Using a USB thumb drive for backups?
in [Mac Systems]
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From: Jolly Roger on 20 Dec 2009 16:07 In article <1jb0x71.1rjb9s71l2di6mN%nospam(a)see.signature>, nospam(a)see.signature (Richard Maine) wrote: > nospam <nospam(a)nospam.invalid> wrote: > ... > > the usb spec requires that the host supply 500ma. if it doesn't, it's > > not compliant. it's that simple. > ... > > apple says their computers are usb compliant, and in order to do that, > > they must meet the requirements of the usb spec. one of those is 500ma > > output on a port. i'm quite certain that apple has measured it. > > Are you sure that Apple says without qualification that all their > computers are USB compliant? That sounds like speculation to me - and > speculation that doesn't match my recollections. Let's see... Well, it > isn't directly from Apple, and I'm too lazy to go chase down a > first-hand citation, but lowendmac is usually pretty reliable (certainly > far more so than random posters on usenet) and I'd suspect they would > not just invent the following. > > See > > <http://www.lowendmac.com/macbookpro/15in-macbook-pro-june-2007.html> > > which is a profile of June 2007 15" MacBook Pro models. Note in > particular the paragraph: > > "Unlike earlier models, where every USB port could provide 500 mA of > power, only a single high-powered device can be attached to the USB > ports, and software will enable one of its downstream ports to supply > 500 mA of power. If a second high-powered device is attached, it will > behave like a normal bus-powered hub and only provide 100 mA per > downstream port." I also doubt that the 500ma output of USB ports on a laptop is always a continuous stream without fluctuation. -- Send responses to the relevant news group rather than email to me. E-mail sent to this address may be devoured by my very hungry SPAM filter. Due to Google's refusal to prevent spammers from posting messages through their servers, I often ignore posts from Google Groups. Use a real news client if you want me to see your posts. JR
From: Richard Maine on 20 Dec 2009 16:11 JF Mezei <jfmezei.spamnot(a)vaxination.ca> wrote: > nospam wrote: > > > i didn't say there weren't. however, some new drive mechanisms use less > > power than older ones. > > A 600 meg disk in the mid 1980s required a 10amp 115VAC circuit for each > drive, and used a 1/3HP motor to turn the disk. And weighed about 175 pounds and cost about $30,000 - at least one model I recall did. I think those were the right numbers. I distinctly recall helping our hardware guy remove one from its rack. I wasn't very smart about how I positioned my hold on it. Once we were holding the full weight, it seemed like the thin metal rail that I was holding it by was cutting through my fingers. I had this horrified vision of dropping and breaking a $30k piece of government equipment. I managed to avoid that, but I still recall the pain. -- Richard Maine | Good judgment comes from experience; email: last name at domain . net | experience comes from bad judgment. domain: summertriangle | -- Mark Twain
From: nospam on 20 Dec 2009 16:18 In article <jollyroger-8820CD.14064120122009(a)news.individual.net>, Jolly Roger <jollyroger(a)pobox.com> wrote: > > the usb spec requires that the host supply 500ma. if it doesn't, it's > > not compliant. it's that simple. > > Yes, and it seems neither you nor I are in a position to say whether or > not all USB ports on every Mac are compliant to that requirement all of > the time. if they want to use the logo and call it a usb 2 device, they're required to meet certain standards. <http://www.usb.org/developers/compliance/> > > you haven't measured anything, yet you are so sure it fluctuates?? > > Neither have you. Are you sure it doesn't? > > Common sense tells me if the battery power level in a laptop happens to > fluctuate, so will the power supplied to USB ports. Are you saying that > cannot ever happen? there's a voltage regulator for changes in the battery voltage, especially since the voltage will drop as it discharges and rise when being charged. nothing is perfect, so nobody can guarantee that something unusual won't ever happen, but in normal use, there's 5v @ 500ma available on the usb ports. that's very little compared to what the rest of the laptop consumes so there should rarely be any problem in providing it. > > apple says their computers are usb compliant, and in order to do that, > > they must meet the requirements of the usb spec. one of those is 500ma > > output on a port. i'm quite certain that apple has measured it. > > Are you certain they have measured it through all possible usage states, > not just at idle? it's a requirement to comply with the spec. here's the checklist: <http://www.usb.org/developers/compliance/check_list/compchksys080205.pd f> as i said before, laptop hard drives will run on 500ma or less for everything except spin-up. in other words, once it's spun up, there should not be any problems due to power issues.
From: nospam on 20 Dec 2009 16:26 In article <1jb0x71.1rjb9s71l2di6mN%nospam(a)see.signature>, Richard Maine <nospam(a)see.signature> wrote: > > apple says their computers are usb compliant, and in order to do that, > > they must meet the requirements of the usb spec. one of those is 500ma > > output on a port. i'm quite certain that apple has measured it. > > Are you sure that Apple says without qualification that all their > computers are USB compliant? if the logo is on the box, they must meet the spec. see links in my other post. > which is a profile of June 2007 15" MacBook Pro models. Note in > particular the paragraph: > > "Unlike earlier models, where every USB port could provide 500 mA of > power, only a single high-powered device can be attached to the USB > ports, and software will enable one of its downstream ports to supply > 500 mA of power. If a second high-powered device is attached, it will > behave like a normal bus-powered hub and only provide 100 mA per > downstream port." that's actually from apple's own documentation, nor does it say anything about voltage fluctuations. usb starts off at 100ma for a device, and if the device needs more power it must request 500ma. that request may be denied. a good example is a bus-powered hub, where you have 500ma (from the host) for *all* of its ports.
From: nospam on 20 Dec 2009 16:26
In article <jollyroger-27E138.15074920122009(a)news.individual.net>, Jolly Roger <jollyroger(a)pobox.com> wrote: > I also doubt that the 500ma output of USB ports on a laptop is always a > continuous stream without fluctuation. it's required. read the usb spec. |