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TPS2372: /BT line

Daniel Campano
Intellectual 470 points
Part Number: TPS2372
Other Parts Discussed in Thread: TPS2373

Hi All,

I have a question about the /BT line on TPS2372. I understand that /BT line indicates if a BT compliant  PSE has been connected to the TPS2372, when that happens /BT is low and it indicates that the PSE is BT complaint. 

On table 3 page 16 of TPS2372 datasheet  we can see demotion cases, where on every case a PSE type 3-4 is forced to send less classification fingers than what it could send ( that is why they are demoted being type 3-4).Even in the cases that the PSE send less than 4 classification fingers the  BT PD sets a cero on  /BT line indicating that the PSE is a BT PSE.   Does it mean that the BT PD can detects a BT PSE even dough the BT PSE  is forced to send less 4 classification fingers?.

In other words a BT PSE will be always detected by a BT PD as BT PSE regardless how many classification fingers the BT PSE send to the BT PD?.

Regards,

Daniel.

 

  • 0
    TI__Mastermind 41770 points
    Hi Daniel,

    That's correct. The number of class fingers relates to the amount of power that the PSE delivers to the PD. And the first long class event (described in the BT standard) will allow the BT PD to distinguish between a BT PSE Type 3 or 4 and a Type 1 or 2 PSE. Please let me know if this helps. Thanks!

    Regards,
    Darwin
  • 0 in reply to Darwin Fernandez
    Intellectual 470 points

    Hi Darwin,

    Can we say then that  a BT PD configured with any Class will always recognize a BT PSE and put the /BT line low?.  Does it mean as well that the /BT line will stay high as long the PSE is not a BT PSE ( it could be a 802.3at PSE, 802.3af PSE or any other non-standard PSE)?.

    Regards,

    Daniel.

  • 0 in reply to Daniel Campano
    TI__Mastermind 41770 points

    Hi Daniel,

    A BT Type 3 or 4 will output a first long class finger independent of the number of class fingers. Speaking for TI's PoE BT PDs, it will process this and /BT will go LOW even if RCLSA is set to Class 0-8. This is shown in the THP/TPL/BT truth table in the TPS2373 and TPS2372 datasheets.

    A Type 1 and Type 2 PSE and non-standard PSE (that I know of) does not implement the first long class finger, so /BT will stay HIGH when connected to TI's BT PDs.

    Thanks!

    Regards,

    Darwin