# Re: [802.3af] voltage offset proposal (was 1-point detection,monotonicity)

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Roger,

The 500 ua was chosen to allow the PD to have some amount of
current offset.  Here is a way to visualize the slope requirement
of the PD:  In the V-I space we define a rectangular area
where the sides are
top:     v = 10.0 volts
bottom:  v = 2.8 volts
left:    I = 0.0 ma
right:   I = 0.5 ma
The requirement on the PD is to have a V-I characteristic that
enters the space through the bottom and leaves through the top,
and inside the rectangle the characteristic must have the required
constant slope.

In order to achieve this characteristic, the PD could theoretically
have a pure voltage offset of up to 2.8 volts.  However, once the
characteristic enters the space it must have the constant slope.

Similarly, the PD could theoretically have a pure current offset
of up to 0.123 ma.

The above is the requirement on the PD.  It does not matter if the
PD has an offset, and it does not matter how the PD achieves an
offset.  It only matters that the slope in the area has the
required constant value.

The requirement on the PSE is to look within the space for the
characteristic.  The PSE is told what the must-detect slopes are,
and the PSE is warned that there might be an offset.

Therefore, the must-detect characteristics for the PSE are:

a) 19K to 26.5K ohm resistance between pairs,
b) no more than 110 nF of capacitance,
c) a voltage offset of at most 2.8 volts DC in the signature,
c') a current offset of at most 0.123 ma in the signature.

John

R karam wrote:

> Hi John
>
> I think the 10.1v Max conflicts with a 500ua max out of the PSE
> during detection for 500ua* max Rsig=26.5k (if i got this right)
> that puts is at 13.25v max.  do you know where the 500ua spec
> came from?
>
> also are we happy with the 2.8v being one VBE (diode drop away)
> say at cold we allow 0.8v per diode that is 1.6v is this too tight?
> thank you for taking the time to do this - there is no shortage of
> confusion on the numbers...
>
> regards
> roger
>
> >I have a proposal for the signature voltage offset requirement
> >as seen by the PSE.  However, before I generate an official
> >comment to the editor, I am sending it to the reflector
> >for comment.  I believe the proposal below is in
> >the spirit of our agreements, and it also eliminates
> >the one-point detection "loophole" in our former discussion.
> >
> >The Proposal (in two parts):
> >Part 1:  Change the valid detection signature maximum
> >voltage offset that the PSE might see to 2.8 volts,
> >instead of 1.5 volts (see p.8, l.11).
> >Part 2:  Change the valid detection signature
> >voltage range for the PD V-I characteristic
> >to 2.7 - 10.1 volts, instead of 2.8 - 10 (see p.16, l.22).
> >
> >Rationale for Part 1: the 2.8 volt offset:
> >Several meetings ago we agreed that the detection
> >test voltage would be 2.8 to 10 volts.
> >The 2.8 number was selected to allow for
> >voltage offsets.
> >
> >Where did the 1.5 number come from?
> >I think this is what happened.
> >The PSE Detection Source Material gave its "must pass"
> >requirements in the form of circuits that could include
> >the "typical" two diodes.  D1.2 put this into words.
> >
> >Rationale for Part 2: the 2.7 - 10.1 PD voltage range:
> >This leaves a 100mV band for qualification testing,
> >and ensures that a slightly-out-of-spec PSE
> >will still detect a slightly-out-of-spec PD.
> >(Thanks, Dave D.)
> >
> >John Jetzt
> >

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