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Re: [802.3af] Diode bridge both on the spare and data pairs

The leakage current concern may very well be an issue in some
implementations. This could perhaps be covered by adding a leakage current
spec and associated test description. However at this late stage this would
only further complicate and delay the standard.

Conversely, mandating bridges on both pair sets would remove the
interdependence between the data and power configurations. This would
simplify the standard and eliminate a potential interoperability issue. I
would say this is worth a diode or two.

My two cents/diodes...

Yair Darshan <YairD@xxxxxxxxxxxxxx> on 04/04/2003
04:47:17 PM

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Subject:  [802.3af] Diode bridge both on the spare and data pairs


I would like to discuss the  benefits in mandating diode bridge at the
input of both data pairs and spare  pairs.


The PD is required to be ready  to accept power from the spare pairs or
from the data pairs.
Typical implementation of Oring  the power from data pairs or spare pairs
could be one of the following  options:
1. Data pairs has diode bridge  and spare pairs using single diode.
2. Data pairs has diode bridge  and spare pairs has diode bridge.
3. Data pairs and spare pairs  has has single series diode each, data pair
should have diode bridge if the PD  is auto-mdi-x.

Now lets consider the following  case:
A multiport system activate  port number  x and send power to the PD.
The PD is configured per option  1 or 3.
Now, there is voltage present  at the output of the oring diode, but, due
to the fact that one of the leads of  the spare pair is directly connected
to one pair data pairs
There is a leakage current path  from the data pairs to the spare pairs
back to the PSE.

This leakage current will find  its way to other ports in the PSE and may
affect the detection  function.
In some bob-smith  termination configurations that was good for a switch
without pse and  are not suitable for switch with pse some ports may see
voltages above  30V even if they are at OFF state.

In order to prevent such  scenarios, option 2 is suggested that keep DC
isolation from the spare pare to  the data pairs and vice versa.

In addition, using diode bridge  at the data pairs will fix the issue
raised by Moti Goldish regarding the  MDI-X/AUTO MDI-X issue.

Mandating diode bridge on both  pairs will ensure powering of the PD in any
PSE configuration and in any cable  type straight or crossed cable
so we can  eliminate  the potential of interoperability problems regarding
the ability to successfully powering the PD.
The data issue is solved by the  definitions for the PSE and PD, by the pin
assignment and polarity for the  MDI/MDI-X/AUTO MDI-X configurations as
described in tables 33-1and table  33-7.
Actually referring to Auto  MDI-X in tables 33-1 and table 33-7 will not be
required  anymore.


The suggested remedy to support  the above is:

Draft 4.2 page  60:
1. Delete the text at  lines 50-51:
"If the interface  is implemented as an MDI-X or Auto-MDI-X per Clause
14,the PD shall be polarity  insensitive "

Replace it with the following  text: "The  interface in Mode A and in Mode
B shall be polarity  insensitive.

2. Consider to delete the  reference for Auto-MDI-X from tables 33-1 and
33-7 as it is not required due to  (1).

I believe that to mandate the  above is required.

Please comment over the above  issue as soon as possible.



 Darshan Yair
Chief  Engineer
PowerDsine Ltd.   -  Powering Converged Networks
1 Hanagar St., P.O. Box 7220
Neve  Ne'eman Industrial Zone
Hod Hasharon 45421, Israel
Tel:   +972-9-775-5100, Cell: +972-54-893019
Fax:  972-9-775-5111

   E-mail: <mailto:yaird@xxxxxxxxxxxxxx>.