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
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 .................... 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/********************************************************************
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
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
Point accepted.
Heh - this was from long before RS-485 was published ;).
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
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.
That is not correct. As noted above, see the other article, as I'm not going to repeat it all in multiple posts.
Telecommunications engineers, working with longer cable runs, need to know even more... :-)
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
No, you absolutely *don't*. See my other posts.
Steve
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
Errr... no, the 100R is there for transmission line matching. It should match the characteristic impedance of the cable.
Steve
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.See the other article I posted explaining that in detail.
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.
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.That is only *one* way to accomplish that.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
You keep saying this, but it's wrong. If you won't believe us, use Google to do some searching. A few seconds of searching turned up:
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:
1 Protective Ground Name: AA Direction: - CCITT: 101This pin is usually connected to the frame of one of the devices, either the DCE or the DTE, which is properly grounded. The sole purpose of this connection is to protect against accidental electric shock and usually this pin should not be tied to Signal Ground.
This pin should connect the chassis (shields) of the two devices, but this connection is made only when connection of chassis grounds is safe (see ground loops below) and it is considered optional.
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.
Since the loops impedance is low, this current can be quite high and easily burn out electric components. Electrical storms could also cause a burst of destructive current across such a loop. Therefore, connection of the Protective Ground pin is potentially hazardous. Furthermore, not all signal grounds are necessarily isolated from the chassis ground, and using a RS-232 interface, especially across a long distance, is unreliable and could be hazardous. 30 meters is considered the maximum distance at which the grounding signals can be connected safely.
I'm confused about what you actually have, see below.
I wasn't getting a clear picture of this yet... and I'm not sure that I am yet! :-)
EQUIP ISOLATOR RS485 RS485 ISOLATOR EQUIP
If you are doing that, with 10 km of cable and *not* grounding the cable shield, it may well work, but it isn't the best way to do it..
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.
EQUIP ISOLATOR RS485 RS485 ISOLATOR EQUIP | | | | \ / \ / | | > On the other side, they are *all* connected to a common
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
... snip ...
Yet use of the old fashioned 20 ma current loop, driven through photo diodes/transistors, is virtually immune to these problems. It is biased in only one place. If you are going to inject signal at every station the interface is harder. For point to point or broadcast you can hardly beat it.
--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
Available for consulting/temporary embedded and systems.
USE worldnet address!
So? You seem to miss the fact that its DC characteristics are just as important a its impedance matching function. That's the same as the 50 Ohm termination on a 10base5 ethernet. Ever try using RG59 cable instead of RG58? It works... as long as you stick with 50 Ohm terminations. Same point...
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.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
I've explained that *in detail*. That *is* the way telephone cable systems are installed.
And you are out of your field...
If they had a melted cable, it wasn't simply the difference in ground potential. (Telecom cables generally are spliced and grounded every 6000 feet, max.)
See the tutorial I posted: Message-ID:
That is fine within one building, and is not the best practice for longer cable runs, for reasons explained in the message listed above.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Impedance is relative. To an 8 Ohm speaker, 600 Ohms is high impedance.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Bullshit.
Why not do some real research, and find out how it is actually done.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
I don't think I ever saw runs longer than 2-3km.
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---/ /-- | +------+ / /// +--+--------/ /-- | ///
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).
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.
--
Grant Edwards grante Yow! Psychoanalysis?? I
at thought this was a nude
visi.com rap session!!!
No, the whole idea is that you *want* that current to flow. In particular it is the 60 Hz power line induced current that makes up most of the current flow.
Keep in mind that the whole idea is to allow the current flow to generate an equal and opposite induction into the signal pairs.
-- Floyd L. Davidson Ukpeagvik (Barrow, Alaska) floyd@barrow.com
I've pointed out several factual errors with your posts. I've demonstrated 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?
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.
Steve
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