rs485 and RTN

There is no universal answer. Connecting the shield to frame ground at the source is, however, a common practice and may have been used in this specific instance.

Probably. Not so much from "danger" issues (one hopes) but to help to keep the A/B lines within the your installation's specified common-mode voltage range.

B&B Electronics has some very readable appnotes and guides for understanding and setting up 422 and 485 systems. Take a look at

--
Rich Webb     Norfolk, VA

1. Correct. This prevents current loops which other wise can cause damage and magnetic coupling of other signals.
2. I shield is a shield is a shield, as it is stated. Just keeps the inside and out side noise apart and not to converge.

Jamie

It's better to chassis-ground the shield on both ends. That prevents EMI and ESD and crud from being coupled into the signal conductors on the end where the shield is floating.

Yes, but if both ends have local earth grounds, the resistance of the RTN pair will get overwhelmed by the much lower ground impedance. But it never hurts.

RS485 is fairly forgiving of common-mode potentials.

John, this is where I must disagree with you. The above assertion is incorrect!

You DC connect only one end, the other, you can use a small signal cap if it makes you feel better, but never tied two different devices together via their shields on a ground path.

Argue all you want, I've been there many times over the years, it's basic 101 practice. If you look in a lot of equipment, the grounding of such transmission lines are cap coupled to the chassis, not direct and for good reason.

Jamie

It's perfectly correct. Imagine a long shielded cable with the shield open on one end. Induce a nasty current spike, EMI or ESD, into the shield at the open end. That will make a big voltage spike in the shield... it's far away, many wavelengths and many ohms and many microhenries, from earth ground. That nasty voltage spike is directly coupled into the signal conductors.

And driving the signal lines will induce voltages into the ungrounded shield, radiating EMI. Bad news all around.

I almost always do. And my stuff works.

In other words, hearsay and superstition.

If you look in a lot of equipment, the grounding of

Disagree, sorry. Shielded Ethernet is grounded on both ends. So are standard shielded RS232 and VGA cables and RCA audio/video, and lots of other stuff. You *want* ground loop current in the shield, because it induces compensating common-mode voltages into the differential signal conductors. The cable acts like a balun at higher frequencies

Why leave one end of the shield open, then add caps to local ground? It's better to just ground it.

I'm having this argument with a major customer now. I'm selling him lots of boxes that interconnect over shielded cables. I'm earth grounding all the PC board ground planes with lots of spacers to the chassis, and also grounding the cable shields at both ends. They want me to float the boards, add an R||C to ground at every board, and half-ground the shields. I'm not gonna do it.

Well, IMHO, you're both wrong.

Partially.

And you're both right, partially.

If you have a setup where the various boxes are fairly close together, the grounding is thought out well enough to allow for ground-loops in the shielding, and the EMI environment isn't so severe that you'll melt the shielding, then having the shielding stay continuous from end to end is a good thing.

Presumably this is the world that John is used to.

If you have a setup where the cable runs are long and ground loops are uncontrolled, then connecting the ground of one machine to that of another two hundred feet away may not just be a bad idea from a signal perspective, it may start a fire.

Presumably this is the world that Jamie is used to.

I've seen different grounding schemes used to varying degrees of success

-- each one has its place, many of them can work well, and few of them work well when you start mixing and matching.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I've had to contend with 60VAC gnd-to-gnd high-rise building-to-building. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

We've never had a fire or melt down but what happens is magnetic coupling occurs in the components under the shield and takes out the inputs/outputs of the devices.

Our lab actually test for these conditions when doing product developing. They apply various levels of current through shields for destructive testing and you'd me amazed what the simplest methods of protection can do for you. You would also be amazed of what kind of crap also makes it through what you think should be a good shield.

Most of the devices I've had open on the bench for view actually support a signal ground, which does not directly connect to the chassis. Those eng must've done their home work because we normally don't have any issues with that aspect of the equipment.

This whole thread reminds of how we had to remove a system ground from a AB power supply due to getting damaged components when ever mother nature decided to pass by us. The next revision of that supply from them had a designed change, I wonder why? Must of been the cost they were incurring on warranty cost.

Jamie

60 to 80 meters, three floors of a building, in the current case. One end is my controller, the other end is a megawatt of pulsed CO2 laser.

If the grounds are that many volts far apart, go fiber; the signals won't work anyhow.

Anybody heard of shielded CAT6 catching fire?

