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Author Topic: Shield grounded at one end only  (Read 7346 times)

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Offline stirling

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Shield grounded at one end only
« on: February 07, 2014, 05:07:48 AM »
In every off the shelf parallel and serial cable I've looked at, both end shells are connected via the shield. I've just double checked my PC and printer and was not too surprised to find that the port receptacles on BOTH were connected to their respective chassis which of course in turn are both grounded. So it seems that off the shelf kit does not follow this ground shield at one end only philosophy.

And yet - when I look it up on tinternet - there it is all over the place - ground shield on signal wires at one end only....

Anyone care to shed some light/wisdom/knowledge/thoughts on this?

EDIT: Just to be clear, I understand the logic behind the one-end-only. My question is why doesn't this appear to be the case in "off the shelf" kit.
« Last Edit: February 07, 2014, 05:53:07 AM by stirling »

Offline RICH

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Re: Shield grounded at one end only
« Reply #1 on: February 07, 2014, 06:17:39 AM »
Shielding is done / required / utilized and even custom designed for a lot of different reasons, but, simplisticaly:
- to keep what is inside, inside the shield
- to keep what is outside from interfering what is inside

So now the question becomes......  What are you trying to control with the shield?

Using a crude water analogy... water may be on a pipe. Where do you want  the water to go ?
Go back to the sanitation sewer, air dry, or go to a designated sump which makes it easier to get into the sewer line or a designated sump system, just contain it ( ie; tank to tank ) or even a combination of two or three of them.

So grounding of the shield could be subjective and only the person looking at the big picture for what was considered may know why
what is being done with the shield is appropriate. But generalizations do come about and are not necessarily always true.

From a non-electrical ....and may be all "wet", just some thoughts,

RICH



                                              


 

Offline Tweakie.CNC

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Re: Shield grounded at one end only
« Reply #2 on: February 07, 2014, 06:36:28 AM »
Just my 2p’s worth…

Basically, in most countries, regulations for mains connected equipment provide that unless a product meets the ‘double insulated’ (or a couple of other) standards all metal parts that we may touch must be bonded to protective Earth.

With our CNC the LPT pins 18-25 are used as ‘signal GND’ and sooner or later, in our build, we connect them to Earth - this is where the potential problems with electrical noise start if we don’t follow the one ended connection rule for shields / screens.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline ger21

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Re: Shield grounded at one end only
« Reply #3 on: February 07, 2014, 07:16:01 AM »
The resident electrical expert at CNC Zone, Al The Man, has been posting information for the last few years that says the currently accepted method is to ground shields at both ends.
Gerry

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Offline Hood

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Re: Shield grounded at one end only
« Reply #4 on: February 07, 2014, 07:46:43 AM »
I can tell you that the Allen Bradley drives I use say to connect the encoders shield at both ends and I have done that on some and not on others and never noticed a difference but I am talking normal incremental 5v square wave differential encoders.
 I also have some motors with sick stegman sine/cosine encoders and they will just not work without a shield or the shield only connected one end, they need the shield connected at both ends.
Now I have seen all sorts of explanations of how to shield and where to connect and some of the explanations have mentioned that it is frequency dependant. I have always tended to connect one end only as that seemed to be current thinking but like everything else, thinking changes and you can never get a clear answer.
 I say connect one end and if you have problems try connecting both, if you still have issues then start crying (would pull my hair out but its already gone :D  )

Hood

Offline Hood

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Re: Shield grounded at one end only
« Reply #5 on: February 07, 2014, 07:52:56 AM »
Current Thinking?
The SmoothStepper was a good example of this, some people suffered from noise problems and some like me never. Well thats a slight lie as I did slightly on one machine when I whacked the E-Stop it would occasionally screw up the SS. So someone found their probs were halted when they remove a resistor  and cap that connected the SS Gnd to the shield of the cable and to the computer, I tried that and it cured my very occasional issue. Greg started sending out SS's without these components and some people still had issues with noise, so resistor and cap were soldered on and their problems stopped.
So why is it it can cure for some and make worse for others? I say there is no hard and fast way to stop the noise getting in and it is all dependant on the rest of your system, so one end may be good on one system where another needs both but I can say for sure no shielding will be the worst.

Hood
Re: Shield grounded at one end only
« Reply #6 on: February 07, 2014, 08:02:54 AM »
From what I've gathered, it's all about the "loop".
If every component of a system is grounded properly, connecting the shield at both ends would drain well.
But due to improper grounding, the drain can act as a path to ground for other components, not just the emi.
One end only is probably best in the "hobby" world.
Professional builds in the industrial world may be different.
AlphaWire is king in this dept.

http://www.newark.com/pdfs/techarticles/alphaWire/USC.pdf

Russ


 
Re: Shield grounded at one end only
« Reply #7 on: February 07, 2014, 08:38:58 AM »
... to reinforce what I've gathered ....

This problem is very prevalent in audio systems.
"One end" connected actually acts as an antenna.

Interesting writing:
http://www.equitech.com/support/wiring.html


(from above link) In virtually every case, with balanced power, balanced audio can be interconnected in a standard way with the shield hooked up at both ends.  The only exception is when a piece of gear has a "dirty" chassis which requires isolation away from the rest of the grounding system.  This can usually be accomplished by lifting the audio ground at the input and isolating the offending chassis from other chassis with insulators.  A "dirty" chassis condition is rare in professional audio equipment and it often is the result of a substandard power supply or the audio ground not being connected to the chassis.  These problems can often be fixed with some effort. In general, wiring and grounding techniques are far simpler with balanced power and it is easier to identify and deal with any offending piece of equipment.

Offline Chaoticone

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Re: Shield grounded at one end only
« Reply #8 on: February 07, 2014, 09:38:44 AM »
I think sometimes it pretty much boils down to some twisted form of black magic. I think it has a lot to do with the location of the component generating the noise. Envision a control cabinet with nothing but low voltage/current components like a PC and BOBs. Leaving that you have a few control wires going to a VFD. I would likely start by breaking convention and grounding the shield at the VFD end only. Most I have seen in industrial controls suggest grounding the control end only but I view that as a suggested starting point. If that doesn't work, try grounding both ends. If that doesn't work try grounding only the end opposite of the controller. Sometimes floating the ground of a power supply helps, others it causes a real mess. I think it boils down to the fact that we simply do not know enough about EMF. Some of it can be extremely difficult to measure and it is hard to analyze data you do not have. EMF is like a fire. Try to contain it as close to the source as possible because it is going to do everything it can to spread to everything in sight.

Most of the time you can narrow it down to the source of the problem pretty quick. Other times it takes for ever and others you never do. The ones that just blow my mind is when you have a machine all wired up and tested and maybe even run for extended periods of time with no issues. Pick it up and move and now noise is a serious problem. Everything I know to measure are dang near identical. Dedicated ground rods megged to within a few ohms of one another. Same wiring, same routing, same input power, same type building, same kind of lighting, relative humidity........ every thing I can think of.  Obviously I'm missing something but what? The only thing I can come up with is that it is simply located at a different spot on this molten Iron core rock we call earth. Could drive a man mad. Most of the time the solution is to fiddle with the shields again. I just find things like this bizarre to say the least and anywhere from humbling right up to embarrassing.  ;D  If any of you guys have seen similar and actually narrowed it down and could put your finger on it how about share it. Or share the most bizarre source of noise you have found and the solution.

Brett
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Offline RICH

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Re: Shield grounded at one end only
« Reply #9 on: February 07, 2014, 10:56:30 AM »
Quote
most bizarre source of noise you have found and the solution

Blinking Christmas decorations.
Just need to waite until after the new year.  ;)

RICH