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 cables.... )
Iīm not sure if the wording of current version of the standard
prevents injecting power backwards from the PD to the
If its not in, it should be added of
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
of Oring the power from data pairs or spare pairs could be one of the
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
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
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 33-7.
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 lines 50-51:
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 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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