For equipotential bonding of the cable shield is a good idea and should be done in at least one place. The master is a natural place to connect the shield(s) to the PEN/PE/TE ground (depending on wiring convention).

What to do with the shields at the slave(s) is a tricky question. Especially if the TN-C wiring convention is used with a common PEN neutral+ground conductor, the potential difference between two PEN conductors, especially in separate buildings can differ with more than

10 Vac. Worst of all, these days, with a lot of electronic loads, the 3rd harmonic is summed into the N or PEN conductor. Thus even with balanced 3 phase loads, the fundamental 50/60 Hz is canceled out, while the return currents are _summed_ at 150/180 Hz.

Think what happens if the master and slave(s) are connected to different feeders with a lot of 50/60 Hz (unbalanced load) and 150/180 Hz harmonics with more than 10 Vac difference. A significant amount of load current (1-100 A) is going to flow in the shield trying to reduce the potential difference. This huge current will induce noise into the signal wires and might even burn some PCB tracks.

The obvious solution would be to leave the shield unconnected at the slave(s), but as other have pointed out, this is not ideal for high frequency interference prevention. One common method is to use a 100 ohm 2-3 W resistor between the cable shield and each slave PEN/PE/TE local ground. With 10 Vac potential difference, this will limit the shield current to 100 mA, dissipating 1 W. The high frequency noise can still be suppressed, by adding a capacitor across this resistor, but of course, the reactance of this capacitor should be greater than

100 ohms at mains and its harmonic frequencies.

With such long distances (500 m) I would definitively recommend using isolated (500-2500 V) floating transceivers at the slave(s) or even fiber optics if the slaves are at different building with power feed and separate lightning grounding electrodes.

With floating slaves, the isolated transceiver islands are held at the same potential as the master transceiver (which could be connected to the real PEN/PE/TE ground).

Trying to use non-isolated transceivers with such distances is just asking for trouble, if the signal grounds at both ends are connected to the PEN/PE at each slave. You are ending with the same problem with the RTN as with the shield in the first case. If the potential difference is say 10 Vac, it will easily exceed the -7 .. +12 V common mode range specified by the standard, thus isolation should be used, if significant potential differences are expected.

I've had to contend with 40VAC ground to ground on one factory floor, on plugs that were ten feet apart -- and it wasn't a factory with honkin' big 500HP motors, either, it was just a moderate-sized light industrial building, albeit one big enough (as we found out) to have two separate electrical systems.

Fortunately all that we burnt up were some fairly easily replaced test fixtures -- but I learned to only trust the "ground all cases together" mantra if you had control over what those cases plugged into.

Actually, I had that in mind when I was responding to the post: the AC potential (and it was low impedance, too -- it wasn't just a "connect 'em together and be happy" thing) between grounds bolsters Jamie's position, and the system under test used a grounding system that included all the cases being electrically tied together, to bolster John's. Hence -- the best answer depends on the circumstances.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I had to do a data link between two OmniComp/GenRad buildings... ended up using Manchester and a transformer termination to avoid the ground difference. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

As you say, it all depends. There's a decent, short paper on some trade-offs over at

--
Rich Webb     Norfolk, VA

In this case, ground the slaves together and use galvanic isolation at the master only, this is the most economical solution. Ground the shields at the master through 100 ohm resistor (and capacitor).

PE and N are most definitely joined together at the main distribution panel, since the underground cable will carry only the PEN conductor anyway (TN-C-S).

Sometimes you need a serious ground strap (think welding cable) between chassis to force the grounds closer together. The resistance of the strap has to be much less than the total resistance of the ground returns in the power system.

When things get extreme, big difference at low impedance, you need isolation in the signal circuits, or fiber.

this is the case in a schematic:

(fidocad at the end - I don't know if you use this software in this ng)

to be more generic, as example, I've put-in 2 kind of slaves: ac/dc powered and own powered (example: battery); if I well understood your suggestion, slaves are "float" respect to master so slave power-supply source isn't a problem; thanks a lot

