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RS485 is bidirectional does it mean it is fullduplex?

Started by Swizi June 15, 2005
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote: >> "Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote: >>>"Tim Mitchell" <timng@sabretechnology.co.uk> wrote: >>>> RS485 is a 2-wire half duplex system where there is one transmitter and >>>> multiple receivers. >>>> >>>> RS422 is a 4-wire duplex system which goes between two devices, each of >>>> which has a transmitter and receiver. >>> >>>While this is essentially correct, I'd add: >>> - Don't forget ground - i.e. 2-wire is actually 3-wire, and 4-wire is >>>actually 5-wire. >> >> RS-485 is a differential (balanced) system, and there is no >> signal ground connection. The cable used might well include a >> frame ground, but that is for noise induction cancellation, not >> signal ground. >> >> Hence it actually is a 2-wire or 4-wire link. > >I'm a bit shocked at the amount of traffic this point has generated. It's >fairly fundamental. > >If we were talking about balanced transformer-coupled audio, you'd be right. >But we're not - we're talking about common-mode voltage, which is >effectively +/-7V for RS-485 (on top of the 0-5V signal range, hence >+12/-7V). This (small) range is fine for connections that are physically >close, but really *not* fine for long lines across disparate grounds. (In >the old days of RS-422, I saw a lot of fried drivers for this very reason.) >In practice, one usually designs in an optically-isolated RS-485 interface, >to allow the 3rd/5th wire to be explicitly connected without >cross-connecting grounds.
Voice frequency circuits on twisted pair cable operate at *much* lower voltage levels over *much* long distances, all without the consideration you claim is necessary.
>So, I repeat: one *has* to consider common-mode. That 3rd or 5th wire must >be there, whether implicitly (via a common local ground) or explicitly (via >a physical cable).
You haven't caught the significance of what I've been saying either. Your "3rd or 5th wire" is *not* the way to deal with ground potential differences. What has been described causes more problems than it cures. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote: >> >It would be unrealistic to assume that the grounding electrodes of two >> >separate buildings would stay within the -7.. +12 V common mode range >> >at all times (especially during thunderstorms), so optoisolation >> >should be used to keep the grounds separate. The real question is, is >> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should >> >a fiber optic cable be used instead. >> >> I don't have a lot of experience with thunderstorms, so I can't >> really comment on that. Otherwise, that simply is not true. > >I have the same experience. I ran an RS-485 network in a factory invonment, >using one twisted pair for data and two other pairs to distribute the power. >Total length appr. 500 meters (1500 feet). Never ever had a problem, all >boards shared the same power supply ground. When a link had to be made >between two buildings, the power was split and only both datalines were >connected. At both ends, the systems we ground. Every thunderstorm blew the >Tx chips.
The obvious conclusion to be drawn is that it was not designed properly, almost certainly due to a lack (then and now too perhaps) of understanding about what caused the damage. That has virtually *nothing* to do with signal ground, frame grounds, power distribution, or for that matter common mode voltages. It has to do with induced voltages from static discharge (lightening) and protection against such surges. You mentioned nothing about that, and I assume there was none provided.
>> You *absolutely do* want a good ground connect at both ends. > >The only think you absolutely want is some means to prevent excessive common >mode voltage outside the range of the TX chips. How you achieve this, is not >important. This could easily be done with the "third wire" to connect both >commons (I explicitly won't call it "ground").
It *is* important. And it isn't, as your example above demonstrates, just common mode voltage equalization. This "third wire" would have done *nothing* to benefit your situation above. The point is that if frame grounds are connected properly the *normal* amount of noise on the cable is reduced. If done the wrong way the noise will be increased. In fact, *either* way will probably work most of the time! That leads to a lot of statements to the effect of "we did that, and it worked". But if you have a nationwide network, with millions of examples, those which are done wrong do show up as being where 90% of the maintenance costs go. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
On Thu, 16 Jun 2005 13:57:44 +0100, "Steve at fivetrees"
<steve@NOSPAMTAfivetrees.com> wrote:

>The second clue is in the fact that many of the old RS-422 drivers had >tristate control inputs. (I would agree, however, that this appears not to >be mentioned in the RS-422 spec.)
