[kwlug-disc] WAS: USB hard drive mount / Electrical

john at netdirect.ca john at netdirect.ca
Thu Jan 21 09:27:04 EST 2010

kwlug-disc-bounces at kwlug.org wrote on 01/21/2010 09:03:44 AM:
> Glenn's question reminded me of one of my own. While USB drives are
> suppose to be plug and play, it seems to me a bad idea to be putting
> anything in the USB port that already has electricity going to it.
> It's almost like pulling memory out without unplugging the system
> unit, it may not appear to hurt the system, but the system still has
> electricity running through it, and damage could be done...

If you look at a USB plug (http://en.wikipedia.org/wiki/File:USB.svg), A 
and B ends are designed so that the power contacts come into contact first 
when plugging in and are removed last when unplugging. I think this is 
done to ensure that the data circuitry is completely disconnected before 
it loses power. This would prevent any stray current from flowing through 
the data circuits.

If, for example, the ground contact became unconnected momentarily before 
the other connectors, the power could flow back through the data ground 
and cause unwanted results.

John Van Ostrand
Net Direct Inc.
564 Weber St. N. Unit 12
Waterloo, ON N2L 5C6
john at netdirect.ca
Ph: 866-883-1172
Linux Solutions / IBM Hardware
Fx: 519-883-8533

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