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Do those usb hard drives work on a lot of pcs with power coming through USB
cable?
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"rb" <rbig[ at ]bellsouth.net> wrote in message
news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
[Quoted Text] > Do those usb hard drives work on a lot of pcs with power coming through
USB
> cable?
>
>
I'd avoid any 'self powered' USB drives
If you plan to purchase one, by all means get the type that has it's own
power supply.
Won't be difficult as that's how most of them are!
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"rb" <rbig[ at ]bellsouth.net> wrote in message
news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
[Quoted Text] > Do those usb hard drives work on a lot of pcs with power coming through
USB
> cable?
All hard drives require a DC power supply (not AC mains.)
Many USB external drives (all of large capacity) are sold
with their own DC converter. In theory any USB device
can be powered through the USB connector, but it is
imprudent to use this for a hard drive (as distinct from a
flash or chip memory unit.)
--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)
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Good responses. I see a ton of the 100-500gb types for sale that say they
run off their own power through USB ports.
With what you've told me, I should stick to ones with their own separate
power source.
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"rb" <rbig[ at ]bellsouth.net> wrote in message
news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
[Quoted Text] > Do those usb hard drives work on a lot of pcs with power coming through
> USB cable?
>
Usually, 3.5" USB drives come with power supplies, but 2.5"
drives run off the USB power. I have a WD My Passport
320GB drive that runs off USB with no problem using a
single USB connection, but I usually use a USB power
splitter that provides power from 2 USB ports. The drive
itself is a 320GB Seagate SATA Momentus 5200.5.
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"philo" <philo[ at ]privacy.net> wrote:
[Quoted Text] >I'd avoid any 'self powered' USB drives
Not sure why you don't like them, but they sure have worked for me.
Not having to drag the miserable wall wart/cord around is fantastic.
Course I'm only a sample of one, but my lil 80G drive has worked on
everything I've ever plugged it into and it even fits in my pocket...
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Ian D wrote:
[Quoted Text] > "rb" <rbig[ at ]bellsouth.net> wrote in message
> news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
>> Do those usb hard drives work on a lot of pcs with power coming through
>> USB cable?
>>
> Usually, 3.5" USB drives come with power supplies, but 2.5"
> drives run off the USB power. I have a WD My Passport
> 320GB drive that runs off USB with no problem using a
> single USB connection, but I usually use a USB power
> splitter that provides power from 2 USB ports. The drive
> itself is a 320GB Seagate SATA Momentus 5200.5.
>
>
Interesting you mention the splitter. I have a small coolmax case I
bought for my old laptop drive and it has a Y on the end of the cable
and 2 USB plugs. One Red and one Black. I don't have to plug them
both in on my laptop but my desktop requires me to plug them both in.
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"AJL" <nomail[ at ]fakeaddress.com> wrote in message
news:8057g4d4f2g0es6h17mpbuc0jdrrlkdjr9[ at ]4ax.com...
[Quoted Text] > "philo" <philo[ at ]privacy.net> wrote:
>
> >I'd avoid any 'self powered' USB drives
>
> Not sure why you don't like them, but they sure have worked for me.
> Not having to drag the miserable wall wart/cord around is fantastic.
> Course I'm only a sample of one, but my lil 80G drive has worked on
> everything I've ever plugged it into and it even fits in my pocket...
I say...
go with whatever works for you.
Just in general there would be less likelihood of problems
with a device that had it's own power source.
Especially if the machine has a lot of USB devices
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"philo" <philo[ at ]privacy.net> wrote:
[Quoted Text] >Just in general there would be less likelihood of problems
>with a device that had it's own power source.
Why would an external USB drive fed by a wall wart be any more
reliable than an external USB drive fed by the computer's internal
power supply? Any facts to back this up?
>Especially if the machine has a lot of USB devices
That shouldn't make any difference if the USB powered devices and the
USB ports that feed them both meet standard specifications.
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"AJL" <nomail[ at ]fakeaddress.com> wrote in message
news:slq7g4tagj95keb9k947a0qlm38uob59c6[ at ]4ax.com...
[Quoted Text] > "philo" <philo[ at ]privacy.net> wrote:
>
> >Just in general there would be less likelihood of problems
> >with a device that had it's own power source.
