Re: [802.3af] 45Kmin instead of 70Kmin ?
Yair, John -
As I recall, the 70k number was intended to prevent two PSEs from detecting
each other if they were erroneously connected together. Rick showed a
presentation in St. Louis (if I remember right) that showed a possibility
of detection if the source impedance was less than 60-some K. At the time I
ran his numbers and agreed with them, but I'd have to look back at my notes
to see how we got to to that number.
We do have text that allows any source impedance if there is a diode in
series (so it's always 1st quadrant). 45K works fine this way.
I'd oppose this change, at least until I get a chance to look back at
At 01:54 PM 1/8/2002 +0200, Yair Darshan wrote:
>We specified that the high impedance option for the detection circuit
>requires 70K min.
>We also specified that the voltage across the port is 2.8V to 10V.
>Now, if we want to get the 10V when we have the following conditions: 70K
>series resistance, Rsig=23.75K (25k - 5%)
>the probe source voltage need to be 10*(70k+23.75k)/23,75k= 10V*3.94=39.4V.
>We have limited the open circuit voltage to 30V max.
>So to get the 10V max we need to reduce the 70k min value to (30V*23.75K -
>10V*23.75K)/10V = 47.5K.
>Having 5% margin we get: 45K min.
>In case of PSE to PSE connection the worst case power dissipation over this
>resistor would be: 57^2/45k=72mW which is <125mW.
>Thus to keep all numbers consistent with the intentions I suggest to change
>from 70K min to 45K min.
>Do you see problems with this change?
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