
My questions are:
1) Should a line be grounded only on one end, or on both ends?
Should it be grounded at all?
2) What about in line taps that go to devices such as a repeater
or bridge. Meter checks show that this makes a solid ground,
so does that count as one of the grounds? In these cases do
we need to add any ground at all?
3) BNC Tee connections to a Ethernet card in a PC seem to make a
very weak ground or no ground at all. Should we ground these
connections?
4) Is it advisable to isolate against all unintentional grounds
as with a connection touching a metal rack by wrapping the
metal part of the connection?
I'm completely confused after talking with Digital Equipment.
Each person I talked to there had a different answer.
Suggestions and comments are appreciated.
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
: Steven Lendt /\ Managers and users were all having a fit :
: Network Manager \/ Thinking their systems the most important bit :
: Department of Computing /\ I said to each one with very small wit :
: Univ of Nebraska @ Omaha \/ What's not physical net, ain't really nothin' :
:............................................................................:childers@avsd.UUCP (Richard Childers) (10/06/89)
network@zeus.unl.edu writes: >I have heard that it is advisable to ground these [ethernet] lines. From what I understand, "ground" is used in slightly different ways in the computer and radio fields. In radio, "ground" represents a convenient sink for excess electrons, as well as a gross but adequate reference level for electrical inter- -actions. In computers and telecommunications, "ground" represents a carefully maintained reference voltage level, a median against which machines on both ends of the cable orient themselves and their interpretation of signals. It is entirely separate from the "ground" that is used as a sink for excess electrons, although conceptually related, and in fact, while the frames of the machines are assumed to be grounded, usually through the power socket(s), the actual power that is delivered into the power supply via power cables is electrically isolated from the ground, even though they are both in the same cable, and is more oriented towards handling power supply problems than it is oriented towards defining voltage levels for telecommunications. It's easiest to understand if you think of the planet Earth as a giant capacitor, with an even number of electrons scattered throughout, as a gigantic reference level for busy humans playing with electrical currents, wherever they are. Things get ugly when you're in a plane or a boat, and have to fake it or make it. I'm not sure how it works in space, I'd assume you need to create some sort of "sink" electronically. That's why cars and trucks drag strips of conductive material, to synchronize their electrical level to the planet, which everyone else is using as a reference. Rubber tires insulate bodies nicely, and without this, as you get out of your car, touching the car door, you may get a shock as the excess electrons make their merry way through your nice, conductive body and down into the ground. So, to summarize : o the "ground" ( or "earth", in Great Britain ) represents a mean reference for all voltage measurements on planet Earth o it also acts as a sink for excess electrons o in telecommunications, while there is a need for a mean reference voltage between machines, there is no need for a "sink", as there are _no_ excess electrons in microcircuitry o such that, while you should ground your machines, you should not ground the cables explicitly, other than the normal ground that is associated with connecting them normally as per specifications. Now, no doubt I'll get snowed by zillions of corrections by EEs, but that's about how I understand it and it seems to work. >1) Should a line be grounded only on one end, or on both ends? Both ends. > Should it be grounded at all? Yes. To the machines using the cable, only. >2) What about in line taps that go to devices such as a repeater > or bridge. Meter checks show that this makes a solid ground, > so does that count as one of the grounds? In these cases do > we need to add any ground at all? You shouldn't. The outside of the connector is ground. >3) BNC Tee connections to a Ethernet card in a PC seem to make a > very weak ground or no ground at all. Should we ground these > connections? They should be adequate, as designed. >4) Is it advisable to isolate against all unintentional grounds > as with a connection touching a metal rack by wrapping the > metal part of the connection? Preventing accidental grounding can rarely hurt, and almost always helps, as a preventative measure. >Suggestions and comments are appreciated. It wouldn't hurt to go find a book on antenna theory and / or ground loops, if you're interested in getting into ham radio or are really dedicated to network management, as I have heard ethernet being explained successfully as an insulated antenna ... >: Steven Lendt /\ Managers and users were all having a fit : >: Network Manager \/ Thinking their systems the most important bit : >: Department of Computing /\ I said to each one with very small wit : >: Univ of Nebraska @ Omaha \/ What's not physical net, ain't really nothin' : -- richard -- * "Domains constitute a futile attempt to defeat anarchy and otherwise * * retard progress." (Steve Bellovin, Peter Honeyman, pathalias(l)) * * * * ..{amdahl|decwrl|octopus|pyramid|ucbvax}!avsd.UUCP!childers *
rpw3@amdcad.AMD.COM (Rob Warnock) (10/08/89)