[FIDOCAD] LI 140 40 20 40 LI 20 40 20 100 LI 20 100 20 145 EV 170 70 160 60 EV 170 85 160 75 EV 170 100 160 90 RV 130 55 155 105 LI 145 145 145 105 LI 140 55 140 40 LI 145 55 145 40 TY 135 95 10 5 90 0 0 * MAX1480 LI 240 65 260 80 LI 260 80 280 65 LI 280 65 300 80 LI 205 80 225 65 LI 225 65 240 80 LI 240 80 260 65 LI 260 65 280 80 LI 280 80 300 65 LI 205 65 225 80 LI 225 80 240 65 LI 340 65 360 80 LI 360 80 380 65 LI 380 65 400 80 LI 340 80 360 65 LI 360 65 380 80 LI 380 80 400 65 MC 160 120 1 0 080 MC 170 120 1 0 170 LI 160 135 170 135 SA 165 135 LI 170 135 170 130 LI 160 135 160 130 LI 160 115 170 115 LI 160 115 160 120 LI 170 115 170 120 SA 165 115 LI 165 100 165 115 LI 155 80 160 80 LI 160 65 155 65 RV 160 60 170 100 LI 205 90 170 95 EV 200 60 210 90 LI 205 60 300 60 LI 205 90 300 90 LI 340 60 400 60 LI 340 90 400 90 PL 195 30 15 30 2 RV 45 65 100 95 LI 100 70 130 70 LI 100 90 130 90 TY 105 60 10 5 0 0 0 * TX TY 105 80 10 5 0 0 0 * RX TY 65 70 20 7 0 0 0 * uC RV 115 190 160 215 TY 130 190 10 5 0 0 0 * + TY 145 190 10 5 0 0 0 * - MC 105 195 2 0 000 RV 115 195 105 195 MC 105 210 2 0 000 RV 115 210 105 210 TY 60 220 10 5 0 0 0 * AC/DC WALL ADAPTER PL 65 240 790 240 1 SA 165 240 LI 165 240 165 260 LI 155 270 160 260 LI 160 270 165 260 LI 165 270 170 260 LI 170 270 175 260 LI 160 260 175 260 LI 150 270 155 260 LI 155 260 160 260 MC 35 105 3 0 010 RV 30 105 70 120 MC 65 120 0 0 040 TY 35 110 10 5 0 0 0 * LM7805 LI 65 105 65 95 MC 80 95 0 0 040 TY 210 40 10 5 0 0 0 * 120 LI 190 145 145 145 LI 190 40 190 145 TY 175 70 10 5 0 0 0 * B TY 175 55 10 5 0 0 0 * A MC 185 65 1 0 080 LI 145 40 190 40 TY 145 130 10 5 90 0 0 * 100 SA 185 80 SA 185 65 LI 170 65 205 65 LI 170 80 205 80 LI 185 80 185 75 LI 185 70 210 45 TY 45 195 10 5 0 0 0 * 230V~/50Hz PL 15 185 195 185 2 LI 195 185 215 205 TY 215 205 10 5 0 0 0 * PLASTIC CASE EV 160 175 170 165 EV 145 175 155 165 EV 130 175 140 165 RV 170 165 130 175 MC 135 165 3 0 010 MC 155 155 0 0 040 LI 150 165 150 155 LI 150 155 155 155 LI 165 135 165 165 LI 140 150 140 105 LI 140 150 145 150 LI 145 150 190 150 LI 190 150 190 180 LI 190 180 20 180 LI 20 180 20 145 PL 15 30 15 185 2 PL 195 185 195 30 2 TY 25 40 10 5 0 0 0 * NON-ISOLATED PCB SIDE LI 135 190 135 175 LI 150 190 150 175 LI 165 175 165 240 LI 400 80 435 80 TY 15 10 20 7 0 0 0 * MASTER LI 410 80 410 115 LI 420 65 420 115 EV 405 115 415 125 EV 415 115 425 125 SA 410 80 SA 420 65 TY 150 40 8 4 0 0 0 * ISOL. PCB TY 160 45 8 4 0 0 0 * SIDE EV 460 155 470 165 RV 460 155 470 165 LI 465 165 465 240 SA 465 240 RV 405 115 435 125 EV 425 115 435 125 LI 400 65 435 65 EV 350 60 335 90 LI 340 65 330 65 LI 340 80 330 80 TY 425 130 8 4 270 0 0 * A TY 415 130 8 4 270 0 0 * B PL 480 175 400 175 2 RV 405 145 450 170 TY 435 130 8 4 270 0 0 * RTN LI 435 65 455 80 LI 455 80 475 65 LI 475 65 495 80 LI 435 80 455 65 LI 455 65 475 80 LI 475 80 495 65 LI 435 60 495 60 LI 435 90 495 90 LI 495 80 530 80 LI 505 80 505 115 LI 515 65 515 115 EV 500 115 510 125 EV 510 115 520 