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

Hi Carsten,
See my comments below.
1. The lekage current at the PD side is limited to 10uA during detection period. See table 33-8 page 63 draft 4.2.
    Here we are discussing on the case in which we supply voltage on the data pairs and measure low current at the spare pairs and vice versa.
3. 4.  -----Original Message-----[Yair Darshan] 
From: Carsten Bode [mailto:CBode@xxxxxxxxxxxxxxx]
Sent: Saturday, April 05, 2003 1:50 PM
To: Yair Darshan
Cc: Stds-802-3-Pwrviamdi (E-mail)
Subject: AW: [802.3af] Diode bridge both on the spare and data pairs

Hello Yair,
the different types of crossover cable are a good reason to put a diode brigde to the spare wires.
You may change the standard to require the diode bridge, that gives the end user the guarantee every device wthat it works with every crossover cable.
[Yair Darshan] Yes this was my intention. 
However, using a single diode is in my opinion not even with the current draft an option, since under certain, perhaps rare,  configurations power may be sent back to the PSE. So the PD cannot guarantee that it draws not more power than the upper limit of approx 12W.
[Yair Darshan] Using a single diode is an option due to the fact that when the standard doesn't say you cant than you can..however
    the user is not force to do so and my comment suggest that we mandate it or at least (if it is too late at this stage) add a cautionary note to make
    the PD designer aware of the potential problems.
By the way, I found no direct statement in the 802.3af that injecting power backwards is forbidden, can you direct me to the right place  ?
Specifically, I found nothing limiting the leakage current. Adding such a limit would perhaps make the life for switch manufacturers easier.
[Yair Darshan] See 33.3.1. line 19 in which PD is required to "The PD shall not source power on its PI." 
I donīt see how the leakage current is significantly reduced by adding diodes. The leakage current of diodes is not so much dependend on the reverse voltage,
the main variance is over temperature.
[Yair Darshan]  When a Switch with a PSE is designed "correctly" in which each port has its single bob smith termination and
        there is no DC current going between ports that passes through the terminations than there is no problem at all regardless if there
        is diode bridge or not at the PD. In some termination configurations the usage of diode bridge in both spare and data pairs
        solve the problem however we still have low leakage current which is  depends on the the diode quality however these
        levels can be handled.
-----Ursprüngliche Nachricht-----
Von: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
Gesendet: Samstag, 5. April 2003 12:57
An: Carsten Bode; Yair Darshan
Cc: Stds-802-3-Pwrviamdi (E-mail)
Betreff: RE: [802.3af] Diode bridge both on the spare and data pairs

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.
-----Original Message-----
From: Carsten Bode [mailto:CBode@xxxxxxxxxxxxxxx]
Sent: Saturday, April 05, 2003 12:13 AM
To: Yair Darshan
Cc: Stds-802-3-Pwrviamdi (E-mail)
Subject: AW: [802.3af] Diode bridge both on the spare and data pairs

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.... )
Iīm 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,
-----Ursprüngliche Nachricht-----
Von: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
Gesendet: Freitag, 4. April 2003 23:47
An: 'David Law (E-mail)'; 'John Jetzt (E-mail)'; 'Fred Schindler (Business Fax)'; 'Scarlson (E-mail)'; 'Fred Schindler (E-mail)'; 'Pat_Thaler (E-mail)'; 'Bob. Grow (E-mail)'; 'Gthompso (E-mail)'; 'Mike S McCormack (E-mail)'
Cc: Stds-802-3-Pwrviamdi (E-mail)
Betreff: [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