In article <2128@avsd.UUCP> childers@avsd.UUCP (Richard Childers) writes: +--------------- | network@zeus.unl.edu writes: | >I have heard that it is advisable to ground these [ethernet] lines. | ...[a lot of stuff]... | Now, no doubt I'll get snowed by zillions of corrections by EEs, but that's | about how I understand it and it seems to work. +--------------- Well, I'm sorry, you've just given not only wrong but dangerous advice. +--------------- | >1) Should a line be grounded only on one end, or on both ends? | Both ends. +--------------- Wrong. A given Ethernet (thick or thin) *must* be grounded *EXACTLY ONCE*, no more, no less. To do other wise can *increase* your noise problems or even -- are you ready for this? -- melt your cable! Remember, the original question was talking about between buildings. I have seen situations where there are MANY [is >10 many enough?] volts difference between two adjacent buildings, and if you ground an Ethernet at both ends you have just attempted to short out that potential difference. Now since such differences generally come from different neutral-line currents in *heavy* equipments [elevators, big air conditioners, etc.], that's a very low impedance voltage you're trying to short out, and you can't. What you will do instead is simply create a current in the shield of your Ethernet of magnitude I = E/R, where R is the resistance of the shield. Oh, the shield is a very low resistance? Well then I guess you're gonna get a *very* high current in your shield! Try 10's of amperes (or more!)... In any case, not very good for your Ethernet. Even if it doesn't melt it, it will induce large voltage differences between the center conductor and the shield [remember that the senter conductor is "grounded", as it were, only by the termination resistors at the ends], and this can make noise margins worse, or even make reception impossible, or even destroy transceivers. +--------------- | > Should it be grounded at all? | Yes. To the machines using the cable, only. +--------------- *NO!* *NO!* *NEVER* to the machines using the cable! Most of the cost of an Ethernet transceiver goes to keeping a very high isolation between the controller side and the cable side. Even with thin Ethernet, notice that little ring of ceramic surrounding the BNC jack? That's an INSULATOR! And it's there for a reason. Ethernet cables should be grounded once, preferably *VERY* close to a good earth ground suitable for lightening protection. That is, low resistance, low inductance, and as short as possible. While that will not protect against a *direct* lightening strike [almost nothing will], the most common lightening damage comes from *nearby* strikes which induce large voltages in ungrounded metal objects. Oh, and that's another reason not to ground a cable more than once. Nearby lightening also makes various parts of the ground be something other than "ground" for a while, and various "grounds" temporarily become very different voltage references from each other. If your cable is grounded more than once, it will try to conduct current to "short" the various grounds together. It will lose. [See the above discussion of grounds between buildings.] +--------------- | >2) What about in line taps that go to devices such as a repeater | > or bridge. Meter checks show that this makes a solid ground, | > so does that count as one of the grounds? In these cases do | > we need to add any ground at all? | You shouldn't. The outside of the connector is ground. +--------------- You know, that actually brings up a good question. Repeaters SHOULDN'T ground the cable on either side, but *a* ground for each thin Ethernet must come from *somewhere*, and I doubt that most network installers think to provide one. [It may be the case that these multi-thinwire repeaters ground the "down leg" thinwires to their own chassis ground, which would be the right thing to do in most cases assuming the chassis ground is connected to the third or "green" wire, but they should never ground the "up leg", i.e., the trunk or backbone connection.] +--------------- | >3) BNC Tee connections to a Ethernet card in a PC seem to make a | > very weak ground or no ground at all. Should we ground these | > connections? | They should be adequate, as designed. +--------------- Right. "As designed" there is *NO* ground! Which is correct! +--------------- | >4) Is it advisable to isolate against all unintentional grounds | > as with a connection touching a metal rack by wrapping the | > metal part of the connection? | Preventing accidental grounding can rarely hurt, and almost always helps, | as a preventative measure. +--------------- Always helps. Failure to do so leads to "ground loops", which can lead to the problems enumerated above. +--------------- | >Suggestions and comments are appreciated. | It wouldn't hurt to go find a book on antenna theory and / or ground loops, | if you're interested in getting into ham radio or are really dedicated to | network management, as I have heard ethernet being explained successfully | as an insulated antenna ... +--------------- Sounds like you need such a book, yourself. ;-} Sorry to be so harse, but you gave some really bad advice there... Rob Warnock Systems Architecture Consultant UUCP: {amdcad,sun}!redwood!rpw3 DDD: (415)572-2607 USPS: 627 26th Ave, San Mateo, CA 94403
goodloe@b11.ingr.com (Tony Goodloe) (10/11/89)
