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Re: PD with a PSE and a wall brick

Brian, Gerard,

I see it this way:

When the wall wart is unplugged, we'd like the PSE to be able to pick up 
the slack seamlessly - this means the PSE must already have detected and be 
providing some amount of power greater than the minimum. It need not cut 
off the wall wart, however.

If the wall wart is plugged in while the PSE is providing power, we could 
drop the PSE power to just above 10mA, or not - it's up to the implementer. 
We don't want to allow the PSE to disconnect, or we'd need to redetect when 
the wall wart went away, and the PD would lose power briefly.

I'd vote that this is implementation-specific and doesn't need to be in the 

Related issue - we discussed in Irvine mandating that the RJ45 be 
environment B with respect to any external terminals, including the wall 
wart plug - this means we can't just diode-or the wall wart with the 
incoming PSE power, right? It needs to be OR'd in after an isolated 
switcher - which means it's part of a separate switching network. Other 

This really is a power issue - did we ever set up a power reflector? Should 
we debate this on the main reflector? Is there anyone who's on the main and 
not on the detection reflector?


At 09:18 PM 4/19/2001 +0200, Gérard Vergnaud wrote:

>See my answers inserted in your mail.
>"Lynch, Brian" a écrit :
> > I haven't seen much activity here lately, so I'll
> > warm up with a couple of questions regarding a PD, a PSE
> > and a wall brick.
> >
> > Scenario #1: If a PD is being powered by a PSE, and a wall brick is
> > then plugged in to the PD.
> >
> > Assuming that
> >         - The PSE had already detected/classified and is in
> >         full power ON mode
> >         -  And the PSE voltage may, or may not be, higher than the
> >         wall brick's output
> >
> > What should happen? The Spec says we can power from only one source.
> > So should the PD block the wall brick? Should it block the PSE?
> > Should the PD actively cause the current from the PSE to drop
> > below minimum so that the PSE shuts off?
>I think that when a power has been detected from the Ethernet link, then
>the power access from a wall brick must be disabled. For instance with
>a MOS-FET switching off the power from a wall brick.
> > Scenario #2: Now plug them in in the opposite order. First power the PD
> > from the wall brick, and then plug in the PSE. I assume we don't
> > want the PSE to detect. Is this correct?
>Same answer : as soon as a power is detected from the Ethernet link, then
>the PD doesn't accept power from a wall brick.
>This is also a mean avoiding motorboating problem at PSE side.
> > Brian
> >

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