RS485 is bidirectional does it mean it is fullduplex?

That's a bit simplistic. The driver signal hits the characteristic Z of the cable first; *that's* what determines the voltage levels. It then travels along the cable, and eventually hits the last receiver, and its termination resistor. Ideally, there is at that instant no change in impedance, and hence no energy to dissipate/find some other how.

In terms of what the receiver sees, however, I agree. But it's not what (common-mode) we've been arguing about ;).

(BTW, it's now 3:38am here in southern UK, and probably a bit warmer than where you are ;). Nice pics on your site.)

Steve

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Reply to
Steve at fivetrees
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I keep saying it because it is absolutely true.

In fact, that discussion is a subset, not at all similar to long comm cables. It helps to have a good understanding about what causes a "ground loop" and what causes noise induction in long cables.

...

Do you understand what that is saying? It's not exactly a good example!

In fact, it's a bit of mumbo jumbo and clearly the person who last edited it has no idea what a ground loop actually is! But one little part almost got the point: "induces an electric potential across the ends of the grounding line". That part is right, but it is *not* from "alternating current on the lines in the cable", and "induces" is the wrong term too.

Here's a ground loop:

Signal Source

o | Rload | +---------> connection =======//======= I repeat: what you keep proposing makes no sense. I can only presume that

It's simply good engineering, instead of magic.

(Where "magic" is defined as stuff that isn't understood understand.)

I mentioned not having a great deal of experience with lightening. I should probably qualify that. I have only a few years of experience with lightening, as opposed to decades with the rest of this.

What you say above isn't true.

If you have no expertise in this topic, please *do not* pontificate.

Wrong.

Wrong.

Wrong.

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

You'll forgive me for laughing out loud here.

Steve

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Reply to
Steve at fivetrees

Like I said, low noise immunity. With a very low common mode dynamic range, for one thing.

If you want something with better noise immunity, look at the DS1 interface specifications.

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

Figured I should clarify that.

Floyd, I'm really trying hard (now) to follow you. I've taken every diagram you've drawn, viewed it with a fixed-width font, and tried hard to understand your point. (Perhaps I'm misunderstanding something.) I've gone back and re-read all your posts. I'm utterly convinced you're sincere, you're passionate, and you know your stuff. I'm equally convinced that said stuff does not include RS-422/485, or medium-haul datacomms in general. I really believe you're missing the point (common-mode).

I'm also enjoying this thread. Keep 'em coming ;).

Steve

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Reply to
Steve at fivetrees

Eh?

If you mean the +/-7V common-mode range, yeah, it's not huge - but it's more than adequate if correctly applied.

Steve

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Reply to
Steve at fivetrees

The voltage on the cable is determined by that 100 Ohm resistance, not by the impedance of the cable.

If you don't believe it, go find a pair of old 10base5 ethernet NICs and connect them together with *any* coax of your choice. It was designed for 50 Ohm coax with a 50 Ohm term on each end. But try it with the commonly available 75 Ohm cables used for cable TV.

It will work just fine... unless you use a 75 Ohm termination. That will kill it dead.

Lacking the ability or opportunity to do that, try reading up on in it. You can find that described in any number of places.

What the receiver while see, no matter where it is on that cable, is the voltage across that 100 Ohm resistor. And yes that is exactly where common mode rejection takes place.

And it happens anyway, that the impedance of the cable will almost certainly be something between 120 and 150 Ohms anyway.

Thank you. It hasn't been updated for awhile. I do have another photo essay that I should finish off and make available. It's on skin "umiaq" boats. (There are also other parts of the web page that aren't accessable via the homepage, where I have some programing available.)

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

And I see you have no idea what the difference between a DS1 interface and a RS-485 interface is when it comes to noise immunity.

As I said, check out a DS1 interface.

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

Okay. That's just fine.

Hmmmm... It just occurred to me that if this goes into customer premise locations, the benefit is obvious. That is *much* less complex than telling someone they have to install an appropriate ground system!

I suspect that was engineered around less than the best customer premises. They can't control the customer's environment, so designing it to avoid that altogether is a smart thing to do.

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

You have yet to point out a single factual error. Just *you* saying it is, without supporting your comments with discussion that explains a bit of it, is indicative.