In article <27682@amdcad.AMD.COM>, rpw3@amdcad.AMD.COM (Rob Warnock) writes: > ... situations where there are MANY [is >10 many enough?] volts difference > between two adjacent buildings When I was taking a noise reduction class (ground loops, shielding, etc) in school, our prof mentioned that in the old EE building they had a heck of time with some equipment. They traced it down to a 60 VOLT difference in ground from one end of the building to the other. WOW! Speaking of grounding, ethernet, and such, a tip for installers that I've heard. If you are seperating two pieces of coax at a barrel, don't hold one connector in each hand as you pull it apart. Hold onto the insulation on the cable. If you do have a large potential difference across the ends of the coax, the path to ground that used to be through the barrel is now through YOU! Thanks for taking the time to clear up the misinformation. tony
childers@avsd.UUCP (Richard Childers) (10/11/89)
rpw3@amdcad.UUCP (Rob Warnock) writes: >Wrong. A given Ethernet (thick or thin) *must* be grounded *EXACTLY ONCE*, >no more, no less. To do other wise can *increase* your noise problems or >even -- are you ready for this? -- melt your cable! Hmm. The thin ethernet cables I'm familiar with all have two ends. Neither end is distinguished from the other. Both have the coaxial braid firmly attached ( grounded ) to the connector ... I don't see how you could only ground it ONCE. It has to be connected to two devices. >Remember, the original question was talking about between buildings. I have >seen situations where there are MANY [is >10 many enough?] volts difference >between two adjacent buildings, and if you ground an Ethernet at both ends >you have just attempted to short out that potential difference. I can see how that might be a problem, but that would seem to require that the equipment be grounded, separately. ( In any case, I don't think ethernet belongs between buildings, I think that's a job for fiber optic. ) >... "As designed" there is *NO* ground! Which is correct! Here's my understanding of [thin] ethernet : ------------------------------------------------------------ [1] ============================================================ [2] ------------------------------------------------------------ [1] [1] coaxial cable, external braid, reference voltage [2] coaxial cable, shielded inner conductor ... where the data is conveyed via [1], and said data is kept from being radiated by [2], which is grounded. [1] connects to the transceiver. [2] connects to the transceiver, perhaps the case, but the transceiver, nonetheless. This is a complementary situation to that found in RS232, where 1 is frame ground, and 7 is signal ground ... and while you may or may not need [1], it being optional, [7] is mandatory, as There Can Be No Signal Without (some sort of) Ground. >| It wouldn't hurt to go find a book on antenna theory and / or ground loops, >| if you're interested in getting into ham radio or are really dedicated to >| network management, as I have heard ethernet being explained successfully >| as an insulated antenna ... >Sounds like you need such a book, yourself. ;-} I have a lot of things to read. _The C Language_ is at the top of the queue, right now ... I haven't seen anything addressing the needs of networking support personnel, though. Any suggestions ? >Sorry to be so harse, but you gave some really bad advice there... Fair enough. But I'd really like to know what I'm not understanding. >Rob Warnock >Systems Architecture Consultant -- richard -- * A CITIZEN: "Who might you be ? Samson ? --" * * CYRANO: "Precisely. Would you kindly lend me your jawbone ?" * * from _Cyrano de Bergerac_, by Edmond Rostand * * ..{amdahl|decwrl|octopus|pyramid|ucbvax}!avsd.UUCP!childers *
rh1m+@andrew.cmu.edu (Rudi Jay Halbright) (10/11/89)
childers@avsd.UUCP (Richard Childers) writes: >rpw3@amdcad.UUCP (Rob Warnock) writes: >>Wrong. A given Ethernet (thick or thin) *must* be grounded *EXACTLY ONCE*, >>no more, no less. To do other wise can *increase* your noise problems or >>even -- are you ready for this? -- melt your cable! >Hmm. The thin ethernet cables I'm familiar with all have two ends. Neither >end is distinguished from the other. Both have the coaxial braid firmly >attached ( grounded ) to the connector ... I don't see how you could only >ground it ONCE. It has to be connected to two devices. >>Remember, the original question was talking about between buildings. I have >>seen situations where there are MANY [is >10 many enough?] volts difference >>between two adjacent buildings, and if you ground an Ethernet at both ends >>you have just attempted to short out that potential difference. >I can see how that might be a problem, but that would seem to require that >the equipment be grounded, separately. ( In any case, I don't think ethernet >belongs between buildings, I think that's a job for fiber optic. ) >>... "As designed" there is *NO* ground! Which is correct! [detail removed] It seems that you're confusing grounding with shielding and the connector attached to the shield wire with "ground". Grounding means attaching to earth ground. This can be either a pipe which is properly grounded or to the third prong on a three prong outlet, ideally the outlet should be tested to make sure the ground pin is properly wired to earth ground and that is not live (i.e. shorted to one side of the power line). In using thin ethernet, one end of the network should be connected to earth ground. The way I do this is to run a wire from the shield of one of the terminating resistors to ground. Simple testors are available to check if ground is properly wired, these are available from many electronics supply houses, including Radio Shack. -Rudi Halbright rh1m@andrew.cmu
johng@trwind.UUCP (John Greene) (10/11/89)
