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Hi Pete, Please see below my inputs. Yair From: Peter Johnson [mailto:peter_johnson@xxxxxxxxx] EXTERNAL EMAIL Lennart: Finally spent a few more minutes thinking about
Ilim/Tlim. A “short circuit” condition is basically any condition that causes the PSE output to meet or exceed
Ilim (Ilim, a PSE-specific variable, of course). Yair: Agree. I agree, and pointed out in some rejected comments, that there is a fundamental inconsistency in text in the following 3 sentences: From 145.2.10.8: If a short circuit condition is detected on a pairset, power removal from that pairset shall begin within TLIM
as defined in Table 145–16.
(I’m taking Tlim to be a PSE-specific
variable here that has a well-defined range.) Yair: This text was meant to be valid for PSE operating voltage. So, I suggest to clarify it in this way. If IPort-2P
exceeds the PSE lowerbound template, the PSE output voltage may drop below VPort_PSE-2P
min. Yair: I believe that this text is confusing and redundant. I don’t remember why we add
it? A PSE in a power on state may remove power from that pairset without regard to TLIM
when the pairset voltage no longer meets the VPort_PSE-2P
specification. Yair: This text is OK if you clarify the 1st and address the 2nd one and it is very important for PSEs. The final of these 3 sentences washes out any behavior implied by the first 2 sentences. Yair: See above. The practical truth is, a PSE limiting output current will drop outside the
Vport_PSE-2P specification in the vast majority of cases. Yair: Correct. This is because the distance between the nominal PSE voltage to the minimum PSE voltage is lower in percentage compared to ILIM-2P/ILIM_2P_min Sentence #2 addresses this. No doubt, this then puts strain on the PSE controller as it dissipates power. That’s why sentence #3 exists. Yair: Agree, so what if we delete sentence #2? A further practical truth is that many PSE’s that hit their
Ilim threshold will sustain power, perhaps with current foldback to preserve the PSE controller, until
at least Tlim_min. Other PSE’s will implement policy that when in current limit and voltage drops, power goes off immediately without regard to
Tlim_min. 802.3at essentially enabled this duality and 802.3bt is continuing it. I do see that sentence #3 here is completely redundant to the sentence in 145.2.10.1: A PSE in a power on state may remove power from a pairset when the pairset voltage no longer meets the VPort_PSE-2P
specification. I would propose that we don’t need this sentence given the overlap. Yair: For sure we don’t need to sentence that say the same but I am not sure which is redundant. Need
to think which to remove and which to keep. -------------------------- Finally, I would propose setting
Tlim_max in Table 145-16 to hard number 75 msec. I don’t think this would break anything and it would eliminate the absurd case of a PSE operating permanently in a current limited state. Yair: Make sense to me. Regards, Pete J Peter Johnson Sifos Technologies 1061 East St. Tewksbury, MA 01876 Sifos is relocating on June 1, 2018 to: 1 Technology Drive Suite 100 Andover, MA 01810 To unsubscribe from the STDS-802-3-4PPOE list, click the following link:
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