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?
Best Answer
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.