On Wed, 23 Jun 2004 11:24:53 -0700, Andrew Warren wrote: > Peter L. Peres wrote: > > > > I have an external 2.5" drive housing .... it has two USB plugs > > > fitted, so you can draw an Amp. > > That arrangement, although not explicitly prohibited by the USB > spec, will not pass USB compliance testing. Maybe so, but it's how a lot of bus-powered external 2.5" disk drive housings, are made... I just dismantled the powered 4-port USB 2.0 hub that I have (the one that powered-down the external 2.5" drive connected to just one USB port, after a few minutes). It'a a RadioShack Cat.No.26-192, shaped like a 3" diameter flying saucer, and quite nicely made! Most of the work is done by a big NEC chip, but there are two little "AP1212L" chips, which I looked up: they are "Dual USB High-Side Power Switch" - designed specifically to supply USB ports with power according to the specs. Each port can be turned on and off by a logic signal, and they perform overcurrent, undervoltage, and thermal shutdown on their own, with a flag pin telling the controller they've done so. There's no mention of the 100mA pre-enumeration current, so perhaps they are generous with the spec, and supply 500mA all the time (except during fault conditions)? The fault current is limited to 1000mA, so it looks like you can abuse it below this level at the mercy of the thermal shutdown, which is presumably what my external drive did, hence the few-minutes' grace before it shut down. I would guess that once the "Fault" flag has siglalled the controller that there's a problem, the latter shuts down that port for the duration. That's what seemed to happen in my case, with only a power-cycle restoring it (several hours powered up but with nothing connected didn't reset it). Since each chip supplies two USB ports, presumably adjacent, I imagine it's better to use USB ports from each half, rather than an adjacent pair, if you're getting creative with drawing power from two ports. This would spread the thermal load (at least) and have more chance of working reliably. Still out of spec, of course! :-) Cheers, Howard Winter St.Albans, England -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics