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RS485 is bidirectional does it mean it is fullduplex?

Started by Swizi June 15, 2005
"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87mzppg39l.fld@barrow.com...
> > The fact is the 100 Ohm load resistor, being of lower resistance > than any other load on the cable, is what determines the voltage > levels. The receivers are high impedance voltage sensing devices.
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 http://www.fivetrees.com
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote: >> That is *not* true in the situation being discussed, where there >> is a cable long enough to have the two ends connected to separate >> ground systems and equipment supplied by separate power systems. > >You keep saying this, but it's wrong. If you won't believe us, use Google to
I keep saying it because it is absolutely true.
>do some searching. A few seconds of searching turned up: > http://www.hw-server.com/rs232_signals.html > >It's about RS-232, but the grounding issue under discussion is the same >(except it's worse with RS-232, since ground differences are seen as >signal). I'll quote a section:
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.
>1 Protective Ground
...
>Ground loops are low impedance closed electric loops composed from ground >conductors. When two grounded devices are connected together, say by a >RS-232 cable, the alternating current on the lines in the cable induces an >electric potential across the ends of the grounding line (either Protective >Ground or Signal Ground), and an electric current will flow across this line >and through the ground.
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 =======//======= <------+ | to cable | Rwire shield | | | | | ----- Earth Ground ----- --- --- - - Okay, there are three sources of current that affect voltages across the two resistors. The "signal" is listed as "Signal Source". Assume that is a current limited source, just to make this more obvious. There is induced current in the cable shield, and there is current from the ground potential difference between the two Earth Grounds. All currents contribute to the voltage across Rwire. The voltage across Rload is affected by the voltage across Rwire, and thus the voltage drop across Rwire and across Rload. Current through the shield affecting the voltage across Rload is noise. If that is significant, it is commonly said to be a "ground loop". And obviously lifting the ground from the right side will stop the current flow, and thus "cure" the problem. However, so will reducing the resistance of Rwire, the common ground wire. Hence you have two choices. One is very easy and has no detrimental effects for *short* cables inside a building that has both ends on the same AC power distribution and a common ground or very low potential difference between two earth grounds. Long runs of comm cable do not fit that description, and therefore use the technique allowing only *very low impedance* common ground connections. (The only common part would have to be very large, low impedance, cable that is preferably short.)
>I repeat: what you keep proposing makes no sense. I can only presume that >you're confusing this issue with something else entirely.
It's simply good engineering, instead of magic. (Where "magic" is defined as stuff that isn't understood understand.)
>Consider what happens when lightning strikes one location - local ground >potentials go *nuts*. Your (apparent) connection between two local ground >systems will try to connect them together, and it will lose. Bigtime.
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.
>You've mentioned telephone system cabling: I'm no expert, but I had thought
If you have no expertise in this topic, please *do not* pontificate.
>telephone cables were current-driven loops, with the power supplied by >a -48V supply and a local ground *at one end*.
Wrong.
>Perhaps this is only the >subscriber loop. For *very* long trunk cable runs, I have seen what you >describe - shield connected to local ground via a dirty great grounding >pole - but I had understood that the series impedance of the shield was >quite high,
Wrong.
>which is what saved it from becoming a very long and expensive >fuse. This is not what we're discussing here (the need for all RS-485 >signals to be within a defined common-mode range).
Wrong. -- 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:87zmtpelyk.fld@barrow.com...
>>You've mentioned telephone system cabling: I'm no expert, but I had >>thought > > If you have no expertise in this topic, please *do not* pontificate.
You'll forgive me for laughing out loud here. Steve http://www.fivetrees.com
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote in message >news:87zmtqgkx2.fld@barrow.com... >> >> 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. > >On the contrary, it has high noise immunity (i.e. the common-mode voltage >range). It's a balanced differential system - any noise induced on one >signal is likely to be induced in equal measure on the other, and hence >cancelled out.
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 <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote in message 
news:p5ydnfE1eqqnoS_fRVnyjw@pipex.net...
> "Floyd L. Davidson" <floyd@barrow.com> wrote in message > news:87zmtpelyk.fld@barrow.com... >>>You've mentioned telephone system cabling: I'm no expert, but I had >>>thought >> >> If you have no expertise in this topic, please *do not* pontificate. > > You'll forgive me for laughing out loud here.
