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Hi Lennart, Christian and all,
Even if the PSE polarity is fixed, you still ned to mandate the "polarity insensitive" requirement in the PD which means diode bridges or equivalent solution.
The reason is that the new 802.3bt need to be able to work with Endspans and Midspans Type 1 and Type 2 PSE as well to be also backwards compatible.
In the box level you can have ports that are configured to af, at and bt. It is impossible to ask the installer to verify which cable type and/or polarity to use. This is not plug&play and not backwards compatible.
802.3bt PDs can be also with lower power that 51W. 4P PD doesn’t only differentiated from other PDs by the power level only. It can require power over all pairs, over two pairs at any power and other differences that are yet to be determined (similar to Type 2 PD that is not differ from Type 1 only due to the power level).
As you say diodes cost are not costing much and add a lot of value by having plug&play. With higher power PDs that you may need (pending implementation and application) active diode bridges (MOSFETs based) to have lower power lost, the cost is justified if the power is increase by a factor of 2-3, the added cost of the active diode bridges is not an issue.
Remember that if Type 2 PD costs X$/W this ratio will stay or will e lower (it will be lower) for 2-3 times the power, so there is no cost issue as well for the PD system level.
This is all true and relevant for the "usual" Ethernet applications.
For lighting equipment, I understand that it is fixed application and wiring can be controlled, lower chances for doing mistakes etc. and yet you need to find a way how to have simple standard with minimum options, maximum flexibility, and how to differentiate lighting PD types from other typical network/data applications. May be to specify special equipment type for lighting only that meets the whole standard and requirements a,b …n are optional for it. BUT THE POLARITY of PSEs stays as it is today since PSEs can be used for all applications and must be defined as it was so far.
From: Lennart Yseboodt [mailto:lennartyseboodt@xxxxxxxxx]
From: George Zimmerman [george@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
I would confirm what Keng-hua reports, that I know of many PHYs with the crossover correction, on both sets of pairs independently. I agree that this feature in the PHYs works data-only, and will not effect powering. However, because of this feature, we therefore do not see any trouble calls from mis-connected crossover cabling. Therefore we do not seem to have a good way to see whether the two sets of pairs are crossed over independently in the field.
One reason that the feature is popular is that the equipment vendor no longer has to worry about the configuration of the wiring (with regards to crossovers). Specifying a specific configuration for 4-pair POE applications seems to be a step backwards, and would lead us back to the days when the end-user would have to have multiple sets of patch cords handy when connecting a new link so that he could make sure he had the right crossover combination at the end point. Remember that there are several connections in the link topology where a crossover may or may not occur (either by mistake or by design).
Principal, CME Consulting
Experts in Advanced PHYsical Communications Technology
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