My point was that it is not mentioned in the standard.
>In any case, multidrop RS-422 was widely used. (All the comms for all the >products from the company I was working for in the late 70s and 80s were >done this way.) It worked.
A question of semantics, should such system be called a multidrop RS-422 system or a 4-wire RS-485 system :-). While this configuration is widely used, claiming that it conforms either to the RS-422 or RS-485 standard would be a bit suspicious. Paul
"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote in message >news:873brik81n.fld@barrow.com... >> Every telephone cable in the country has the shield connected >> to ground at both ends. > >I can assure you that the screen of my telephone cable is NOT connected to >ground on my side.
Then I assure you that 1) you are either talking about a drop cable, which does not have a sheild, or 2) who ever installed it was incompetent.
>Which is logical too, because you want to keep any current through the >screen as low as possible and that can onlybe achieved by connecting the >screen at only one side.
Wrong. You want to have any current induced into each individual pair to be *more* significant in the shield than in the pair. The shield, being grounded at both ends, will have current flow that will set up an *opposing* field to the original source, and since it is fairly closely coupled to the cable pairs, it will in fact cause at least some cancellation of noise on the pairs.
>> And absolutely unnecessary too. RS-422 works on two pairs. The >> master transmitter and all slave receivers are on one, and the >> master receiver and all slave transmitters are on the other. > >And all those slave transmitters should be disabled, except the one that's >talking to the master.
They are high impedance devices. The only "disable" they need, is to not be sending.
>> They are *all* high impedance devices, relatively. There has to >> be a 100 Ohm load resistor on each pair. >> >> >hard. However, both RS-232 and RS-422 can be used in a multidrop >> >configuration with a few diodes, but this would reduce the noise >> >margins, since the bus would only be actively driven into the Space >> >state, while passively pulled by bias resistors to the Mark (which >> >also is the idle state). >> >> Unnecessary for RS-422. > >So, if all slaves have a dominant Mark, how is one slave going to drive a >Space on the line?
If all of them try to send at one time, you get garble. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Floyd L. Davidson wrote:

> "Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote: >>"Floyd L. Davidson" <floyd@barrow.com> wrote: >>> >It would be unrealistic to assume that the grounding electrodes of two >>> >separate buildings would stay within the -7.. +12 V common mode range >>> >at all times (especially during thunderstorms), so optoisolation >>> >should be used to keep the grounds separate. The real question is, is >>> >the 0.5 - 2.5 kV isolation found on many RS-485 cards enough or should >>> >a fiber optic cable be used instead.
Having designed a comms interface for one of the grottiest, noisiest working environments I have ever come across and ensured that it would survive the ordeal, I can confirm that management of the energy is where you need to focus in order to prevent such failures occurring.
>>> I don't have a lot of experience with thunderstorms, so I can't >>> really comment on that. Otherwise, that simply is not true.
You can get similar problems with a 20MA single turn secondary running from a primary at 1kV which, when the circuit unexpectedly disrupts, releases very large transients back through almost all the systems. Even the most sensitive of our inputs (<10nV) is protected for at least 8.4kV short duration transients. We only ever connect screens at one end (usually the instrumentation cabinet) as, with that much energy around, the three different earths can separate by as much as 75V for a short time. It is not fair on cable screens to carry the sort of current that would imply (it would probably cause a fire anyway).
>>I have the same experience. I ran an RS-485 network in a factory >>invonment, using one twisted pair for data and two other pairs to >>distribute the power. Total length appr. 500 meters (1500 feet). Never >>ever had a problem, all boards shared the same power supply ground. When a >>link had to be made between two buildings, the power was split and only >>both datalines were connected. At both ends, the systems we ground. Every >>thunderstorm blew the Tx chips.
You probably need to look at including more serial resistance and heavier diode clamping at the intgerface connections. Also, control the impedance of the connection of your power supply 0V to the chassis within each module. Really think about the path that the surge/transient energy is going to take.
> The obvious conclusion to be drawn is that it was not designed > properly, almost certainly due to a lack (then and now too > perhaps) of understanding about what caused the damage.
I'd concur with that conclusion.