For those of you that are still interested, the IEEE 802.3 specification
addresses this subject. It states that for thick Ethernet, the shield should
be 'earth' grounded at exactly one point. Connectors should be insulated to
prevent accidental grounding at more than one point. It also says that this
only applies to cable contained within a building. Outsides runs are beyond
the scope of the specification and need "special" consideration. All
transceivers are to be AC grounded to the shield in such a way that it has a
high impedance at 60 hertz (> 250 k-ohm) and not greater than 15 ohms between
3 MHz and 30MHz.
One small point on the 75-ohm cable subject: most transceivers that I have
seen "sink" current from the cable rather than "source" current into the
cable. But hey, what's a little minus sign amongst friends? ;-)
--
John E. Greene "People are just like frankfurters....You have to decide
if you're going to be a hot dog or just another wiener" DLR
TRW Information Networks Division 23800 Hawthorne Blvd, Torrance CA 90505
ARPA: johng@trwind.ind.TRW.COM USENET: ..trwrb!trwind!johnggoodloe@b11.ingr.com (Tony Goodloe) (10/11/89)
In article <2140@avsd.UUCP>, childers@avsd.UUCP (Richard Childers) writes: > Hmm. The thin ethernet cables I'm familiar with all have two ends. Neither > end is distinguished from the other. Both have the coaxial braid firmly > attached ( grounded ) to the connector ... I don't see how you could only > ground it ONCE. It has to be connected to two devices. Only one end of the coax may (and should be) tied to "earth". If you do ground it in more than one place you set yourself up for ground loops. I think part of the national electric code (I'm sure I'll be corrected if I'm wrong :) allows only one end to be grounded. > I can see how that might be a problem, but that would seem to require that > the equipment be grounded, separately. If the equipment is in different parts of the building it probably will be using different grounds. > ( In any case, I don't think ethernet > belongs between buildings, I think that's a job for fiber optic. ) can't say that too loudly! tony goodloe
pat@hprnd.HP.COM (Pat Thaler) (10/12/89)
> > >Wrong. A given Ethernet (thick or thin) *must* be grounded *EXACTLY ONCE*, > >no more, no less. To do other wise can *increase* your noise problems or > >even -- are you ready for this? -- melt your cable! > > Hmm. The thin ethernet cables I'm familiar with all have two ends. Neither > end is distinguished from the other. Both have the coaxial braid firmly > attached ( grounded ) to the connector ... I don't see how you could only > ground it ONCE. It has to be connected to two devices. The connector is attached to the shield of the cable, but the connector is not ground. Ethernet devices are required to provide DC isolation between the cable shield and ground. The IEEE 802.3 standard does specify a low AC impedance to ground, but there are to be no DC ground loops. The cable is only to be grounded in at most one place. The connectors are to be protected from inadvertant contact with ground in other places. There is an ECMA LAN Safety Standard ECMA 97 which recommends the level of isolation depending on the situation. In a nutshell, when in a single building with one ground system, 500 V. When going between buildings, 2250 Vdc (or alternate AC or impluse tests). See the standard for elaboration. > Stuff about ground potential differences within a building deleted. > > Here's my understanding of [thin] ethernet : > > > ------------------------------------------------------------ [1] > ============================================================ [2] > ------------------------------------------------------------ [1] > > [1] coaxial cable, external braid, reference voltage > [2] coaxial cable, shielded inner conductor > > ... where the data is conveyed via [1], and said data is kept from being > radiated by [2], which is grounded. > > [1] connects to the transceiver. [2] connects to the transceiver, perhaps the > case, but the transceiver, nonetheless. Actually, the shield of the cable is not used as a "shield" in the strict sense of the word. It is signal return. The transmitter sends a current into the inner conductor and it returns on the shield. There is no radiation (well, little radiation) because the two currents are balanced. Since there might be some imbalance, an _AC_ ground is provided but not a DC ground. A true shield would not carry signal currents and would require using triax or some form of cable with two conductors inside a shield. > > This is a complementary situation to that found in RS232, where > > 1 is frame ground, and > 7 is signal ground > > ... and while you may or may not need [1], it being optional, [7] is mandatory, > as There Can Be No Signal Without (some sort of) Ground. > The difference is that RS232 uses a single-ended signal. That is, the receivers look at the signal received relative to their ground. LANs normally use differential signals since they are designed to cover greater distances and link multiple devices. RS-422 is also differential. A differential receiver looks at the potential difference between two signal lines. Most coax Ethernet receivers do this by having the internals of the transceiver isolated from the chassis ground and referenced to the shield of the coax. Most twisted-pair Ethernet transceivers use an isolating transmformer at the twisted-pair to remove any common mode offset. Pat Thaler