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RS485 is bidirectional does it mean it is fullduplex?

Started by Swizi June 15, 2005
Paul Keinanen <keinanen@sci.fi> wrote:
>On Wed, 15 Jun 2005 16:17:56 +0100, "Steve at fivetrees" ><steve@NOSPAMTAfivetrees.com> wrote: > >>"Tim Mitchell" <timng@sabretechnology.co.uk> wrote in message >>news:uAXCZAWeYCsCFAHz@tega.co.uk... >>> 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. > >This is generally true in practice.
It *can't* be true in practice. The presumption is to provide a signal ground, and it simply doesn't. Any attempt at providing a signal ground is merely going to connect frame ground between the two locations, which will very likely cause more noise induction into the signal pairs than anything else.
>However, if you are using floating devices and the "fail safe" >termination, the system will work with 2 resp. 4 wires, since the >"fail safe" resistors will force the receiver power supply Vcc and Gnd >potential close to the line potential and thus, within the common mode >range of the receiver (-5..+12 V). Treat the terminated system as a >bidirectional current loop (through the terminating resistors) and it >should be easy to analyze how this works.
The circuit impedance is 100 Ohms. It *is* a current loop... :-)
>> - RS-422 is also multidrop (10 max drops, IIRC). > >The RS-422 impedance levels allows for multiple receivers, but I do >not see how a multidrop bidirectional system could be implemented >within the RS-422 specification.
What would prevent it? It's a fairly simple 4-wire arrangement with a single master and 10 slaves plus a single 100 Ohm termination on each cable. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
On 2005-06-15, Floyd L. Davidson <floyd@barrow.com> wrote:

> 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.
In my experience, the third/fifth wire is required to limit the common mode voltage seen by the receivers. In that respect, it is a signal ground. IIRC, most receivers them can only tolerate 8-12V common-mode DC. If you let the two devices float with respect to each other, you can get fairly high common-mode voltages and the recievers will stop working. -- Grant Edwards grante Yow! I know how to get the at hostesses released! Give visi.com them their own television series!
Grant Edwards <grante@visi.com> wrote:
>On 2005-06-15, Floyd L. Davidson <floyd@barrow.com> wrote: > >> 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. > >In my experience, the third/fifth wire is required to limit the >common mode voltage seen by the receivers. In that respect, it >is a signal ground. IIRC, most receivers them can only tolerate >8-12V common-mode DC. If you let the two devices float with >respect to each other, you can get fairly high common-mode >voltages and the recievers will stop working.
That is a frame ground, and not a signal ground. It will carry no signal current at all. And any variation of current seen will be strictly noise. 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. What kind of distances have you tried that with? I'd expect that across the room or around the bend might be just fine (and wouldn't be needed because the offset between the ground systems wouldn't be high enough to be a problem). But if this went down the road 3000-4000 feet, and you actually did get a ground offset high enough to be a problem, using a single wire in the same cable to equalize the ground potential should add enough noise to your cable run to make it a real problem. A proper ground on each would be much better. And a cable sheath that is properly grounded at *both* ends, to the same single point building ground that the RS-485 equipment is tied to, would be the preferred way to make sure there wasn't too much common mode difference. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Thought I'd throw my 2 cents in. I used to design systems like this back in 
the 80's and you should be aware that there is always the potential that you 
could cross power grids. I worked on a project where the same company had 2 
buildings across the street from each other and they were on seperate power 
grids. The grid different was not in volts, but in 10's and 100's of volts. 
It was something we never expected but it's real, and your ground wire won't 
protect against this.

