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>@majordomo.ieee.org on 04/04/2003
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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
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.
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