> That has virtually *nothing* to do with signal ground, frame > grounds, power distribution, or for that matter common mode > voltages. > > It has to do with induced voltages from static discharge > (lightening) and protection against such surges. You mentioned > nothing about that, and I assume there was none provided.
Actually, you have to consider both the grounding issues and the static/transient discharge issues in concert with each other. Some things that may be very good for one are absolutely awful for the other. If you need to minimise noise pick-up as well then you have quite a bit of work to do. When I tell you to manage the energy, it is all of the energy you need to consider (which includes your desired signal as well. PSpice can be a tremendously good tool for sorting these issues out.
>>> You *absolutely do* want a good ground connect at both ends.
No, just the one end and preferrably the instrument rack/master end of the cable. In the rare circumstance that you cannot isolate the screen at the remote end put a break in the screen at a convenient point close to the remote end and make sure it stays broken.
>>The only think you absolutely want is some means to prevent excessive >>common mode voltage outside the range of the TX chips. How you achieve >>this, is not important. This could easily be done with the "third wire" to >>connect both commons (I explicitly won't call it "ground"). > > It *is* important. And it isn't, as your example above > demonstrates, just common mode voltage equalization. This > "third wire" would have done *nothing* to benefit your situation > above. > > The point is that if frame grounds are connected properly the > *normal* amount of noise on the cable is reduced. If done the > wrong way the noise will be increased. In fact, *either* way > will probably work most of the time! That leads to a lot of > statements to the effect of "we did that, and it worked". But > if you have a nationwide network, with millions of examples, > those which are done wrong do show up as being where 90% of the > maintenance costs go.
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. I shall have to look up the equipment build standard that details all these issues and post the number. All embedded systems engineers should really know this stuff anyway. -- ******************************************************************** 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/ ********************************************************************
On Thu, 16 Jun 2005 10:27:48 -0800, floyd@barrow.com (Floyd L.
Davidson) wrote:

> >>> And absolutely unnecessary too. RS-422 works on two pairs. The >>> master transmitter and all slave receivers are on one, and the >>> master receiver and all slave transmitters are on the other. >> >>And all those slave transmitters should be disabled, except the one that's >>talking to the master. > >They are high impedance devices. The only "disable" they need, is >to not be sending.
You seem to confuse the RS-422/485 with CAN bus. In CANbus, the transmitter is only actively sending the dominant state ("0"), while passive pull-ups put the bus into the recessive state ("1"). All inactive transmitters are constantly "sending" the recessive state and only the active transmitter and only when sending the dominant state is actually sinking/sourcing current to the bus. Prior to dedicated CANbus transceivers, ordinary RS-485 transceivers were used, with the transmitter input tied constantly to "0" and the data stream connected to the transmit enable pin ("0" enabled the transmitter) to generate the dominant state on the bus. Similar multidrop dominant/recessive state behaviour can be created with RS-232 and RS-422 without transmit enable using one or two diodes. Paul
Grant Edwards <grante@visi.com> wrote:
>On 2005-06-16, Floyd L. Davidson <floyd@barrow.com> wrote: > >>>> That is a frame ground, and not a signal ground. It will carry >>>> no signal current at all. >>> >>>It doesn't carry any signal current, but it is the ground to >>>which the receiver's input signal range specs are references. >>>It's the ground that defines what "0V" is for the signal >>>inputs. I call that the signal ground. >> >> It's a frame ground. > >Sorry, never heard that phrase before. I assuem "frame" and >"chassis" were the same. In the installations I've dealt with >the RS-485 common is certainly not chassis ground on either end.
Frame ground is "chassis ground". For example, Pin 1 on the 25 pin RS-232 connector is variously labeled as "Chassis", "Protective", "Shield", or "Frame" ground. Pin 7 is "Signal Ground".
>> If course, the receiver typically cannot tolerate a common mode >> voltage greater than some specified voltage. That that is not >> a signal voltage in any way. It just biases the devices out of >> their useful dynamic range. > >And you've got to somehow guarantee that the recievers common >mode DC voltage is within spec. If the only DC connections to >the outside world are the A/B signal lines, how is that >accomplished?
