Ethernet & Interference.


Thread Starter

Steven Landau

I am using MBE (Modbus over Ethernet) at a Plant and am having interference with the ethernet with Some VFD's.

All the ethernet cables are in separate conduit (rigid XP) and we are using shielded twisted pair. We are getting ethernet collisions/errors when the drives are running.

We figured that the conduit & shielded wire should solve the problem. But no luck. When we run a 100 spare cable along the floor out the hub we have no problems.

Any suggestions on what to try next?

Steve Landau

Hullsiek, William

Get a Fluke Lan tester, and run traces on the shielded pair. Check to where the shielded pair is grounded. Verify that you are in compliance with the EIA/TIA and BICSI guidelines on bonding and grounding.

Check on where the drives are grounded, and where the shielded wires are grounded.

You might find you have better noise immunity with un-shielded twisted pair but with more twists.

I like to find a local consultant who is BICSI certified for the installation of network cabling in an industrial setting.

The Fluke, Anixter, Leviton websites are useful sites for education. Remember, if it works its a Fluke !!

William F. Hullsiek
Software Engineer
Renewal by Andersen
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It has to do something with earthing. Check what's the difference in connecting the spare cable and the orginal cable. Try to isolate the drives from your common earth by making a common earth for the drives.
Good luck.

Met vriendelijke groeten,

Onno Moret

Imtech Projects West
Control Systems

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Any opinions expressed in this message are not necessarily those of the

Curt Wuollet

Hi Steve

Have you tried plastic conduit? Quite often, putting balanced transmission lines into condiut turns them into something else. Another solution would be to use the small, inexpensive, media converters and use coaxial cable. The coax limits you to 10 mbps, but will often tolerate being run im conduit. For more money, of course, you can do fiber, that is just about a guaranteed
solution and preserves the bandwidth.


Curt Wuollet
Wide Open Technologies.

Anthony Kerstens

I assume you Ethernet equipment is using 120vac power. Are your drives isolated? Have you done a harmonic analysis.

It sounds as though the cabling is probably well shielded, but electrical noise can find its way in though the power systems.

Anthony Kerstens P.Eng.

Al Pawlowski, PE

Don't know what XP is, but if it is not ferromagnetic (steel) and it runs close to some of the drive power lines, you may have magnetic
induction. In this case, either re-route the cables to get distance between them or enclose one of them (power or signal) in steel.

If already in steel, you probably need to low-pass filter the power lines to your signal equipment. And, maybe, the same (choke) on your drive input power lines.

Be careful with grounding too. The drive local ground may need to be separated from the signal equipment ground and suitable isolation
installed between them if they have interconnections.

Basically, you have to look at the total EM interconnection of the equipment from a HighFrequency point of view.
Not sure if this will help or not, but i had an experience where I was talking from a plc to a 10base hub local at the plc. I would plug into the 10base hub with a laptop and go online. This would saturate the connection, causing the computer to lose connections to the drives on the server. I replaced the 10base hub with a dual speed hub, allowing the computer to connect at 100base and the plc to keep it's 10base connection, and the problem went away.

If any of your equipment wants to talk 100base, try putting in a dual speed hub ( I use a 4 port from NetGear)


Marc Goodwin

Is the spare cable you run (along the floor) UTP or STP?
Have you checked the installation of the STP cable in particular the connection of the shielding to earth? a bad connection would defeat any advantage in using shielded cable and infact be worse than UTP.



Leon McClatchey

At our plant, we use MB+ to control the lines and ethernet to communicate between the lines. We use Fix Dmacs version 7, but, Dmacs has networking capabilities built in. One of the pluses to this is that we don't have the problem of drive noise interfering with the ethernet:) Another advantage is the line isolation from the
ethernet as well. Each line has several modicon devices which controls different processes and they are all linked together via modbus Plus. Communication is faster (1 megabaud vs 9600) so
we can maintain tighter control over the entire process. You may want to look at doing something similar as well:)

cya l8r
Leon McClatchey
mailto:[email protected]
Linux User 78912 (SuSe62 Box)

Do the drives have isolation transformers? Sounds like the drives are affecting the ground or power sources. You can evaluate the power sources for any noise with rented power analyzers and eliminate it or just follow all of the
suggested solutions for eliminating noise from AC drives. Make sure your grounds are good earth grounds and you can even run grounding straps around the drives cabinet from backpanel to backpanel to help.

