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Re: unable to enumerate USB device



Jim wrote:
> Mikkel L. Ellertson wrote:
>> Jim wrote:
>>  
>>> To get a hub to work with all devices, hubs must have their own power
>>> source externally.
>>> Hubs that get there power source from the computer are the ones that
>>> give you problems, because of voltage drop across the cable to computer.
>>>
>>>     
>> Considering the amount of current we are talking about, I do not
>> think voltage drop is a problem. On the other hand, with a bus
>> powered hub, all the devices, plus the hub itself, are limited to
>> the max current of the port you are plugged into. A limit of
>> 500ma/port is common, but it could be less.
>>
>> Voltage drop = current x resistance
>> .5v = 500ma x 1 ohms.
>>
>> Mikkel
>>   
> I have had a number of bad experiences with bus powered hub, devices
> that would not work in hub I plugged directly to
> computer and they work fine, so i purchased external powered hubs and
> have had no more problems
> As far as the voltage drop goes it depends a lot on how the USB device
> was designed as to how much current it's going to draw. I'm quite
> familiar with OHMS Law.
> 
>  E
> ------
> IxR
> 
But the voltage drop, when you get one, is not caused by the cable.
There is not enough resistance in the cable to cause this. You run
into capacitance problems degrading the signal long before you get
enough resistance to matter.

On the other hand, there is a current limit in the USB driver
circuitry. If you try and exceed this, you may get voltage drop, or
the port may shut off. I remember a discussion a while back about
the USB drivers in Linux enforcing the port limits while Windows
didn't, so devices that would work in Windows wouldn't work in Linux
when plugged in the same way. Linux was shutting down the port. Each
USB device is supposed to report the maximum power requirements as
part of its ID response. (USB powered devices like lights and fans
don't report anything...)

The reason bus powered hubs don't work with a lot of devices is that
you have to share the bus power from the computer with all the
devices plugged in, as well as the hub itself. All it takes is one
high load device to max out the port the hub is plugged into. This
is why self powered hubs are preferred.

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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