I had a quick thought
I agree, a PD should not send
out power it receives on the other wires.
This may cause a problem in
the switch. If the power sources for individual ports are connected in the switch, it may also cause problems in other devices attached to the
However, I think if the data
pair has a diode bridge, and the spare pairs have a diode in each
pair ( a half bridge) ,
the problem you described
So there is no need to mandate polarity insensitivity on the spare
pairs per se.
Such a polarity insensitivity would make sense only
we consider a cabling that swaps the spare pins is considered as a scenario
that needs to work with power over lan. ( May be using Gigabit crossover
I´m not sure if the wording of current version of the standard prevents
injecting power backwards from the PD to the PSE.
If its not in, it should be added of course.
I would like to discuss the
benefits in mandating diode bridge at the input of both data pairs and spare
The PD is required to be
ready to accept power from the spare pairs or from the data
Typical implementation of
Oring the power from data pairs or spare pairs could be one of the following
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
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
There is a leakage current
path from the data pairs to the spare pairs back to the
This leakage current will
find its way to other ports in the PSE and may affect the detection
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
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
Actually referring to Auto
MDI-X in tables 33-1 and table 33-7 will not be required
The suggested remedy to
support the above is:
Draft 4.2 page
1. Delete the text at
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
2. Consider to delete the
reference for Auto-MDI-X from tables 33-1 and 33-7 as it is not required due
I believe that to mandate
the above is required.
Please comment over the
above issue as soon as possible.
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