Regarding grounding of STP cables I was facing with contrasting recommendations. I believe this topic is much more general than Ethernet networking. One camp claims you should only ground on one end of the cable to prevent a ground loop. The other camp claims to ground both ends in order to have an effective way to prevent EMI entering the cable. To be sure with a ground loop for my case I mean a loop in the ground/earth wire of my residential mains electricity installation. And I think it is useful for this case to state that I live in the Netherlands.
Note that
neither camp is wrong. But their actions are exclusive.
If you are leaving one end open, you are letting in
100% of the EMI on the cable.So why even bother getting the shield in the first place? It's literally not doing anything.
The only thing a single-grounded shield can protect against is magnetic induction between the signal lines, which the twisted pair already deals with reasonably well, and electric induction, which the differential lines don't care about. It cannot deal with electromagnetic (wave) induction, i.e. where the offending frequency is higher than the electrical length of the cable.
To avoid ground loop, you might compromise by using bypass capacitors around the ground. This retains galvanic isolation (low frequency -- mains and DC -- ground loop voltages are allowed) while shunting EMI. But this isn't a connector-level thing you can do, and I'm not aware of anything other than board-level design, that handles this.
Points against the ground loop camp are:
1. Who cares if the EMI is doing ground loops? If the shield is complete, EMI isn't getting into your signals! It's fine, end of story. (Resonances are common -- the low impedance connections allow cables to act as resonators -- but this is easily tamed with a ferrite bead or two.)
2. If you have ground loops that are so bad they're actually causing problems, you have bigger concerns than mere network wiring -- namely, your building wiring is
fucked and needs to be repaired immediately before the defective grounding starts a fire or something!
Note that #1 is predicated on what is "fine". Ground loops are offensive in audio applications because the standard signaling method is single-ended, and the signal is low frequency where shields are ineffective. That is, the shield's resistance dominates, and ground loop and signal currents are allowed to mix. The resulting voltage drop is additive to the signal.
This goes away with a differential signaling method, like XLR audio jacks, or Ethernet which is differential and high frequency (AC, transformer coupled).
Tim
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