>
> Why would an external USB drive fed by a wall wart be any more
> reliable than an external USB drive fed by the computer's internal
> power supply? Any facts to back this up?
>
> >Especially if the machine has a lot of USB devices
>
> That shouldn't make any difference if the USB powered devices and the
> USB ports that feed them both meet standard specifications.
The machine's USB ports can only handle so much power...
True, if you only have a single USB device, the self-powered drives may be
fine...
but most of my "customers" have more than one USB device...
Just as an example: A photographer that I've built a machine for
has four external drives, a film scanner, a flatbed scanner and a printer.
He often has three external drives in use at once...
plus a scanner.
If all the drives were getting their power exclusively from the USB ports,
They would not be able to supply enough current.
I still recall the day I set him up with his first external drive...
we did not figure he'd be adding drives later...
but he soon filled it up.
Though it's true, the self-powered drive may have worked out fine for you...
I prefer to err on the side of caution.
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[Quoted Text] > "rb" <rbig[ at ]bellsouth.net> wrote in message
> news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
>
>> Do those usb hard drives work on a lot of pcs with power coming
>> through USB cable?
>
> All hard drives require a DC power supply (not AC mains.)
> Many USB external drives (all of large capacity) are sold
> with their own DC converter. In theory any USB device
> can be powered through the USB connector, but it is
> imprudent to use this for a hard drive (as distinct from a
> flash or chip memory unit.)
The USB 2.0 specification can be found at the link. For reference, a
link to the USB 1.1 specification can be found at the bottom of the
referenced page. Besides a data link, USB provides 5 +/- 0.25 volts to
power an otherwise unpowered device or charge a battery in a self
powered device.
A USB controller or hub is required to power one unit load (100 mA, a
low power load).
It can optionally power up to 5 unit loads (500 mA, a high power load).
You take those numbers and compare them to the requirements of each hard
drive you want to power. 500 mA (1/2 Amp) isn't a lot of power and
isn't going to power very many hard drives, plus if you approach the 500
mA limit and a burst throws the limit above that, the power will be
removed until the load returns to be less than 500 mA and ... well, you
have trashed data on that drive now, possibly a completly corrupted
drive.
If one hard drive is the ONLY USB equipment used, then it may work
fine. Just may, not will. Then, as you start adding keyboards and all
the rest of the possibilities, you can easily begin to approach the
limit of the USB power possibilities.
IMO hard drives on USB power are bad and shouldn't be used; data is
too fragile as it is.
Twayne
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[Quoted Text] > "rb" <rbig[ at ]bellsouth.net> wrote in message
> news:%23DEZ0glNJHA.588[ at ]TK2MSFTNGP06.phx.gbl...
>> Do those usb hard drives work on a lot of pcs with power coming
>> through USB cable?
>>
> Usually, 3.5" USB drives come with power supplies, but 2.5"
> drives run off the USB power. I have a WD My Passport
> 320GB drive that runs off USB with no problem using a
> single USB connection, but I usually use a USB power
> splitter that provides power from 2 USB ports. The drive
> itself is a 320GB Seagate SATA Momentus 5200.5.
Caveat:
Unless that "splitter" is taking power from two different USB
controllers, you're wasting time & effort. Each controller can put out
500 mA, period, whether it goes to one or 4 USB connectors; that is the
limit - period. You do not improve anything by taking power from two
USB connectors off the same controller. It all comes from the same pool
of 500 mS available..
So, you don't end up with 1,000 mA: The limit is still 500 mA.
Plus, to do so requires at least diode isolation between the two so
they can't back feed each other, which in turn lowers the voltage by the
diode conduction voltage, which in most cases is going to put the 5V
below spec even for running a 3V DC converter.
That's for USB2 specs: 1 specs were even less robust.
Fortunately you seldom see USB powered hard drive anymore and that's
good. A hard drive is NOT the place ot take chances with data,
especially if it's valuable data.
Also, USB powered means the devices are equally at risk of
surge/lightning damage applied to the computer, most often via telephone
connections. The bricks dont' have that phone connection and often are
self-limiting to a degree in protection; fast surges don't to through
transformers well. They do switching supplies, however, which leans
toward the more bulky, larger bricks.
It's a pita but IMO it's best to avoid USB powered storage devices like
hard drives that are more or less permanently connected. In my case my
two 500 Gig WD drives are only connected to power & the PC for backups:
They're on a demand-controller switch. Another waste most likely, but
.... so far I'm sucked into liking it.
