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[802.3_4PPOE] Diode Bridges


I have received a couple of requests from participants of this task force about Auto-MDIX and wiring questions in general related to polarity and pair swaps.

The PHY companies would be the ones to talk to, but in my experience many have bits to ID which pair is connected to which MDI PHY pair connection, and they also can detect/correct polarity flips and ID them as well. Of course, this means that some wiring defects/configurations are tolerated, and thus remain operational without user repair/replacements.

As I understand it, the question is whether or not pair/polarity swaps occur and at what rate, and if so, whether or not diode-bridges are required. My thought is that some relatively simple diagnostics could be done in the DC domain to ID the state of the link prior to applying power. I wouldn't want things to *POP* regardless of the statistical aspects of network wiring configurations.

Also, many MDIs either short their 4,5 and 7-8 pairs, or they terminate them to ground with 75 ohm resistors, or they terminate them to a virtual ground with 75 ohm resistors to a 50V cap to ground (150 ohms between wires). For Gigabit, most terminate to a virtual ground on all pairs with 75 ohms, a DC block between the powered pairs, and a 2Kv cap to ground. They may not provide DC blocking between the pairs that power is not anticipated on.

So this means you really have to think about a diagnostic process prior to power application regardless of the traditional wiring config. Most likely a high-impedance application of voltage to each wire/pair and measurement of the response on all other pairs would be required.

I hope this helps the group.

Dan Dove
Chief Consultant
Dove Networking Solutions
530-906-DOVE - Mobile