RS485 is bidirectional does it mean it is fullduplex?

It's a frame ground. A signal ground would be a return path to ground for the signal. RS-232 has such a signal, and is "single ended", hence all signal lines share the same common ground return line.

Differential ciruits are "balanced", the signal is between the two wires of a pair. Neither of them is at ground, so neither is called a "signal ground". The signal does not depend on any relationship to 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.

Optical isolators are are nice because they have a significantly higher "useful dynamic range". Otherwise, the signal on the cable is still the same.

Induced current into that ground wire accompanying the signal pairs. That's why you don't want it to be one small wire if it in the same bundle as the signal pairs.

It is a bit complex. Many people who work with cables don't understand it very well. But any one who sits down and studies it a bit can understand it. It isn't even rocket science! ;-)

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.

That's a pretty good run for RS-485.

What kind of cable was this? Cable you installed, or telco cable?

The cable still has to be grounded at both ends. That connects your two frame grounds together too.

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.

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. Each comm cable entrance would be considered just like an individually isolated rack, and would have its own ground cable going to the grounding point for that floor. (What this says is that the cable is *not* connected to a rack. That a rack in one row is *not* connected to a rack in a different row. That no two rows share a single cable going to the ground point. But often racks in one row share a ground cable, and often equipments mounted in one rack share ground wires.)

Both ends of the cable should be grounded in that fashion.

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

There is no point in repeating what I just said.

Likewise, there is no point in explaining the basics of differential comparitors.

...

And this has what to do with the comment you quoted and are discussing?

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

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 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").

Meindert

I can assure you that the screen of my telephone cable is NOT connected to ground on my side. 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.

And all those slave transmitters should be disabled, except the one that's talking to the master.

So, if all slaves have a dominant Mark, how is one slave going to drive a Space on the line?

Meindert

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.

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).

Steve

The first clue is in the RS-422 spec; I quote: "The driver has the capability to (...) drive up to 10 parallel connected receivers."

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.)

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.

Steve

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.

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?

Are you talking about "inducing" a DC voltage?

What "noise"? I'm talking about controlling common-mode DC level difference between the RS-485 transmitter and receiver.

I think the spec is 10km for decent twisted pair and low baud rates (>> I'd expect that across the room or around the bend might be

Cable an electrician installed.

The shield may be grounded at one end or the other, but the RS-485 common is not.

What are "frame grounds" and what do they have to do with the RS-485 bus???

You keep talking about "frame grounds" and earth and stuff.

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.

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.

--
Grant Edwards                   grante             Yow!  Someone is DROOLING
                                  at               on my collar!!
                               visi.com

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).

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.

Peace everyone =p

-kunil

Yes.

I don't see how you can use A or B as a "common" line.

What happens when nobody is driving the bus is a (mostly) different issue. Usually solved by pulling one line to the reference/common node and the other to 5V (with respect to the reference node).

The reference node connection between the two ends is required to keep the A/B signal values being output by the transmitter within the common-mode voltage range spec for the receiver.

You don't need to earth either one, as long as the transmitter/receiver reference nodes at the two ends are tied together.

--
Grant Edwards                   grante             Yow!  Bo Derek ruined
                                  at               my life!
                               visi.com

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.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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.

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

My point was that it is not mentioned in the standard.

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

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.

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.

They are high impedance devices. The only "disable" they need, is to not be sending.

If all of them try to send at one time, you get garble.

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

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.

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 (>I have the same experience. I ran an RS-485 network in a factory

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.

I'd concur with that conclusion.

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.

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.

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 ....................
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/********************************************************************

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

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".

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.

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.

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 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".

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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.

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.

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.

--
Grant Edwards                   grante             Yow!  I'm having an
                                  at               emotional outburst!!
                               visi.com

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 |

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           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

to

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.

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.

Yes, but the pair inside the shield cannot "see" the original source, so nothing is cancelled there.

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.

Yes, that's why they have a control input to take the transmitter off line.

Meindert