Twayne
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"philo" <philo[ at ]privacy.net> wrote:
[Quoted Text] >The machine's USB ports can only handle so much power...
If the ports are configured to the standard specs *and* the device is
configured to the standard specs, the computer will handle one device
per port with no problems. Most all of the off the shelf products
comply with the standards.
>Just as an example: A photographer that I've built a machine for
>has four external drives, a film scanner, a flatbed scanner and a printer.
If his computers design has 7 standard spec ports and these devices
meet the USB specs, this would work fine.
>If all the drives were getting their power exclusively from the USB ports,
>They would not be able to supply enough current.
Again, if correctly designed, the computer supply would handle the
drive current just fine. If not, or if there is not enough ports on
the machine, a powered hub would be a good way to go. That way the USB
powered drives can still be easily unplugged and carried to the home
computer for more use if needed.
>I prefer to err on the side of caution.
Caution is fine if there are facts back it up. Otherwise it will just
shackle your customer for no reason...
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"AJL" <nomail[ at ]fakeaddress.com> wrote in message
news:o5a9g4hcekvgm6edanv374sq2qe9e65es5[ at ]4ax.com...
[Quoted Text] > "philo" <philo[ at ]privacy.net> wrote:
>
> >The machine's USB ports can only handle so much power...
>
> If the ports are configured to the standard specs *and* the device is
> configured to the standard specs, the computer will handle one device
> per port with no problems. Most all of the off the shelf products
> comply with the standards.
>
> >Just as an example: A photographer that I've built a machine for
> >has four external drives, a film scanner, a flatbed scanner and a
printer.
>
> If his computers design has 7 standard spec ports and these devices
> meet the USB specs, this would work fine.
>
> >If all the drives were getting their power exclusively from the USB
ports,
> >They would not be able to supply enough current.
>
> Again, if correctly designed, the computer supply would handle the
> drive current just fine. If not, or if there is not enough ports on
> the machine, a powered hub would be a good way to go. That way the USB
> powered drives can still be easily unplugged and carried to the home
> computer for more use if needed.
In theory that might be true
but I live in the real world
> >I prefer to err on the side of caution.
>
> Caution is fine if there are facts back it up. Otherwise it will just
> shackle your customer for no reason...
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[Quoted Text] > "philo" <philo[ at ]privacy.net> wrote:
>
>> The machine's USB ports can only handle so much power...
>
> If the ports are configured to the standard specs *and* the device is
> configured to the standard specs, the computer will handle one device
> per port with no problems. Most all of the off the shelf products
> comply with the standards.
>
>> Just as an example: A photographer that I've built a machine for
>> has four external drives, a film scanner, a flatbed scanner and a
>> printer.
>
> If his computers design has 7 standard spec ports and these devices
> meet the USB specs, this would work fine.
Since the specs are a total of 500 mA, as long as the 7 "standard",
whatever that means, devices require less than 500 mA total, then you're
correct. However, if you have two hard drives that want to suck 250 mA
each, then that's your limit; not 7.
Since a controller is specced to serve 4 USB ports, that's how many
devices will draw power from the controller. Up to the limit of 500 mA
after which the supply voltage begins to fall and eventually will shut
off rather then allow damage to occur.
>
>> If all the drives were getting their power exclusively from the USB
>> ports, They would not be able to supply enough current.
>
> Again, if correctly designed, the computer supply would handle the
> drive current just fine. If not, or if there is not enough ports on
> the machine, a powered hub would be a good way to go. That way the USB
> powered drives can still be easily unplugged and carried to the home
> computer for more use if needed.
I don't know what your'e about with this "correctly designed", but it's
making no sense. EVER device has a power requirement of xxx mA at xx
volts. Exceed those specs, and the device will no longer receive power,
along with any others attached to the same controller.
>
>> I prefer to err on the side of caution.
>
> Caution is fine if there are facts back it up. Otherwise it will just
> shackle your customer for no reason...