125 SA 505 80 SA 515 65 EV 555 155 565 165 RV 555 155 565 165 RV 500 115 530 125 EV 520 115 530 125 LI 495 65 530 65 EV 445 60 430 90 TY 520 130 8 4 270 0 0 * A TY 510 130 8 4 270 0 0 * B PL 575 175 495 175 2 RV 500 145 545 170 TY 510 155 8 4 0 0 0 * OWN PSU TY 530 130 8 4 270 0 0 * RTN LI 530 65 550 80 LI 550 80 570 65 LI 570 65 590 80 LI 530 80 550 65 LI 550 65 570 80 LI 570 80 590 65 LI 530 60 590 60 LI 530 90 590 90 LI 600 80 600 115 LI 610 65 610 115 EV 595 115 605 125 EV 605 115 615 125 EV 650 155 660 165 RV 650 155 660 165 RV 595 115 625 125 EV 615 115 625 125 EV 540 60 525 90 TY 615 130 8 4 270 0 0 * A TY 605 130 8 4 270 0 0 * B PL 670 175 590 175 2 RV 595 145 640 170 TY 605 155 8 4 0 0 0 * OWN PSU TY 625 130 8 4 270 0 0 * RTN LI 560 165 560 240 LI 655 165 655 240 SA 560 240 SA 655 240 PL 400 100 480 100 2 PL 495 100 575 100 2 PL 590 100 670 100 2 PL 400 100 400 175 2 PL 480 100 480 175 2 PL 495 100 495 175 2 PL 575 100 575 175 2 PL 590 100 590 175 2 PL 670 100 670 175 2 LI 590 65 610 65 LI 600 80 590 80 LI 400 60 425 50 LI 425 50 435 60 LI 495 60 520 50 LI 520 50 530 60 SA 425 50 SA 520 50 MC 610 105 2 0 080 SA 610 105 SA 600 105 LI 330 85 400 85 TY 630 105 10 5 0 0 0 * 120 LI 605 105 630 110 LI 620 85 620 115 LI 430 85 430 115 LI 400 85 430 85 SA 430 85 SA 525 85 LI 525 85 620 85 LI 525 115 525 85 LI 430 85 525 85 TY 210 110 8 4 0 0 0 * RTN=3DFLOAT LI 215 85 210 95 LI 215 85 215 95 LI 215 85 210 110 LI 330 85 325 95 LI 330 85 330 95 LI 330 85 325 110 TY 325 110 8 4 0 0 0 * RTN=3DFLOAT LI 590 60 595 50 TY 595 40 8 4 0 0 0 * SHIELD=3DFLOAT LI 590 55 590 60 LI 590 60 595 55 TY 210 50 10 5 0 0 0 * SHIELD BE 315 95 305 80 330 55 315 50 LI 215 85 305 85 MC 465 145 0 0 045 MC 560 145 0 0 045 MC 655 145 0 0 045 RV 400 185 445 210 TY 415 185 10 5 0 0 0 * + TY 430 185 10 5 0 0 0 * - MC 390 190 2 0 000 RV 400 190 390 190 MC 390 205 2 0 000 RV 400 205 390 205 TY 345 215 10 5 0 0 0 * AC/DC WALL ADAPTER TY 330 190 10 5 0 0 0 * 230V~/50Hz LI 420 185 420 170 LI 435 185 435 170 TY 485 210 10 5 0 0 0 * METAL TY 580 210 10 5 0 0 0 * METAL LI 585 205 590 175 LI 490 205 495 175 LI 480 175 490 205

Might be a javascript issue. This one may do it:

--
Rich Webb     Norfolk, VA

By using such straps, you are messing around with the electric distribution network. I hope you understand how the electric network is constructed in a particular place and how that strap might effect the protection system and what happens, if there is a ground fault on one side.

If you are making some general purpose systems to me installed to locations not yet known possibly by local people, you can avoid a lot of trouble by using galvanic isolation in all cases, even if it is not needed in some locations.

Standard Modbus system has pullup to supply and pulldown to RTN to be able to determine fault state (cable broken). So the RTN wont nessesarily be floating around due to the construction of the supply for the interfaces and the leakage currents.

Regards

Klaus