Dale Witman
Even though the communication link is shielded pair (single or multiple-pair), and presumably effectively grounded at only one point to a ground bus, the cable sheath/jacket may not be physically intact. Sheath/jacket could have been damaged when pulled through conduit raceway. Thus unintentionally grounding the shield to its surrounding explosion proof (XP) conduit (heavy-wall, galvanized, with threaded joints, etc).

Disconnect grounding/bonding connection between shield of suspect communication cable(s) from its (their) equipment ground bus. Check for
continuity and/or voltages between shield and equipment ground or grid. Also, if cable passes through junction or marshalling boxes, make sure
shield is made continuous, and not inadvertently grounded.

Phil Corso, PE
Trip-A-Larm Corp
We have Ethernet interface with Modicon PLC. The cable type we use is CAT5 cable it is not shielded and is a standard requirement for all 10/100 M Ethernet communication interface.


Pravin Fatnani

Why should the metal conduit create any problems? On the contrary, you earth it properly (at one point) and it should provide very good shielding, better than that available from a braided / aluminium foil shields. Many twisted pair cables in a conduit would be very close to multi-core twisted pair cable with overall shield( in which case the cross talk among different pairs might be the problem).

Pravin Fatnani

Curt Wuollet


I didn't say it didn't shield, I said it could change a balanced transmission line into something else. Random placement inside of it causes imbalances and it can absorb energy, resonate, and do all sorts of things that are not
consistant, predictable, or constant with frequency. Twisted pair Ethernet works because the line is characterized and the drivers and receivers are tuned to the characteristics of the line. Change the characteristics and it may or may not work. It's not a simple shielding problem. That's why one on the floor works and one in a ferrous shield doesn't. Shielded twisted
pair can work because the shielding maintains a constant relationship with _both_ wires in a pair and consistant distributed reactance per unit
length. Stick it in a pipe and it's a crapshoot. You may get lucky and it works. If you have standing waves on the conduit and ground it, if the end happens to be a voltage node at some frequency that frequency will be sucked out. Pulses are the base frequency and all the odd
harmonics ( and some even ones with non-ideal waveforms ). To get a pulse out the other end the line has to have equal attenuation for all those frequencies. S/UTP does not, so the signals are pre-emphasised to compensate. Sheilding part of the line. or wrapping it around objects, etc.
can change the formula enough that the waveforms are distorted and it quits working. STP is better in this regard because the close shield
predominates, but, it's simply not perfect. Cat3, Cat5, define construction that (hopefully) controls these characteristics so cables can be
interchanged and still work. Pulling it through a pipe introduces a lot of variables. I suggested plastic pipe because it will physically space the
wire away from objects that disturb the characteristics and it still provides some protection. Bundling TP with other wires, or even itself, isn't a good idea, especially in tight bundles, you can usually get away with it
but I've fixed more than one problem installation by cutting the wire ties.

At high frequencies, a wire is a complex network, I'm surprised that ignoring that works as often as it does..


Curt Wuollet,

Wide Open Technologies.

Trevor Ousey

I not sure I'd agree here...
IMHO steel conduit is only capable of providing better mechanical protection. With most conduit installations, the condiut would be earthed at
multiple points (ie saddles, clamps, entries) thereby creating circulating earth currents. I have had trouble with one installation, and went to the trouble of insulating all mounting points and fitting an earth separately. This didn't fix the problem, but the hash that was displayed on the CRO was reduced. If there is an interference problem, maybe go back to basics. Ensure the cable is of an adequate quality, spaced a reasonable distance from any power cables,etc., and shields connected correctly.

Trevor Ousey
Sticking stp (cat5) in a metal conduit will not significantly alter the characteristic impedance, wheras utp (cat3) will be easily influnced.

However, correct earthing IS important, and this can be complicated by the use of a metal conduit.

A simple experiment, (if you are still having problems with STP, try passing the entire cable through the eye of an RF chocke toroid.

Steven Landau

I appreciate everybody's help on this. I heard suggestions

Drive Grounding
Conduit Material
Conduit Grounding
Cable Shield
Cable Terminations.

If I had to check all these things I would go out of business. The best suggestion which I actually too, and installed was one word.


It works great, the only problem is the fiber/copper converters we used are blinking out due the to heat generated in our purge cabinet. I have increased the air flow of the purge and we will see if it takes care of it. Does anybody use Industrial Fiber/Copper converters? Please recommend a vendor.


Steve Landau
92 Montvale Ave
Stoneham MA 01890

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