Facts! Man, you are a good one to talk about specs! All that verbosity
and not a specification anywhere, but lots of vague, uneducated talk
about "standard" this and that stuff. If you're capable of any reading
comprehension at all, the USB specs are easy to find, easy to read, and
easy to understand. Go check them out and put some actual facts behind
your vagueries and nonsense here. Whatever your point may be, you
failed at making it. In fact, it appears the whole thing is a guess
and/or a rationalization on your part with a couple of buzz words thrown
in for good measure.
Get some facts and you'll go far. Ignore then and you go nowhere. It's
not even close to rocket science, which as a matter of fact is not that
complex either; it just sounds good so a lot of people like to say it.
Cheers,
Twayne
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"philo" <philo[ at ]privacy.net> wrote:
[Quoted Text] >In theory that might be true
I'm glad we finally agree.
>but I live in the real world
Don't we all... ;)
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"AJL" <nomail[ at ]fakeaddress.com> wrote in message
news:3fm9g4tushgmvkkonkpe3oh8c7s6n5ak96[ at ]4ax.com...
[Quoted Text] > "philo" <philo[ at ]privacy.net> wrote:
>
> >In theory that might be true
>
> I'm glad we finally agree.
>
> >but I live in the real world
>
> Don't we all... ;)
Well with the current world situation
the world we live in is beginning to seem a bit unreal actually !!!! <G>
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"Twayne" <nobody[ at ]devnull.spamcop.net> wrote:
[Quoted Text] >Since the specs are a total of 500 mA,
The USB low power bus mode specifies a maximum of 100 ma per port.
>However, if you have two hard drives that want to suck 250 mA...
USB powered drives stay within the 100 ma per port requirement. If
they draw more than the 100 ma limit they supply a two port cable and
suck power from 2 ports.
>EVER device has a power requirement of xxx mA at xx
>volts. Exceed those specs, and the device will no longer receive power,
>along with any others attached to the same controller.
A correctly designed computer will be able to supply 100 ma to each
USB port. A correctly designed USB powered drive will draw no more
than 100 ma from each USB port. No problems no matter how many ports
involved. Not a hard concept to understand.
> It's not even close to rocket science,
I would agree.
You boys can have the last word...
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[Quoted Text] > "Twayne" <nobody[ at ]devnull.spamcop.net> wrote:
>
>> Since the specs are a total of 500 mA,
>
> The USB low power bus mode specifies a maximum of 100 ma per port.
>
>> However, if you have two hard drives that want to suck 250 mA...
>
> USB powered drives stay within the 100 ma per port requirement. If
> they draw more than the 100 ma limit they supply a two port cable and
> suck power from 2 ports.
>
>> EVER device has a power requirement of xxx mA at xx
>> volts. Exceed those specs, and the device will no longer receive
>> power, along with any others attached to the same controller.
>
> A correctly designed computer will be able to supply 100 ma to each
> USB port.
or 500 mA to one port.
A correctly designed USB powered drive will draw no more
> than 100 ma from each USB port.
No such thing as a "correctly designed" ...
No problems no matter how many ports
> involved. Not a hard concept to understand.
A controller can only source 500 mA total, period. That's IT. Put a
meter on it and a resistor, and do some measurements; you'll see it for
yourself. Two connectors to ports on the same controller will do no
good, nada, nothing, zero, etc..
>
>> It's not even close to rocket science,
>
> I would agree.
>
> You boys can have the last word...
Good thing, because: You need to read some specs I'm afraid. 100 mA *
4 # 500, BTW.
Nuff said though; this has gotten boring.
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[Quoted Text] > "Twayne" <nobody[ at ]devnull.spamcop.net> wrote:
>
>> Since the specs are a total of 500 mA,
>
> The USB low power bus mode specifies a maximum of 100 ma per port.
>
>> However, if you have two hard drives that want to suck 250 mA...
>
> USB powered drives stay within the 100 ma per port requirement. If
> they draw more than the 100 ma limit they supply a two port cable and
> suck power from 2 ports.
>
>> EVER device has a power requirement of xxx mA at xx
>> volts. Exceed those specs, and the device will no longer receive
>> power, along with any others attached to the same controller.
>
> A correctly designed computer will be able to supply 100 ma to each
> USB port. A correctly designed USB powered drive will draw no more
> than 100 ma from each USB port. No problems no matter how many ports
> involved. Not a hard concept to understand.
>
>> It's not even close to rocket science,
>
> I would agree.
>
> You boys can have the last word...
