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RE: [802.3af] Late comment


I agree that if all ports are floating than you may have 2*Vport between any
two ports however these ports would never touch each other under normal
operating conditions and installation.

In any case the preferred lead to be ground is the positive lead from
practical point of view (easy to design when the + is the common for a
single power supply) and from system point of view (positive lead to system

Currently in our spec we didn't defined if:
a. we stay floating in both leads or
b. ground the positive or 
c. use the positive lead as a common point without grounding it to the
system ground/chassis.
d. ground the negative or 
e. using mixture of possibilities.

It is clear that option e is disaster to the standard.

option d is bad from practical design aspects and cost issues. (P channel
mosfets etc)

Option c : the best option by having a positive common point which is
floating too.

Option b is proffered if we find good reasons why we should ground the port.

Option a is very good due to max completely floating outputs.


I suggest the following text to the standard:

1.	The PSE port outputs shall be isolated from system ground, frame
ground and data circuits.
2.	In multi port system all ports common point shall be the positive
output (+Vport).


-----Original Message-----
From: Dave Dwelley [mailto:ddwelley@xxxxxxxxxx]
Sent: Tuesday, January 14, 2003 3:47 AM
To: Geoff Thompson
Cc: stds-802-3-pwrviamdi@xxxxxxxx
Subject: Re: [802.3af] Late comment

All -

Geoff is right. There's nothing to stop someone from building a system with 
a +/-48V supply, and wiring half the ports to -48 and half the ports to 
+48. The fact that the supply is floating relative to earth ground doesn't 
matter - there are at least two terminals that are 96V apart, and something 
could short them together.

Technically, because we're isolated, it's completely arbitrary which end we 
treat as "ground". Practically, there are arguments both ways:

- most non-telecom engineers (read: networking engineers) are most 
comfortable with ground on the bottom, i.e., +48
- most telecom engineers are used to -48
- most engineers of both stripes like buying 100V NFets in preference to 
PFets, suggesting switching the negative rail is best.

I'd vote we specify that the more positive rail (environment A only) is 
"common to all ports" (don't call it "ground"), and the more negative rail 
is switched. If we refer to that switched rail consistently as "isolated 
-48V", it would be even more clear.

Now, what to call the rail that the logic on the isolated side runs from? 
-44.7V? :-)


At 10:10 AM 1/13/2003 -0800, Geoff Thompson wrote:
>If you put a voltmeter between "hot" leads from 2 different PSEs,
>then is the max voltage between the leads (fault conditions included):
>         VportMax
>         VportMax - VportMin
>         Zero
>         2 X VportMax
>I contend that any answer above except [2 X VportMax] is OK
>We currently have no text to preclude  [2 X VportMax].