> > 3. (the conspiracy theory ;-) ) It may be a kind of serial interface
> > used to authenticate the original HP power supply to prevent using
> > other, cheaper adapters.
> 
> AFAIK it's actually #3, it's often used to confirm "authenticity",
> although in some cases it's used to determine how strong a supply it
is
> (on some Dells for example there are two supplies available, one is
> strong enough just to power the laptop, while the second can power the
> laptop and charge the battery, or something like that).

Yes the concentric one is #3 (see my first post on this thread). The two
sizes of supply are laptop only, and laptop + desk stand. However the
later models of laptop seem to need the higher power supply just for the
laptop.
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