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 http://www.fivetrees.com
"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87vf4del81.fld@barrow.com...
>>On the contrary, it has high noise immunity (i.e. the common-mode voltage >>range). It's a balanced differential system - any noise induced on one >>signal is likely to be induced in equal measure on the other, and hence >>cancelled out. > > Like I said, low noise immunity. With a very low common mode > dynamic range, for one thing.
Eh? If you mean the +/-7V common-mode range, yeah, it's not huge - but it's more than adequate if correctly applied. Steve http://www.fivetrees.com
"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote:
>"Floyd L. Davidson" <floyd@barrow.com> wrote in message >news:87mzppg39l.fld@barrow.com... >> >> The fact is the 100 Ohm load resistor, being of lower resistance >> than any other load on the cable, is what determines the voltage >> levels. The receivers are high impedance voltage sensing devices. > >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
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.
>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 ;).
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.
>(BTW, it's now 3:38am here in southern UK, and probably a bit warmer than >where you are ;). Nice pics on your site.)
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 <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 in message >news:87vf4del81.fld@barrow.com... >>>On the contrary, it has high noise immunity (i.e. the common-mode voltage >>>range). It's a balanced differential system - any noise induced on one >>>signal is likely to be induced in equal measure on the other, and hence >>>cancelled out. >> >> Like I said, low noise immunity. With a very low common mode >> dynamic range, for one thing. > >Eh? > >If you mean the +/-7V common-mode range, yeah, it's not huge - but it's more >than adequate if correctly applied.
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. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Grant Edwards <grante@visi.com> wrote:
>On 2005-06-17, Floyd L. Davidson <floyd@barrow.com> wrote: > >> EQUIP ISOLATOR RS485 <===/ CABLE /===> RS485 ISOLATOR EQUIP >> >> If you are doing that, with 10 km of cable > >I don't think I ever saw runs longer than 2-3km. > >> and *not* grounding the cable shield, it may well work, > >The cable shield was grounded. What wasn't grounded was the >RS-4xx driver/transceivers. > >----+ +------+ +------------/ /-- Shield > | | Opto | +-----+ \ > uP|-Tx--| Iso |--Tx--| 485 |--|---A-----/ /-- > |-Rx--| |--Rx--| xcvr| | > |-dir-| |-dir--| |--|---B-----/ /-- Mirror >-++-+ | | +-++--+ | Image > |`--Pwr--| Pwr |-IsoPwr-'| | > `---Gnd--| Iso |-RScom---+-----|---com---/ /-- > | +------+ / > /// +--+--------/ /-- > | > ///
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!
>Or something pretty close to that. In some pieces of gear the >uP was floating (uP ground was RS485 common). In all cases the >RS-485 interface was floating with respect to earth/chassis >ground. > >I don't remember if people were told to ground the shield at >one end or both. There were A, B, common, and chassis ground >lugs on each of our bits of gear (in addition to some other >stuff unrelated to the discussion). > >> You don't need to use a cable pair to connect the common mode >> grounds on the RS485 drivers, because the shield should have >> a separate ground strap going to a building ground at each end. >> The RS485 common mode ground points should also be connected >> to those same building grounds, with a separate cable. > >That's sure not the way I remember it in the process control >systems I worked with (it's been 6-7 years). The RS-485 bus was >pretty much always floating w/ respect to ground, with A, B and >a floating common wire between the two RS485 transceivers. >There may have been people that grounded the RS-485 common node >at some point, but it was expected to work if it was floating.
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 <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: >> >>>As I've just said elsewhere, this is simply nonsense. >> >> And you are out of your field... > >I've pointed out several factual errors with your posts. I've demonstrated
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...
>several areas where your understanding of RS-422/485 is, at best, >incomplete, and at worst, downright wrong. I (and many others) have provided >you with ample details of why what you're saying is just plain wrong. > >"Out of your field", you say?
Apparently quit a ways out.
>To be real clear about all this: I'm sure you're sincere, but you sound >confused. RS-422/485, and its practical applications, happens to be an area >I know *real* well. I'm a bit embarrassed for you.
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. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com