• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 434 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

IEEE 802.3af standard problem using the TPS23750PWP

Jelle
Prodigy 10 points
Other Parts Discussed in Thread: TPS23750

Hello,

I have a problem with my currently designed PD device. I used at this moment the spare lines for getting power for my pd device. The problem is that a Endpoint (end-span) switch needs to see the pd on the spare-lines and data-lines. To make our system accept power from the datalines isn't possible because the transformer isn't suitable for POE current.

The only way to let the switch believe that there is a correct PD device present, is to place a 25k Ohm resistor between the transformer points of the data lines. But is this possible and does this work or is this allowed??

Hopefully you can answer my question.

Thanks,

Jelle

  • 0
    TI__Expert 7990 points

    I am assuming that you want to make a generic, standard compliant PD.  Since you are using the TPS23750, I am assuming that your PD input power is 13W or less.  I am also assuming that your PD only extracts power from the spare pair (4-5/7-8).

    A PSE must power the same pair-set that it detects a valid PD on.  There is no requirement that an individual PSE detect both pair sets.  A PSE may have been designed to power either pair set (1-2 / 3-6 OR 4-5/7-8).  A compliant PSE may not power both pairs.  The PSE is not required to see a signature on both pair sets.

    A compliant PD must be capable of detecting and powering through either pair set - only one at a time is reqired.  This maintains compatiblity between PSEs and PDs. 

    As I understand it, your PD design is not currently compliant to the standard.  If you simply put a 25K signature on the data pairs with no provision to extract power, you will find that your PD will not work with many PSEs.  The PSE could detect and power  the PD on the data pair set, but  the PD will not run.

    When 802.3af was brought out,  transformer manufacturers found that they did not have to increase the size of their magnetics (used same core size).  There are lots of vendors making 13W magnetics and a number making jacks with internal magnetics that should fit the same space on your board.  You might actually find that your existing magnetics might work.

    If you want a compliant (and compatible) PD, your best solution is to change the magnetics and add the second bridge.  If you are controlling both the PSE and PD, you can make a system that works but is non-compliant.