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I've got a project coming up that's to be doing RS-485 communications over shielded twisted pair cable. The project will involve cable length runs of many 10s of ft and the cables will probably have to be laid in a single conduit containing 36VDC wiring to motors. It is unlikely the motor power cable will have a shield.

Regarding the shield in the twisted pair cable, should it be tied to ground at one end, both ends or not at all? What practical difference with this make?

Then if tied to ground, should it be to the board's electrical ground or the chassis earth?

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If both devices have their own ground reference then you should only attach the shield at one end. This way you do not produce what is called a ground loop.

When you do attach it I would attach it to the chassis as this is the return path you want noise to take, the chassis to earth, if you attach the device that I assume is inside the metallic case and then that attaches out to the earth reference you are bring noise inside the box that you do not need to.

If one device is operating lets say PoE and has no earth shield reference you want its shielding/chassis connected to the shield to act as its connection to earth for noise mitigation.

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  • \$\begingroup\$ OK I think Ive got it, connect one end only of the shield to the chassis ground, as that's where we want the noise to be "sunk" to. Grounding at both ends would cause ground loops so don't do that. The case I am using has a ground lug nut where the ground from the AC power is connected, would that be a good place to ground the cable's shield? \$\endgroup\$ – fred basset Jun 4 '12 at 16:11
  • \$\begingroup\$ @fredbasset, The name is eluding me, but we have our connectors mounted to our chassis so that the cable when it has its shield connected properly in the cable end(sometimes called termination much to my frustration) is connecting the shield to the chassis itself. You can take the time to run the shield down to this lug but it might be easier just to connect it to your boards ground plane and make sure that is connected to your lug. chances are you will not have any major issues due to this but you can always test it and find out if you are currently installing it. Might be faster. \$\endgroup\$ – Kortuk Jun 4 '12 at 16:19
  • \$\begingroup\$ And the main purpose of the shield is to protect the twisted pairs inside from outside electromagnetic fields and noise and then also channel this noise to the chassis ground where it can be sunk? \$\endgroup\$ – fred basset Jun 4 '12 at 16:47
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    \$\begingroup\$ @fredbasset, noise forms paths inside a room it might couple to your wires and find a path to return(closed loop as always) back all the way to the source through neutral, if instead it passes through the shield it and to the earth it can stop it from affecting internal circuits. \$\endgroup\$ – Kortuk Jun 4 '12 at 17:01
  • \$\begingroup\$ +1. But one note. If it is 4 wires: +, -, ground, shield. Then shield should be grounded on one end, and "ground" wire should pass through 2x100 ohm resistors to chassis on both boxes. \$\endgroup\$ – user924 Jun 4 '12 at 23:25

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