I notice, for example, that you don't want to respond to the detailed discussions I've provided on cable grounding. Instead you pick other posts and just claim it isn't true. If you see an error in the explainations I posted, please point it out. I do make mistakes. I post when tired, I type too fast, and don't always read it well enough. Plus sometimes I just have the wrong idea. But you are going to have to support it with more than pontification...

Apparently quit a ways out.

Could be Steve, but since the discussion has been about

*cables*, and *ground systems*, I don't see what value your practical applications have to the topic when you don't seem to understand the *specific* part that it is about.
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Floyd L. Davidson           
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com
Reply to
Floyd L. Davidson

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.

Paul

Reply to
Paul Keinanen

In the TN-C electric distribution system, the "Signal source" is the mains phase voltage (100-240 V depending on country), Rload perhaps

10-1000 ohms, Rwire perhaps 0.2 ohm. An AC current up to 25 A could flow though Rload and through Rwire. The voltage drop across Rwire would be up to 5 V.

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.

The magnitude of this "ground loop" current depends on the ratio of the Rwire compared to the combined resistance of the cable shield and the right hand side equipment grounding resistance. Especially, if the right hand side equipment is close to the main distribution panel, the total path resistance is dictated by the shield resistance. If the alternate path total resistance is as low as Rwire, more than 10 A could flow in the cable shield, which could even cause a fire hazard.

Paul

Reply to
Paul Keinanen

You should also realize that 'signal source' above includes a myriad of possibilities, including lightning, pools of acid forming batteries, whatever. We can never really get at a true ground except through some sort of impedance. Luckily most (but not all) of our controllable signal sources are imposed between the marked signal point and the junction of Rwire and Rload.

Also the effects of Rwire, and the IR drop across it, can be present at either the near or far end, or both. Those R things are not necessarily resistive, nor passive.

--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Reply to
CBFalconer

I think that was the general idea.

The people who installed this stuff were much more used to installing 4-20mA current loop instruments, which were always isolated from ground, and the 4-20mA current loop could be grounded at any point in the loop (but hopefully only at one point).

RS-485 was pretty darned high-tech at the time. We were using it to replace a weird proprietary busd that used 48V differential signalling (with a floating common) that ran at two different baud rates. The "high speed" version was 250 baud, IIRC. A big advantage of RS-485: it doesn't hurt nearly as much as 250Hz at 48V. The 48V was current-limited but it still stung.

--
Grant Edwards                   grante             Yow!  Okay... I'm going
                                  at               home to write the "I HATE
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Reply to
Grant Edwards

I just looked up the dictionary definition of "pontification". It has two meanings, but I'm ignoring one, as I have no immediate plans to become the next pope. The other involves opinions.

The only opinion I've expressed in this thread is that you're misguided. Everything I've said about RS-422/485 is factual, and you are very welcome to check these facts against the published specs. (I have them here, and am very familiar with them. You don't appear to have read them.)(I can't point you at them on the web; the specs are not in the public domain.)

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.

Finally, look around. Do you see anyone agreeing with your somewhat strange point of view? Does this not tell you something?

Whatever it is you're talking about, it's certainly not RS-422/485.

Steve

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Reply to
Steve at fivetrees

No.

"Signal Source" means whatever your *desired* signal is. Not the AC power distribution... simply because how much that is perterbed is of no significance.

Across Rload... is your Hifi Amp, your telephone set, the PA system, computer data, gas tank level sensor output, whatever...

However, we could add that and many other sources of voltages. It makes the diagram and the discussion more complex, but the one particular example you chose is an excellent one because it is commonly seen and often has significant effect.

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.

We need to make the distinction that this is a ground system fault, not an inherent characteristic of the indicated connections. And it can happen whether the arrangement is properly designed or not.

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

I hate to mention this, but there is a difference between comm cable and power cable. The Neutral in three phase power systems is *not* the equivalent of the shield on a comm cable.

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

So you declare yourself correct, eh? No discussion, no facts, nothing but pontification.

If it is demonstrable, why don't you!

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

I repeat: please see the RS-422/485 specs.

I, and others, have been trying. I'm about ready to give up.

Steve

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Reply to
Steve at fivetrees

IMHO, This is related to the difference between mechanical and electronic ground. In the systems we made years ago, the electronic ground was isolated from the mechanical ground. A jumper was provided that enabled the customer to tie both grounds if he wished. He was responsible for the earthing of the system.

You will find perhaps the following useful:

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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

Reply to
Lanarcam

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