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[FAQ]How to design PSE port polarity to be IEEE802.3bt compliant

Part Number: TPS23880

Hi!

I'm testing this new EVM with several different PDs on it's 2-pair and 4-pair outputs.

Single signature on 4-pair works and turns on signal and spare pairs. However dual-signature on 4-pair only spare pair turns on, and signal pair does not turn on. Seems like it's not checking signatures properly. I also verified this with a split cable too, and only spare pair comes up. Is anyone else having the same issue?

Any help is appreciated. Thanks,

-Paul

  • Hi Paul,

    Can you please provide more information about your test condition.

    1. What is the power allocation(0x29) set for the 4 pair port?
    2. What's class level of each PD for the dual signature PD?
    3. Can you send me a waveform of port voltage and port current(odd channel's port voltage& current, even channel's port voltage&current)?
    4. Can you also send me a screen shot of TPS23880EVM GUI when you have this problem?

    Thanks.

    Best regards,

    Penny

     

  • In reply to Penny Xu:

    Hello Paul. Does this thread still require our help?

    It has been nearly 2 weeks since our initial reply. I will be closing thread within 24 hours if not hearing back from you.

    Thanks for considering PoE from Texas Instruments,

    Thomas Lewis

    PoE Applications & Marketing Manager

    www.ti.com/poe

  • In reply to Thomas Lewis:

    Hi Thomas,

    I figured out the problem, and it had to do with the polarity of signal pair on this TI EVM being backwards from the standard ethernet cable wiring (which is allowed, of course, just unexpected), so I added a bridge rectifier to the PD I'm working with, and it seems to come up fine now on signal pair, spare pair. and both together in 60W tests.

    Thanks for your help and checking in,

    -Paul

  • In reply to Paul Kottas:

    Paul, Thank you kindly for the reply and for explaining the root cause of your issue.

    As other might experience this same issue, we will consider adding a note to the User's Guide or, perhaps, publishing a FAQ here on E2E.

    Thank you for considering PoE form TI,

    Thomas Lewis

    PoE Applications & Marketing Manager

    www.ti.com/poe

  • In reply to Paul Kottas:

    Hi Paul,

    Our EVM port polarity followed IEEE802.3bt standard. For Type 4 ports(right two 90W ports), you have to follow this rule: pair 3&6 is positive, pair 1&2 is negative; pair 4&5 is positive and pair 7&8 is negative. Please read this white paper to understand more details about IEEE802.3bt standard before the standard is published.

    Best regards,

    Penny

     

  • In reply to Penny Xu:

    Hi Penny,

    So just to clarify..for .3bt type 4, pair 1&2 must be negative, and pair 3&6 must be positive
    by definition of the standard?

    I only ask this because prior standards allowed it either way.

    Thanks for the info,

    -Paul
  • Hi Paul,

    The IEEE802.3at standard only allows 2 pair power delivery and it allows both polarity for 2 pair Alt A . But 4 pair power delivering is only allowed in IEEE802.3bt standard. If the PSE is Type 4, you have to follow the polarity defined for Type 4 which I described above. If you only use 2 pair (<=30W), it is considered as Type 3, Type 2 or Type 1, you can use both polarity as the picture shows. Thanks.

    Best regards,
    Penny

     

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