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I am working in the oil field with tools which I need to connect by cables (which "should be shielded") to computers. Most of the time when I communicate with my tools, I face grounding issues. I tried to ground the computer but it didn't always work, even when am trying to use the laptop to minimize that problem.

Does anybody know how I can reduce or cancel that noise to get better communication with my tools?

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  • \$\begingroup\$ Ahmed, I've attempted to improve your question by fixing grammar and English mistakes. Hopefully your intended meaning is intact. \$\endgroup\$ – JYelton Sep 17 '14 at 5:04
  • \$\begingroup\$ There may be other alternatives to better grounding, e.g. optical fiber or differential signalling. What is the nature of the signalling on the cables? Maybe a link to a commercial web page for one of the tools? \$\endgroup\$ – markt Sep 17 '14 at 6:16
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Twisted pairs(usually with differential signaling), common-mode chokes, ferrite beads, decoupling capacitors, shielding are common solutions. It is difficult to address your problem without the exact setup and attempts to solve the problem, but I would like to add to the above comments with some good shielding practices, since that is the solution you are using.

  • When shielding, grounding on one end will only help against interference from electric fields. Grounding both ends will help against both electric and magnetic fields, but will introduce ground loops, so extra measures must be taken to prevent/compensate these(you should try to ground everything to the same reference point in a 'star' grounding configuration andyou should try to keep the enclosed surface between shield and ground as small as possible).
  • Prevent grounding your shield with a long wire (pigtail grounding), because this wire itself can radiate EMI.

Shielding done wrong can actually backfire, so I would personally use twisted pairs as a starting point against EMI.

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If you have an electrical device ("tool" in your description) that is grounded at a distant point but is then also connected to a computer that is grounded separately (or not earth grounded at all), that can be the source of a lot of ground noise. It's called a "ground loop". Google it.

To fix a ground-loop, you need to run a separate wire from the same grounding point as the computer out to each device. Meaning all devices and the computer all converge on the same grounding point.

More specifically, use a separate power-strip just for the computer and devices connected to it. So you will have a power-strip plugged into one clean high-power supply and then all of the power cords fan-out from that one point.

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    \$\begingroup\$ The other option might be to use isolated communications. If you are using RS-232 for example, you could get an off the shelf converter box to change it to isolated RS-422 which does not suffer from the same ground loop issues. \$\endgroup\$ – Oliver Oct 17 '14 at 7:00
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    \$\begingroup\$ Isn't it true that for a shielded cable, the shield should only be connected to the ground from one end, not from both ends? \$\endgroup\$ – PkP Dec 16 '14 at 8:37
  • \$\begingroup\$ Yes generally that's true so as to avoid the ground loop in question. \$\endgroup\$ – lm317 Jan 15 '15 at 23:01
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You should have a look to common and differential mode noise in EMC.

If the cable is shielded, the signal still may be polluted as you experience it. You might try to twist your wires as a first step. Maybe use coax cables instead of two shielded signal cables.

Finaly, you should find many ways to reduce the noise in EMC lessons on the internet, like : http://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx

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