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Re: usb-sticks not recognized

At 11:15 AM +0100 11/11/05, Joerg Mertins wrote:

Concerning a possible hardware problem with overcurrent: Does the apple machine protect itself aginst damages caused by plugging in a faulty device?

All powered ports have to have overcurrent protection to be spec compliant. The method of protection is not specified and the spec allows quite a wide range. The port can not overcurrent at less than the configured current (ie 500mA) but must overcurrent if the current draw reaches 5A.

Some machines overcurrent nearer the 500mA end of things, particularly portables where power is a constraint, and some machine overcurrent nearer the other end of the range, particularly on desktops where power is not so much of a constraint. The device is still out of spec and subject to overcurrent restrictions whenever it draws more than 500mA. So if it works on one machine, its not a problem (with the host) if it overcurrents on another.

Overcurrent tends to be implemented in one of 2 ways, either switching the port off, in which case the over current dialog pops up. Or current limiting, where the voltage drops to limit the current draw. The switching is good in that the user is warned of the problem, but bad in that there is no good way of recovering from this and the host usually needs to be rebooted for the port to recover. The current limiting is bad in that the user is not warned of the problem, the device will just act abnormally, but good in that it is self correcting as soon as the device is unplugged.

All Apple machines have overcurrent protection, the method and limit are not specified.
--
Barry Twycross
email@hidden
---
USB, it's not a Dyslexic BUS. (Thanks to TC.)
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References: 
 >Re: usb-sticks not recognized (From: Barry Twycross <email@hidden>)
 >Re: usb-sticks not recognized (From: Barry Twycross <email@hidden>)
 >Re: usb-sticks not recognized (From: Joerg Mertins <email@hidden>)



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