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[FAQ] TPS23861: What’s Alternative A (ALT A) and Alternative B (ALT B)? Can TPS23861 support both Alternative A and Alternative B wiring?

Part Number: TPS23861

What’s Alternative A (ALT A) and Alternative B (ALT B)? Can TPS23861 support both Alternative A and Alternative B wiring?

  • There are four pairs in one single Ethernet cable.  For IEEE802.3at application, we only use two of them to deliver power. ALT A/B defines through which two pairs the power is delivered. In ALT A, power is delivered through pair 1&2, 3&6 and in ALT B, power is delivered through pair 4&5, 7&8. For either Endspan PSE or Midspan PSE, both ALT A and ALT B work. The only difference is the backoff time (when PSE fails to detect a valid PD, ALT A PSE should complete a second detection in less than Tdbo min(2 seconds) after the beginning of the first detection; ALT B PSE should back off for at least Tdbo min before attempting second detection).  

    TPS23861 is designed to only support Alternative A.

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