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RS485 is bidirectional does it mean it is fullduplex?

Started by Swizi June 15, 2005
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote in message 
news:d8sihj$ffk$1$830fa7a5@news.demon.co.uk...
> > 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.
Absolutely. Connecting to local ground at both ends of the run is a recipe for HUGE ground currents. I've seen (very) melted ground wires. It is indeed a safety hazard. See my other post re isolated comms interfaces. Steve http://www.fivetrees.com
Floyd L. Davidson wrote:

> 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
If you really do need to connect at both ends then you may need to consider inserting some impedance in the screen connections at both ends (usually a capacitor and resistor in parallel). The resistor is large enough to prevent high current flows but needs to be small enough to provide an effective electrostatic drain. The capacitor provides a low impedance at higher frequencies. -- ******************************************************************** 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/ ********************************************************************
"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87d5qmi3i3.fld@barrow.com...
>>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.
I'm afraid I'm not just not catching you, I'm not following you at all. Unless I'm missing something, you seem to be suggesting dangerous practices, e.g. connecting a long cable to local ground at both ends. At that point I started skipping your posts... As I've just said elsewhere, this is simply nonsense. Consider: you're trying to short out a part of the mains utility distribution system, which may be carrying significant leakage currents over long distances, with a flimsy little signal wire. Ground potential differences can be significant, and the source impedance is very low indeed - certainly capable of delivering tens of amps. I've seen guys who should know better staring at melted cables, scratching their heads... Please see my (and others') post(s) re isolated comms interfaces. Use one ground; isolate from the other - i.e. keep both Tx and Rx relative to one ground. Steve http://www.fivetrees.com
"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:874qbyi2cb.fld@barrow.com...
>>> 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've just confirmed my suspicions that you have no clue what you're talking about. RS-422 and RS-485 drivers are low-impedance in both MARK and SPACE state, and do indeed have a third (tristate) condition. Steve http://www.fivetrees.com
"Paul Keinanen" <keinanen@sci.fi> wrote in message 
news:hgf3b1pip56k28vcq79itdsam37tkaak2h@4ax.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.
Point accepted.
>>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 :-).
Heh - this was from long before RS-485 was published ;).
> While this configuration is widely used, claiming that it conforms > either to the RS-422 or RS-485 standard would be a bit suspicious.
Agreed. Although we were using standard drivers, with tristate control inputs, from reputable companies (e.g. TI), and not kludging anything - just completely following and using the datasheet. Also, we could kinda see RS-485 coming... Steve http://www.fivetrees.com
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
>Floyd L. Davidson wrote: >>>> 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.
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. See the other article I posted explaining it in detail. In short, grounding at one end is an expedient design that works for *short* cables within a single building.
>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.
That is not correct. As noted above, see the other article, as I'm not going to repeat it all in multiple posts.
>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.
Telecommunications engineers, working with longer cable runs, need to know even more... :-) -- 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:877jguk8ql.fld@barrow.com...
> > You *absolutely do* want a good ground connect at both ends.
No, you absolutely *don't*. See my other posts. Steve http://www.fivetrees.com
"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. Steve http://www.fivetrees.com
"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:873brik81n.fld@barrow.com...
> 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. > They are *all* high impedance devices, relatively. There has to > be a 100 Ohm load resistor on each pair.
Errr... no, the 100R is there for transmission line matching. It should match the characteristic impedance of the cable. Steve http://www.fivetrees.com
"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote:
>"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.
I can assure you it is *not* "just ... done" that way in the Netherlands any more than it is here. If for no other reason than surge suppression, there *is* a ground and the main cable *does* have a grounded shield. There is almost *certain* to be a ground at your interface to the telephone company. Whether you can see it or not, I certainly don't know.
>> 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.
See the other article I posted explaining that in detail.
>> 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.
The shield has no such magical effects. At 1000Hz it will reduce a noise signal by about 3 dB, and 60Hz it has about 0.04 dB effect. Which is to say that shielding T1 leads has significance (and indeed, inside a building ABAM shield cable is used), but where the primary noise induced will be power line frequencies and harmonics, shielding as such has little effect.
>> 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.
RS-422 is specified as maximum open voltage +/- 10 V, when loaded with 100 Ohms, +/- 2V. Driver output resistance, 100 Ohms, maximum current 150 mA. Receiver input resistance 4000 Ohms, common mode +/- 7 Volts, and sensitivity +/- 200 mV. Hence, if there are 10 transmitters on the line, the requirement is more than 200mV with less than 150mA... which is a very low resistance. Even if the 100 Ohm output resistance represented a load resistance, that would make a total of 11 such loads across the line, and the impedance would be just less than 10 Ohms. That 150mA of current would generate well over a volt of signal. Regardless, I'm *not* positive what does or does not work. I don't recall ever using RS-422. In searching the web I find reliable sources (Maxim, for example) who say that 1 master and 10 slaves, which they diagram as having both transmit and receive functions, can be used. On the other hand, I've also seen very explicit statements that RS-422 can handle 1 master that can transmit and receive, and *all* of the slaves are receive only. Or statements that a "true" multidrop cannot be done with RS-422.
>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.
That is only *one* way to accomplish that. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com