Well, the opposite side of that would be "somehow guarantee use of receivers that can handle the existing common mode voltage excursions". (Note that I am specifically not limiting that to DC.) The point is not that there is no DC connection to the outside world, but that is has to be done *correctly*. And that is not accomplished via a single ended one wire loop added to the required pairs.
>>>> The trick is to get the induction into the ground wire to >>>> then, in the cable between the ground wire and the signal >>>> pairs, cancel the induction into the signal cables. > >Are you talking about "inducing" a DC voltage?
I'm not talking about DC. I'm talking about how to reduce *noise* in a communications cable. Very few such cables operate in an environment where there is no significant power line influence, not to mention other noise sources. If the ground system is properly designed, the noise in the cable is reduced. If not done right, it can be substantially increased. And it can exceed ground potential difference by several times, too. There is no point in reducing the DC ground potential from 10V to 0V, and in the process acquiring 20 VAC in the process.
>> Yes. You also have to be very careful about the currents >> induced into said ground connection. Do it the wrong way, and >> it adds noise to the signal pairs; do the right way and it >> will help cancel noise induced from the same source into those >> signal pairs. > >What "noise"? I'm talking about controlling common-mode DC >level difference between the RS-485 transmitter and receiver.
The RS-485 signals are carried on a cable. Any influence on the output which is not the input signal, is noise. It is impossible to avoid (particularly 60 Hz power influence). One reason RS-485 was only specified for 4000 feet is because it isn't very immune to noise. DC common mode offset is just another noise...
>>>> What kind of distances have you tried that with? >>> >>>A couple kilometers. >> >> That's a pretty good run for RS-485. > >I think the spec is 10km for decent twisted pair and low baud >rates (<1M).
It was originally spec'd at 4000 feet. Better line driver technology has extended that.
>> What kind of cable was this? Cable you installed, or telco >> cable? > >Cable an electrician installed.
If he had significant experience with comm cables, which is most likely, then your cable had a properly grounded shield and had surge protection installed at both ends.
>> The cable still has to be grounded at both ends. > >The shield may be grounded at one end or the other, but the >RS-485 common is not.
The RS-485 at each end is connected to the same ground that the cable is connected to. But there should *not* be a cable pair dedicated to connecting the two.
>> It *should* be connected to earth ground at both ends. And that >> ground point *should* be a single point where *all* frame >> grounds for the entire building go. > >What are "frame grounds" and what do they have to do with the >RS-485 bus???
Ground that is not a signal path. E.g., common mode ground.
>> Typically equipment bays in a single row are strapped together, >> though sometimes individual racks will have separate grounds. >> There should be a single cable from each row (or each rack if >> some racks are isolated) to a common grounding point on each >> floor of a building. > >You keep talking about "frame grounds" and earth and stuff.
It's necessary to grasp the difference in what "ground" is, and I'm not really aware of what your exposure to it is. I was assuming that since you wanted to talk about RS-485 at the hardware level that you'd probably been exposed to all of this, but wouldn't necessarily have remembered it or found any of it significant. In that case, simply using the vocabulary correctly will enforce a proper set of definitions on the discussion. But if you aren't into electricity for the sake of electricity... yeah, this starts getting to sound like word soup! Sorry about that.
>The RS-485 systems I'm talking about are all optically isolated >from frame, chassis, and earth. If you don't connect the RS-485 >commons together with the cable, then you end up with >common-mode voltages out of spec. Study all you want, that's >what happens in practice.
What you are doing will result in equalizing the common mode DC offset from different grounds. It is *not* the best way to do it, simply because it can (not necessarily, but *can*) cause just as many problems as it solves. Done properly, you don't have trouble with 1) common mode offset, 2) induced AC and other transient, or 3) lightening surges. But any of those can be handled in other ways... which increase the potential for trouble with one of the others. The shorter the cable run, and the fewer hazards it is exposed to, the fewer problems. Hence it can easily be done in ways that are not the best, and yet work very well for years. But that doesn't mean those methods are "correct".
>I don't care what you do with the cable shield, and frame >grounds and chassis grounds, but they aren't connected to >RS-485 common.
