Re: [8023-POEP] What's the max voltage drop thru a midspan?
Guys,
Chad, I agree with your assumptions.
In addition, this debate reminds me discussions in the early days of
802.3af.
The issue back then was how existing PCB design can hold 350mA in patch
panel etc.? Do you remember this one?
We got some reports of excessive heat at 350mA and people spent time on
this issue which was a temporary problem and not real one.
In reality most designer use margins.
Those who don't will revise their design to be in the market.
And all this is what if question type which address one operating
configurations of many others.
The bottom line is:
It should not affect the standard at all.
We should do the right thing for the future in the standard which is to
allow all these operating configurations and let designer and owner of
current equipment to worry about this.
After 1 year or so this question will be solved and forgotten assuming
vendors want to be in the market.
Yair
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Chad Jones
(cmjones)
Sent: Monday, June 12, 2006 8:26 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
Martin summarized it well by stating that the midspan would have to
carry the endspan currents in one of these (not recommended)
installations. Unfortunately, that means that legacy midspans may have
only been tested to carry 350mA - if this was tested at all.
On the flip side, 800mA flowing through a 5 mil trace of 1oz Cu on an
external layer will only result in a 17C temperature rise. I am
assuming that these traces are on an outer layer and that most people
use 1oz Cu on outer layers and that most traces are 5 mils or larger.
-Chad
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of David Law
Sent: Monday, June 12, 2006 11:35 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
Hi Steve,
I see your point but the text states that the Mid-span 'shall not alter
the transmission requirements of the permanent link.' so it really
depends if the DC Current carrying capacity and the Resistance are
considered 'transmission requirements' or not.
Regards,
David
owner-stds-802-3-poep@xxxxxxxx wrote on 12/06/2006 16:00:15:
> David,
> Unfortunately I don't think this text solves the problem. My
interpretation
> is that it does the following:
> 1. The midspan must pass data thru it without degrading data
integrity.
> 2. As the midspan injects power on the spare pares, it must block
> that power from going back to the endspan.
> So it doesn't say anything about passing power thru on the data pairs.
> Steve
> -----Original Message-----
> From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx]
> On Behalf Of David Law
> Sent: Monday, June 12, 2006 3:46 AM
> To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
> Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
> Hi Steve,
> Do you think the following text contained in the third from last
paragraph
> of subclause 33.4.8 'Midspan PSE device additional requirements' of
> IEEE Std 802.3-2005 covers what you are looking for:
> ---oo000oo---
> Configurations with the Midspan PSE in the cabling channel shall not
alter
> the transmission requirements of the "permanent link." A Midspan PSE
> inserted into a channel shall provide continuity for the signal pairs.
> A Midspan PSE shall not provide DC continuity between the two sides of
> the segment for the pairs that inject power.
> ---oo000oo---
> Regardless, I agree that the IEEE 802.3at specification will need to
> be written carefully to ensure that, for example, it doesn't
retrospectively
> place a higher current carrying requirements on existing IEEE Std
> 802.3-2005 compliant Mid-Span PSEs.
> Regards,
> David
>
> owner-stds-802-3-poep@xxxxxxxx wrote on 10/06/2006 05:34:39:
> > Guys,
> >
> > As you all know, we're planning to run current from the endspan thru
the
> > midspan in a 4P system.
> >
> > Although I think its unlikely that we'll see Af-midspans with fused
> traces
> > or delaminated boards, technically we can't prove that it won't
> > happen because 802.3af doesn't specify a min current rating for this
path.
> There
> > seems to be no max limit for dc resistance either. (If these specs
are
> in
> > 802.3af and I've missed them, then someone please point me to the
> applicable
> > table or paragraph.)
> >
> > We should make sure 802.3at specifies both these parameters. (But
only
> for
> > midspans that output >15.4W so we're not retroactively putting new
> > requirements on Af-midspans.)
> >
> > Any comments?
> >
> > Steve
> >
> > [attachment "C.htm" deleted by David Law/GB/3Com]