[8023-POEP] Mark High vs Mark Low
Joe,
Mark voltage high or mark voltage low is doable. Each
has + and -'s.
We decided mark voltage low because with mark voltage
high one can have the following problem:
Consider a class 3 .af PD which is micro power from
21V to 30V.
.at PSE drives PD to -18V and classes.
.at PSE drives PD to -25V to create mark.
.at PSE attempts to drive PD to -18V.
But....PSEs can only drive PD more negative.
Only PD can discharge itself from in this case -25V to
-18V. (PDs are isolated by diodes and PSEs usually
only pull down.)
The above mentioned PD might stay up above class
voltage and stay in micropower mode, thereby reporting
class 0, or it might drop into class part way through
the PSE class measurement, thereby fooling the PSE
into reporting some class value between 0 and class 3.
Alternately, if using mark low voltage, after the PSE
classes, an .af PD may or may not discharge down into
the signature range. Worst case would be a class 0 PD
which might not discharge the port. However, when the
PSE goes to class a second time, however far the PD
has discharged the port, the PSE will just drive it
back to -18V. All is well.
With respect to operating memory in the signature
range, if we keep the mark voltage at say 7-10V, then
at 7V, 25K draws 280uA. The memory element probably
uses little current compared to 280uA.
Clay
--- Joseph DeNicholas <Joseph.DeNicholas@xxxxxxx>
wrote:
> hi Clay -
> I had to drop off for a little while, the last I
> heard we didn't want the
> trigger to be in the signature range because the PD
> is in such a low power
> draw mode that it would be difficult to maintain the
> state if we went back
> into signature. Isn't this still a concern? To
> hold state the PD
> probably has to keep a bias regulator up, all on
> less than 10uA, no?
> thanks,
> joe d.
>
>
>
>
>
>
> "Clay Stanford (LTC)" <claystanfordltc@xxxxxxxxx>
> Sent by: owner-stds-802-3-poep@xxxxxxxx
> 06/21/2006 09:21 AM
> Please respond to "Clay Stanford (LTC)"
>
>
> To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
> cc:
> Subject: [8023-POEP] 6-21-06
> Classificaiton Ad Hoc Results
>
>
> We made some decisions in today's meeting:
>
> 1. There will be an additional class between 7W and
> 15.4W.
>
> 2. We will focus on using the Ping-Pong
> Classification
> method until an unsolvable problem arises.
>
> 3. The "mark" voltage will be down in the signature
> range. For example, it may be 7-10V.
>
> Next week, we are going to concentrate on the
> voltage
> and timing issues surrounding Ping-Pong.
>
> thanks
>
> clay
>
>
>
>
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