OK, here you go, since you're too lazy to look for yourself. It's
simple if you can just bother to READ and comprehend, not just the parts
you like, but ALL of the specs.
....
A USB device specifies its power consumption expressed in 2mA units in
the configuration descriptor which we will examine in detail later. A
device cannot increase its power consumption, greater than what it
specifies during enumeration, even if it looses external power. There
are three classes of USB functions,
a.. *Low-power* bus powered functions aka 100 mA
b.. *High-power* bus powered functions aka 500 mA
c.. *Self-powered* functions
*Low power* bus powered functions draw all its power from the VBUS and
*cannot draw any more than one unit load.* The USB specification defines
*a unit load as 100mA. Low power* bus powered functions must also be
designed to work down to a VBUS voltage of 4.40V and up to a maximum
voltage of 5.25V measured at the upsteam plug of the device. For many
3.3V devices, LDO regulators are mandatory.
*High power* bus powered functions will draw *all* its power from *the
bus* and cannot draw more than one unit load *until it has been
configured*, after which *it can then drain 5 unit loads (500mA Max)*
provided it asked for this in its descriptor. High power bus functions
*must* be able to be detected and enumerated at a minimum 4.40V. When
operating at a full unit load, a minimum VBUS of 4.75 V is specified
with a maximum of 5.25V. Once again, these measurements are taken at the
upstream plug.
Self power functions may draw up to 1 unit load from the bus and derive
the rest of it's power from an external source. Should this external
source fail, it must have provisions in place to draw no more than 1
unit load from the bus. Self powered functions are easier to design to
specification as there is not so much of an issue with power
consumption. The 1 unit bus powered load allows the detection and
enumeration of devices without mains/secondary power applied.
No USB device, whether bus powered or self powered can drive the VBUS on
its upstream facing port. If VBUS is lost, the device has a lengthy 10
seconds to remove power from the D+/D- pull-up resistors used for speed
identification.
Other VBUS considerations are the Inrush current which must be limited.
This is outlined in the USB specification paragraph 7.2.4.1 and is
commonly overlooked. Inrush current is contributed to the amount of
capacitance on your device between VBUS and ground. The spec therefore
specifies that the maximum decoupling capacitance you can have on your
device is 10uF. When you disconnect the device after current is flowing
through the inductive USB cable, a large flyback voltage can occur on
the open end of the cable. To prevent this, a 1uF minimum VBUS
decoupling capacitance is specified.
For the typical bus powered device, *it can not drain any more than
500mA* which is *not* unreasonable. So what is the complication you ask?
Perhaps Suspend Mode?
....
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"Twayne" <nobody[ at ]devnull.spamcop.net> wrote in message
news:e%23$ano9NJHA.2912[ at ]TK2MSFTNGP03.phx.gbl...
[Quoted Text] >> "Twayne" <nobody[ at ]devnull.spamcop.net> wrote:
>>
>>> Since the specs are a total of 500 mA,
>>
>> The USB low power bus mode specifies a maximum of 100 ma per port.
>>
>>> However, if you have two hard drives that want to suck 250 mA...
>>
>> USB powered drives stay within the 100 ma per port requirement. If
>> they draw more than the 100 ma limit they supply a two port cable and
>> suck power from 2 ports.
>>
>>> EVER device has a power requirement of xxx mA at xx
>>> volts. Exceed those specs, and the device will no longer receive
>>> power, along with any others attached to the same controller.
>>
>> A correctly designed computer will be able to supply 100 ma to each
>> USB port. A correctly designed USB powered drive will draw no more
>> than 100 ma from each USB port. No problems no matter how many ports
>> involved. Not a hard concept to understand.
>>
>>> It's not even close to rocket science,
>>
>> I would agree.
>>
>> You boys can have the last word...
>
> OK, here you go, since you're too lazy to look for yourself. It's simple
> if you can just bother to READ and comprehend, not just the parts you
> like, but ALL of the specs.
>
> ...
