Author Topic: Extra conductors in cable - ground or NC?  (Read 2856 times)

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Offline shadewindTopic starter

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Extra conductors in cable - ground or NC?
« on: August 24, 2012, 10:54:13 pm »
I'm connecting two boards together through a connector with 8 conductors but I only need 7. Should I make the extra connector an extra ground or should I just leave it unconnected?
 

Offline Rerouter

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Re: Extra conductors in cable - ground or NC?
« Reply #1 on: August 24, 2012, 11:03:54 pm »
make it a ground, it will help suck up a little bit more of the noise in your signals
 

Offline Psi

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Re: Extra conductors in cable - ground or NC?
« Reply #2 on: August 25, 2012, 01:38:08 am »
Yes, make it ground.  Less resistance in the ground path can only be good for you.
Tie them to ground at both ends.

Or, if you actually want separate grounds, be sure you check out which is best to use for what. eg, high current ground vs signal ground.
You want the ground conductor with the least resistance to be used for power ground, which is often the mesh screen.
However you also might want the mesh screen to be signal ground to screen all the other conductors.
So you need to think about how you want to do it.

« Last Edit: August 25, 2012, 01:45:26 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline SeanB

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Re: Extra conductors in cable - ground or NC?
« Reply #3 on: August 25, 2012, 05:08:23 am »
If you do not use it use as ground, and preferably haveit next to the other, reduces the loop area to pick up common mode current. If you have a high frequency signal set use alternate conductors as ground with them.
 

Offline EEMarc

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Re: Extra conductors in cable - ground or NC?
« Reply #4 on: August 25, 2012, 05:36:11 am »
Using more than one conductor in a cable as ground is okay if they are terminated together at both ends.

There is a hazard for ground loops for other configurations. Particularly for multiple cables that don't terminate in the same location on the board.

The key for ground loops is the area of the loop of wire. The larger the loop, the easier it is to introduce a loop current. The more loop current, the larger the voltage difference at various points of the loop. If you have multiple cables, there is both the potential of a larger ground loop current and the voltage difference at different points in the circuit can cause havoc. Thus a ground loop problem. In the same cable, the loop is small. Even if it wasn't, the termination at both ends of the cable limits the effect of a ground loop current.

As SeanB mentioned, minimize the loop area. Also, if you have a high speed differential signal, keep the pair adjacent and twist the pair if possible.
 

Offline AndyC_772

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Re: Extra conductors in cable - ground or NC?
« Reply #5 on: August 25, 2012, 07:48:51 am »
What are the two boards, and what kinds of signals are the other 7 wires carrying?

Offline shadewindTopic starter

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Re: Extra conductors in cable - ground or NC?
« Reply #6 on: August 27, 2012, 11:17:56 pm »
I've made a Nixie driver board which drives two Nixie tubes. It has edge mounted horizontal headers so that you can place multiple ones edge to edge to daisy chain them without any cables. There will be a ribbon cable from the first board in the chain going to the board which controls the whole thing.
 


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