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A thought
to consider. If schottky diodes
are used for this discussion, it should be remembered that schottky’s can be
very leaky in the reverse mode.
Sometimes several milliamps can flow in a back biased schottky. So if
leakage is a direct concern, this is an important parameter. From a
Motorola spec: 60V 3A MBRD360 Max Irev = 20mA @ 125°C Ed Walker Analog Product Specialist Texas Instruments
Incorporated HC66 Box 203 Mountainair, NM 87036 INTERNET: ed_walker@xxxxxx
WEB SITE:
<http://www.ti.com> Office = 505-847-0576 Fax = 413-280-0812 -----Original Message----- Hi, 1. If the spare pair have diode on each
pair i.e. a diode on 4,5 and a diode on 7,8 than the problem I have described
can not happen, however using a full bridge on the spare per had the additional
advantage of being polarity insensitive and free us from being dependent on
what type of cable is being connected. 2. The are few types of crossed cables: Type 1: only pins
1,2,3,6 are crossed. Type 2: all pins are
crossed. Therefore I find
mandating diode bridge on the spare pair is useful too. 3. The wording of the current draft
prevents injecting power backwards from the PD to the PSE however, we are
discussing here very low leakage
current that may be enough to mass up the detection function. Yair. -----Original Message----- Hello together, I had a quick thought about Yairs
point. 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 switch. 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 cannot happen. 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.... ) 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. Best regards, Carsten -----Ursprüngliche Nachricht----- Hi, I would like to discuss the benefits in
mandating diode bridge at the input of both data pairs and spare pairs. Background 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. Summary: 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. Thanks Yair. Darshan Yair E-mail: <mailto:yaird@xxxxxxxxxxxxxx>. |