They should be. But you've got two different circuits you are talking about too. One on each side of the optical isolation. On one side the common mode range is narrow, and on the other is is very high. The isolators are used over the cable, so ground potential offset is not a problem (because the offset voltage will never approach the common mode limit for the optical isolators). On the other side, they are *all* connected to a common ground, if they are properly engineered. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
On 2005-06-16, Floyd L. Davidson <floyd@barrow.com> wrote:

>>The RS-485 systems I'm talking about are all optically isolated >>from frame, chassis, and earth. If you don't connect the RS-485 >>commons together with the cable, then you end up with >>common-mode voltages out of spec. Study all you want, that's >>what happens in practice. > > What you are doing will result in equalizing the common mode DC > offset from different grounds.
Except that none of the RS-485 circuits were grounded. They were all optically isolated and the RS-485 transmitter and receiver grounds were floating.
> It is *not* the best way to do it, simply because it can (not > necessarily, but *can*) cause just as many problems as it > solves. Done properly, you don't have trouble with 1) common > mode offset, 2) induced AC and other transient, or 3) > lightening surges. But any of those can be handled in other > ways... which increase the potential for trouble with one of > the others. The shorter the cable run, and the fewer hazards > it is exposed to, the fewer problems. Hence it can easily be > done in ways that are not the best, and yet work very well for > years. But that doesn't mean those methods are "correct". > >>I don't care what you do with the cable shield, and frame >>grounds and chassis grounds, but they aren't connected to >>RS-485 common. > > They should be.
They aren't. The product spec required that the RS-485 bus be isolated. It's pretty common (at least in the type of gear I saw) for RS-485 interfaces to be optically isolated. The RS-485 transmitters and receivers had floating grounds.
> But you've got two different circuits you are talking about > too. One on each side of the optical isolation. On one side > the common mode range is narrow, and on the other is is very > high. The isolators are used over the cable, so ground > potential offset is not a problem (because the offset voltage > will never approach the common mode limit for the optical > isolators).
No, the opto-isolators were between the RS-485 transmitters/receivers and the rest of the gear. The Rs-485 transmitters and receives were galvanically isolated from earth, chassis, and supply ground. Our experience was that connecting two "floating" RS-485 interfaces together without a common (connecting just the data lines) resulted in a lot of problems.
> On the other side, they are *all* connected to a common > ground, if they are properly engineered.
-- Grant Edwards grante Yow! I'm having an at emotional outburst!! visi.com
"kunil" <kunilkuda@gmail.com> wrote:
>May I summary this ? > >I think RS485 problem is 2 : > *) Noise over long distance cable and > *) Faulty data because of different node voltage reference > >To prevent noise over long distance cable, we can earth one side of the >cable shield (refer to Ott, Henry, Noise Reduction Techiques in >Electronic Systems).
On a "long distance cable", you'd better ground *both* ends. First, understand that a "shield" has virtually no effect at 60 Hz. The reduction in induced signal at 1000 Hz is about 3 dB when a shield is added. At 60 Hz the difference is about 0.04 dB! (Which says, we don't put a shield on the cable to necessarily reduce noise in the obvious way! It has other effects, if used correctly.) A "ground loop" is caused by having a *common* ground path for two signals. Hence if the "ground" for a cable shield is provided by attaching it to the equipment, and most particularly if it is attached in a way such that from the connection to some other point there is a shared path with the signal, current in the cable shield will affect the signal to the degree that it can cause a voltage drop across the distance of that common connection. That can be significant at higher impedances. That is the *wrong* way to ground a long cable. I've emphasized a separate ground cable is required, and that buildings require a single point ground system, just to avoid said ground loops. Here it is graphically. This is an incorrectly grounded cable shield, causing a ground loop with each equipment. All currents induced into the cable shield share the common connection to ground *through* the equipment. +-------+ +-------+ | | >--------- tx wire/pair ---------> | | | EQUIP | <--------- rx wire/pair ---------< | EQUIP | | | ========= cable shield ========= | | +-------+ | | +-------+ | | | | | | | +---+ +---+ | o o | | ----- Earth ----- Earth --- Ground --- Ground - - By the expedient of removing the ground at one end, several things are accomplished. One is the removal of the ground loop. It also, however, removes common mode DC equalization, and it reduces the current flow in the cable shield, which