dbrown






"Floyd L. Davidson" <floyd@barrow.com> wrote in message 
news:87u0jzjav2.fld@barrow.com...
> Grant Edwards <grante@visi.com> wrote: >>On 2005-06-15, Floyd L. Davidson <floyd@barrow.com> wrote: >> >>> 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. >> >>In my experience, the third/fifth wire is required to limit the >>common mode voltage seen by the receivers. In that respect, it >>is a signal ground. IIRC, most receivers them can only tolerate >>8-12V common-mode DC. If you let the two devices float with >>respect to each other, you can get fairly high common-mode >>voltages and the recievers will stop working. > > That is a frame ground, and not a signal ground. It will carry > no signal current at all. > > And any variation of current seen will be strictly noise. 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. > > What kind of distances have you tried that with? I'd expect > that across the room or around the bend might be just fine (and > wouldn't be needed because the offset between the ground systems > wouldn't be high enough to be a problem). But if this went down > the road 3000-4000 feet, and you actually did get a ground > offset high enough to be a problem, using a single wire in the > same cable to equalize the ground potential should add enough > noise to your cable run to make it a real problem. > > A proper ground on each would be much better. And a cable > sheath that is properly grounded at *both* ends, to the same > single point building ground that the RS-485 equipment is tied > to, would be the preferred way to make sure there wasn't too > much common mode difference. > > -- > Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> > Ukpeagvik (Barrow, Alaska) floyd@barrow.com
"David Brown" <david.brown_spamnot@vertronix.com> wrote:
>Thought I'd throw my 2 cents in. I used to design systems like this back in
I'm retired now David, but I spent 34 years making systems like this work, in a variety of environments that would best be described as mind boggling.
>the 80's and you should be aware that there is always the potential that you >could cross power grids.
Not in this post, but in another I said: "it probably wouldn't make any difference at all if the cable run is relatively short. (Unless it is between two locations on separate power distributions, and one of them has a bad ground.) Note the kicker at the end of that, about a bad ground. That can mean a faulty ground system, or it can simply mean that a "good ground" is simply not available.
>I worked on a project where the same company had 2 >buildings across the street from each other and they were on seperate power >grids. The grid different was not in volts, but in 10's and 100's of volts.
Lets not get too far from reality here. If it was 100's of volts somebody has a *very* serious fault in the electrical system. On the other hand, up to 10 volts is not rare at all.
>It was something we never expected but it's real, and your ground wire won't >protect against this.
I'm not sure which "your ground wire" you are referencing. A single wire in the same sheath as the twisted pairs used for data, won't "protect" against it for the reasons that I stated. It will probably add more noise to the data circuits, and do little else. If the ground systems are really bad it might actually equalize the offset though. A well grounded cable sheath, at both ends, almost certainly will correct the problem. Which is to say, I've never seen it fail, but have seen instances where it was not as good as we'd have liked to see. Invariably that has to do with inability to get a good ground connection. But it *is* good enough for RS-485, as long as the ground systems for equipment on both ends are in fact connected to the ground system the cable is attached to. Multiple grounds won't do, even if they are relatively good. The cable and the equipment both must be tied separately to a single building ground. Typically, telephone equipment cannot be adjusted for more than about 20 volts of ground difference. That's the range of adjustment provided on most equipment (typically that would be something referred to as an emx unit, or as dx signaling). And of course that is with much slower data and much higher voltages than RS-482. -- 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:
> Grant Edwards <grante@visi.com> wrote: >>On 2005-06-15, Floyd L. Davidson <floyd@barrow.com> wrote: >> >>> 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. >> >>In my experience, the third/fifth wire is required to limit the >>common mode voltage seen by the receivers. In that respect, it >>is a signal ground. IIRC, most receivers them can only tolerate >>8-12V common-mode DC. If you let the two devices float with >>respect to each other, you can get fairly high common-mode >>voltages and the recievers will stop working. > > 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.
> And any variation of current seen will be strictly noise.
What current?
> 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.
I really don't understand what you're talking about. The differential receiver inputs can deal with only a few volts of common mode DC voltage. You have to use a ground that's common between the transmitters and receivers to make sure that the common-mode DC voltage seen by the receivers is within spec.
> What kind of distances have you tried that with?
A couple kilometers.
> I'd expect that across the room or around the bend might be > just fine (and wouldn't be needed because the offset between > the ground systems wouldn't be high enough to be a problem). > But if this went down the road 3000-4000 feet, and you > actually did get a ground offset high enough to be a problem, > using a single wire in the same cable to equalize the ground > potential should add enough noise to your cable run to make it > a real problem.
It didn't seem to.
> A proper ground on each would be much better.
Not allowed for safety reasons. The RS-485 transceivers at both ends are optically isolated from earth.
> And a cable sheath that is properly grounded at *both* ends, > to the same single point building ground that the RS-485 > equipment is tied to, would be the preferred way to make sure > there wasn't too much common mode difference.
Nope. The cable sheild is earth ground at one end or the other and can't be electrically connected to the RS-485 signal or "ground" signals. -- Grant Edwards grante Yow! A shapely CATHOLIC at SCHOOLGIRL is FIDGETING visi.com inside my costume...
On Wed, 15 Jun 2005 15:40:22 -0800, floyd@barrow.com (Floyd L.
Davidson) wrote:

