RE: [802.3af] Contradiction in Table 33-5
I separated my response from the original text, it was getting confusing
I think we will need a comment as, even if I agree that current limiting is
the "proper" behaviour, Table 33-5 item 1 requires that you maintain a
floor of 44 volts - its a hard requirement. I would find it hard to limit
output current if I can't vary the output voltage (if the load is
constant.) There is no exception for item 1 that allows you to drop the
output voltage to accomplish current limiting. Granted, I want to end this
thing and print, that is why I'm hitting the reflector first, but doing
nothing is not the right answer.
I don't understand the reason why any of the following:
1.1) PD current spikes
1.2) Ripple over 400mA
1.3) PSE voltage steps
require current limiting. PDs aren't allowed to exceed 400mA at any time
so a 451mA hard shut-off would allow margin for the leading edge of current
spikes and/or ripple on the allowed 400mA. I don't see why every PSE needs
to be support switching over to batteries as I don't remember mandating
backup power, allowing some current limiting might be nice as a mechanism
to facilitate PSEs which contain batteries, but you're burdening every PSE
for one special application.
33c.4 is in an informative annex, it carries no weight in the evaluation of
what is compliant and what is not.
2.1) Item 11 is the timing for what to do while the output current is in
the range described in time 10. If the current is outside the range in
item 10, item 11 is irrelevant.
2.2) Figure 33c.4 is in an informative annex and does not carry any weight
in determining what is actually required by the spec. Further, the test
cases do not describe drawing more than 400mA , nor for that matter does
the follow on test case ever say you try to exceed 450mA, but that is also
just informative. The drawing 33c.6 requires that you maintain at least
30V at Vport - how can you do that if it is a hard short while limiting the
current to less than 450mA?
If anything, the second behaviour, of going past 450mA and shutting off, is
the only thing that really meets the specification. I would interpret Item
10 and 11 to be the definition of a short circuit, any PSE observed power
draw in that range is defined to be in the "short circuit range". Once the
PD exceeds the ranges in the spec, the PSE is free to do as it chooses, and
it has faithfully maintained the output voltage requirement until the PD
drew it offside and beyond the spec. Current limiting a true short circuit
(not an excessively thirsty PD) will force the PSE to fall outside the
output voltage range and therefore violate the spec.
While current limiting the PD may be something we allow, to require the PSE
to force compliance on a PD that throws a crowbar across the line, with a
5% duty cycle of course, is a bit much for me.