happens to have a negative effect, as I'll show. Note that this is *very* appropriate for use with cable existing within a single building. The benefit is the same, but the negatives are of negligible effect. +-------+ +-------+ | | >--------- tx wire/pair ---------> | | | EQUIP | <--------- rx wire/pair ---------< | EQUIP | | | ========= cable shield ========= | | +-------+ | +-------+ | | | | | +---+ | o o | | ----- Earth ----- Earth --- Ground --- Ground - - However, if the cable is a long run, and particularly if there is exposure to power lines, if the ground potential is different at the two ends, or if there are any other sources of induced noise in the cable, this arrangement has the best effect: +-------+ +-------+ | | >--------- tx wire/pair ---------> | | | EQUIP | <--------- rx wire/pair ---------< | EQUIP | | | ========= cable shield ========= | | +-------+ | | +-------+ | | | | | | | | o------+ +------o | | ----- Earth ----- Earth --- Ground --- Ground - - Note the minimum common path to ground. If correctly sized there will be no significant voltage drop across that small section. (Which is to say, that should probably be copper strap between a copper terminal plate and the actual ground system connection.) Hence, there is no "ground loop" effect. However, the two grounds are connected electrically and the voltage is equalized between then. The second benefit is that voltages induced into the cable by exposure to electro magnetic fields will have a low impedance circuit path, and will therefore conduct current. Just as it does in a transformer, current changes in one direction cause a opposite voltage to be induced into a coupled conductor. Hence, we have the external field causing a voltage in the shield and the pairs which is identical. The current that flow is the shield causes an exactly *opposite* voltage to be induced into the cable pairs. The externally induced voltage and the shield induced voltage cancel to some degree in the cable pairs, thus reducing external noise induction into signal pairs. This effect *requires* both ends be grounded (with quality connections presenting a relatively low impedance to the noise current).
>To prevent faulty data because of different node voltage, we can use >common line. But since RS485 is a differential mode protocol, we can >use either A or B line as our common line. > >However, if we connect using this fashion, when the master is in the >idle mode, there will be floating voltage between A or B line (since >nobody is driving the bus). Therefore, we connect pull-up/pull-down >resistors in the A and B line (to give at least definite voltage level >when nobody's driving the bus). > >To make this "definite voltage level" same at the receiver / >transceiver point, we need to earth their voltage reference node at the >both side. > >I think everyone is correct here. Just the naming convention that makes >confusion.
That is indeed a significant part of the problem. The other part is just not being exposed to the full expanse of what is involved in data transmission over longer lengths of twisted pair cables. -- 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:874qbyi2cb.fld@barrow.com...
> "Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote: > >"Floyd L. Davidson" <floyd@barrow.com> wrote in message > >news:873brik81n.fld@barrow.com... > >> Every telephone cable in the country has the shield connected > >> to ground at both ends. > > > >I can assure you that the screen of my telephone cable is NOT connected
to
> >ground on my side. > > Then I assure you that 1) you are either talking about a drop > cable, which does not have a sheild, or 2) who ever installed it > was incompetent.
Well, 1) someone from the telephone company installed it and 2) is does have a shield. This is just the way it is done in the Netherlands.
> Wrong. You want to have any current induced into each > individual pair to be *more* significant in the shield than in > the pair.
How can that be? An induced current in the shield will cause, given high enough frequency and long enough cable, induce almost the same current in the pair, as common mode. Just look at the principle of a transmission line transformer. The goal is to induce as little current as possible in the shield. And that can only be achieved by grounding it on one side.
> The shield, being grounded at both ends, will have > current flow that will set up an *opposing* field to the > original source,
Yes, but the pair inside the shield cannot "see" the original source, so nothing is cancelled there.
> They are high impedance devices. The only "disable" they need, is > to not be sending.
They are NOT high impedance devices. Go check a datasheet. For instance, the MAX485 has an open voltage of 5V, and 2V when loaded with 50 Ohms. That represents an internal impedance of 75 Ohm. Not sending is not the same as being disabled. Not sending means to be in an idle (mark) state, but still driving the line into that state. Disabling means shutting down the driver to become high impedant.
> If all of them try to send at one time, you get garble.
Yes, that's why they have a control input to take the transmitter off line. Meindert