>
> A USB device specifies its power consumption expressed in 2mA units in the
> configuration descriptor which we will examine in detail later. A device
> cannot increase its power consumption, greater than what it specifies
> during enumeration, even if it looses external power. There are three
> classes of USB functions,
>
> a.. *Low-power* bus powered functions aka 100 mA
> b.. *High-power* bus powered functions aka 500 mA
> c.. *Self-powered* functions
> *Low power* bus powered functions draw all its power from the VBUS and
> *cannot draw any more than one unit load.* The USB specification defines
> *a unit load as 100mA. Low power* bus powered functions must also be
> designed to work down to a VBUS voltage of 4.40V and up to a maximum
> voltage of 5.25V measured at the upsteam plug of the device. For many 3.3V
> devices, LDO regulators are mandatory.
>
> *High power* bus powered functions will draw *all* its power from *the
> bus* and cannot draw more than one unit load *until it has been
> configured*, after which *it can then drain 5 unit loads (500mA Max)*
> provided it asked for this in its descriptor. High power bus functions
> *must* be able to be detected and enumerated at a minimum 4.40V. When
> operating at a full unit load, a minimum VBUS of 4.75 V is specified with
> a maximum of 5.25V. Once again, these measurements are taken at the
> upstream plug.
>
> Self power functions may draw up to 1 unit load from the bus and derive
> the rest of it's power from an external source. Should this external
> source fail, it must have provisions in place to draw no more than 1 unit
> load from the bus. Self powered functions are easier to design to
> specification as there is not so much of an issue with power consumption.
> The 1 unit bus powered load allows the detection and enumeration of
> devices without mains/secondary power applied.
>
> No USB device, whether bus powered or self powered can drive the VBUS on
> its upstream facing port. If VBUS is lost, the device has a lengthy 10
> seconds to remove power from the D+/D- pull-up resistors used for speed
> identification.
>
> Other VBUS considerations are the Inrush current which must be limited.
> This is outlined in the USB specification paragraph 7.2.4.1 and is
> commonly overlooked. Inrush current is contributed to the amount of
> capacitance on your device between VBUS and ground. The spec therefore
> specifies that the maximum decoupling capacitance you can have on your
> device is 10uF. When you disconnect the device after current is flowing
> through the inductive USB cable, a large flyback voltage can occur on the
> open end of the cable. To prevent this, a 1uF minimum VBUS decoupling
> capacitance is specified.
>
> For the typical bus powered device, *it can not drain any more than 500mA*
> which is *not* unreasonable. So what is the complication you ask? Perhaps
> Suspend Mode?
>
> ...
>
What about laptops? My HP laptop will provide up to at least 1000 mA
from it's own internal power. I did some measurements. Up to 500 mA
the USB hub provides the power according to Device Manager. Go
over 500 mA, and the USB hub current drops to a residual 10 mA, and
the USB port still provides the power, obviously on a switch over to
power directly from the laptop 5V bus.
I have a 2.5" HD in a Vantec external case, which came with an extra USB
power connector on the USB cable. On my laptop, I don't need the extra
connector. If I connect it, it draws no current from that USB hub. On the
other hand, if I connect the USB HD to the front ports of my Antec case,
that are connected to an Asus P5B Dlx MB, with the single connection,
the drive won't power up until I plug in the extra USB connector. Then it
powers up and runs normally, so between the two case front USB ports
over 500 mA is available.
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[Quoted Text] > "Twayne" <nobody[ at ]devnull.spamcop.net> wrote in message
> news:e%23$ano9NJHA.2912[ at ]TK2MSFTNGP03.phx.gbl...
>>> "Twayne" <nobody[ at ]devnull.spamcop.net> wrote:
>>>
>>>> Since the specs are a total of 500 mA,
>>>
>>> The USB low power bus mode specifies a maximum of 100 ma per port.
>>>
>>>> However, if you have two hard drives that want to suck 250 mA...
>>>
>>> USB powered drives stay within the 100 ma per port requirement. If
>>> they draw more than the 100 ma limit they supply a two port cable
>>> and suck power from 2 ports.
>>>
>>>> EVER device has a power requirement of xxx mA at xx
>>>> volts. Exceed those specs, and the device will no longer receive
>>>> power, along with any others attached to the same controller.
>>>
>>> A correctly designed computer will be able to supply 100 ma to each
>>> USB port. A correctly designed USB powered drive will draw no more
>>> than 100 ma from each USB port. No problems no matter how many ports
>>> involved. Not a hard concept to understand.
>>>
>>>> It's not even close to rocket science,
>>>
>>> I would agree.
>>>
>>> You boys can have the last word...
