Question about Shielding Cables - Signal / Power

[digi001]

I am sending four RS-485 differential signals, and Power+GND over a cable from one PCB to another. The power is made on the master PCB and the Slave PCB will be floating.

The cable has 6 twisted pairs and a shield so i have a couple of options.
The obvious choice for signals is to send all across the twisted pairs.

But for Power is it better to 1.) send across a twisted pair or 2.) send power across a single wire and have the return (GND) come back on the shield wire?

 

[chuckey]

It is better if the shield is dedicated to chassis ground. It might become necessary to float the PSU connections at some time.
Frank

 

[digi001]

In this testing setup there is no chassis. Just 2 PCBs, 1 power supply with 1 GND. Power supply is floating.

 

[edpalmer42]

I am sending four RS-485 differential signals, and Power+GND over a cable from one PCB to another. The power is made on the master PCB and the Slave PCB will be floating.

The cable has 6 twisted pairs and a shield so i have a couple of options.
The obvious choice for signals is to send all across the twisted pairs.

But for Power is it better to 1.) send across a twisted pair or 2.) send power across a single wire and have the return (GND) come back on the shield wire?

I would use the twisted pair for power and keep the shield just for shielding. The twisted pair will help to contain any power supply noise. If you use the shield for power ground, you might radiate more electrical noise either outside the unit or into the signal pairs. Since this sort of thing is something of a 'black art', you might also want to try grounding the shield only at one end.

Ed

 

[siongboon]

Totally agree with edpalmer42.

I would use the twisted pair for power and keep the shield just for shielding. The twisted pair will help to contain any power supply noise. If you use the shield for power ground, you might radiate more electrical noise either outside the unit or into the signal pairs. Since this sort of thing is something of a 'black art', you might also want to try grounding the shield only at one end.

Ed