>The specification includes the maximum ground offset voltage >permissable. (I don't recall what it is, or how realistic it is >for common 4000 foot runs of twisted pair cable.)
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.
>But attempting to supply a separate "signal ground", for example >via a single separate cable pair, will almost certainly result >in a poor frame ground connection instead!
You do not want a good ground connection at least not at both ends in this situation. It is quite common to use a 100 ohm resistor between the signal ground wire and the frame ground (PE) to limit the loop current. Preferably, at least one end of the connection should be floating (optoisolated) in which case the signal ground wire goes only to the C terminal of the floating interface, but there is no connection between the signal ground and frame ground (PE) at that end. Paul
On Wed, 15 Jun 2005 18:26:09 -0800, floyd@barrow.com (Floyd L.
Davidson) wrote:

>That is a frame ground, and not a signal ground. It will carry >no signal current at all.
The ground wire does not carry any signal current. The signal ground C is required to supply the bias current. Assume that the receiver input stage consists of a differential pair made up of NPN transistors. These NPN transistors require that a bias current flows into the base in order to get a collector current flowing and thus a meaningful output voltage from the differential pair. If you just connect the A wire to one base and the B wire into the other base, there would be at least one reverse biased junction in the path between A and B and no bias current would be available and the receiver would not work. If there is the ground return, the bias current from either A or B wire would flow into the NPN transistor base, then through the constant current resistor and back to C and the stage will now operate. The other alternative is that the "fail-safe" termination is used, in which case a large resistor is connected from receiver Vcc to one base of the differential stage, the transmission line termination resistance (typically 100-120 ohms) is connected between the transistor bases and a large resistor is connected from the other base to local DC ground. There is a small current flowing through the voltage divider biasing the differential stage properly. When the A and B wires are connected to the ends of the terminating resistor, a large signal current will flow in either direction depending of the signal being transmitted and hence the voltage between the transistor bases will also change and the signal can be recovered. The large bias resistors from Vcc to one input and from the other input to DC ground will help to keep the floating receiver DC supply close to the signal pair average potential and hence within the common mode range. In this configuration only two wires are required. No signal grounds wires nor any connection to local frame ground are required, but the transceiver supply must be floating. The problem with this system especially in multidrop systems is that the terminating resistors are at the end of the bus, but the bias resistors must be used at each station and are effectively in parallel, loading the bus. This may limit the number of stations connected to the bus. Paul
On Wed, 15 Jun 2005 16:04:56 -0800, floyd@barrow.com (Floyd L.
Davidson) wrote:


>>>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. >> >>This is generally true in practice. > >It *can't* be true in practice. The presumption is to provide a >signal ground, and it simply doesn't. > >Any attempt at providing a signal ground is merely going to connect >frame ground between the two locations,
DO NOT connect the frame grounds together with the data cable (shield). This cable shield can conduct 1-100 A of AC current, which should have otherwise gone through the neutral wire.
>which will very likely cause >more noise induction into the signal pairs than anything else.
10 A of dirty 50/60 Hz AC current with usually a lot of odd harmonics (especially at 150/180 Hz in three phase systems with electronic loads) can indeed cause a lot of interface to the data within the cable. Exactly for this reason, the signal cable shield should NOT be grounded at both ends.
>>> - RS-422 is also multidrop (10 max drops, IIRC). >> >>The RS-422 impedance levels allows for multiple receivers, but I do >>not see how a multidrop bidirectional system could be implemented >>within the RS-422 specification. > >What would prevent it? It's a fairly simple 4-wire arrangement >with a single master and 10 slaves plus a single 100 Ohm >termination on each cable.
IIRC, the original RS-422 specification did not contain specifications for tri-stating the transmitter, so doing it directly would be a bit 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). Paul
Paul Keinanen <keinanen@sci.fi> wrote:
>On Wed, 15 Jun 2005 15:40:22 -0800, floyd@barrow.com (Floyd L. >Davidson) wrote: > >>The specification includes the maximum ground offset voltage >>permissable. (I don't recall what it is, or how realistic it is >>for common 4000 foot runs of twisted pair cable.) > >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.
>>But attempting to supply a separate "signal ground", for example >>via a single separate cable pair, will almost certainly result >>in a poor frame ground connection instead! > >You do not want a good ground connection at least not at both ends in >this situation.
You *absolutely do* want a good ground connect at both ends.
>It is quite common to use a 100 ohm resistor between >the signal ground wire and the frame ground (PE) to limit the loop >current.
There is no "signal ground wire". What are you talking about?
>Preferably, at least one end of the connection should be >floating (optoisolated) in which case the signal ground wire goes only >to the C terminal of the floating interface, but there is no >connection between the signal ground and frame ground (PE) at that >end.
I'm sorry, that is just not true, other than optisolation is not a bad thing. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@barrow.com