>>
>> OK, here you go, since you're too lazy to look for yourself. It's
>> simple if you can just bother to READ and comprehend, not just the
>> parts you like, but ALL of the specs.
>>
>> ...
>>
>> A USB device specifies its power consumption expressed in 2mA units
>> in the configuration descriptor which we will examine in detail
>> later. A device cannot increase its power consumption, greater than
>> what it specifies during enumeration, even if it looses external
>> power. There are three classes of USB functions,
>>
>> a.. *Low-power* bus powered functions aka 100 mA
>> b.. *High-power* bus powered functions aka 500 mA
>> c.. *Self-powered* functions
>> *Low power* bus powered functions draw all its power from the VBUS
>> and *cannot draw any more than one unit load.* The USB specification
>> defines *a unit load as 100mA. Low power* bus powered functions must
>> also be
>> designed to work down to a VBUS voltage of 4.40V and up to a maximum
>> voltage of 5.25V measured at the upsteam plug of the device. For
>> many 3.3V devices, LDO regulators are mandatory.
>>
>> *High power* bus powered functions will draw *all* its power from
>> *the bus* and cannot draw more than one unit load *until it has been
>> configured*, after which *it can then drain 5 unit loads (500mA Max)*
>> provided it asked for this in its descriptor. High power bus
>> functions *must* be able to be detected and enumerated at a minimum
>> 4.40V. When operating at a full unit load, a minimum VBUS of 4.75 V
>> is specified with a maximum of 5.25V. Once again, these measurements
>> are taken at the upstream plug.
>>
>> Self power functions may draw up to 1 unit load from the bus and
>> derive the rest of it's power from an external source. Should this
>> external source fail, it must have provisions in place to draw no
>> more than 1 unit load from the bus. Self powered functions are
>> easier to design to specification as there is not so much of an
>> issue with power consumption. The 1 unit bus powered load allows the
>> detection and enumeration of devices without mains/secondary power
>> applied. No USB device, whether bus powered or self powered can drive
>> the
>> VBUS on its upstream facing port. If VBUS is lost, the device has a
>> lengthy 10 seconds to remove power from the D+/D- pull-up resistors
>> used for speed identification.
>>
>> Other VBUS considerations are the Inrush current which must be
>> limited. This is outlined in the USB specification paragraph 7.2.4.1
>> and is commonly overlooked. Inrush current is contributed to the
>> amount of capacitance on your device between VBUS and ground. The
>> spec therefore specifies that the maximum decoupling capacitance you
>> can have on your device is 10uF. When you disconnect the device
>> after current is flowing through the inductive USB cable, a large
>> flyback voltage can occur on the open end of the cable. To prevent
>> this, a 1uF minimum VBUS decoupling capacitance is specified.
>>
>> For the typical bus powered device, *it can not drain any more than
>> 500mA* which is *not* unreasonable. So what is the complication you
>> ask? Perhaps Suspend Mode?
>>
>> ...
>>
> What about laptops? My HP laptop will provide up to at least 1000 mA
> from it's own internal power. I did some measurements. Up to 500 mA
> the USB hub provides the power according to Device Manager. Go
> over 500 mA, and the USB hub current drops to a residual 10 mA, and
> the USB port still provides the power, obviously on a switch over to
> power directly from the laptop 5V bus.
>
> I have a 2.5" HD in a Vantec external case, which came with an extra
> USB power connector on the USB cable. On my laptop, I don't need the
> extra connector. If I connect it, it draws no current from that USB
> hub. On the other hand, if I connect the USB HD to the front ports
> of my Antec case, that are connected to an Asus P5B Dlx MB, with the
> single connection, the drive won't power up until I plug in the extra
> USB connector. Then it powers up and runs normally, so between the two
> case front
> USB ports over 500 mA is available.
No doubt you're right, and thanks for the information. Sounds like they
are monitoring what hte controllers are doing and jumping in with an
assist ckt when it's needed. Sounds like a good design, actually, and
I'd imagine it's catching on to more and more computers.
In that light, I should probably make the USB controller "specs" more
prominent. I wonder if that's typical of only front ports and why not
all of them? Mmm, maybe to keep from overtaxing the internal PSU cktry.
Dunno. Maybe I'll take a look at the USB3 specs & see if they address
that.
Thanks!
Twayne
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