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

Andrew Kohlsmith (Mailing List Account) aklists at mixdown.ca
Thu Jan 21 09:55:00 EST 2010

On January 21, 2010 09:03:44 am Charles M wrote:
> > So it's the order.  Turn on, plug in, no go.  Plug in, turn on, works.

> 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...

The case of plugging an already-powered USB device into an alread-powered 
computer is part of the USB specification.  Essentially you have two things to 
consider on the USB plug: the USB power (V+) coming from the computer and the 
D+/D- (differential signalling lines) that USB communicates over.

The USB specification states that any self-powered USB device must be able to 
handle the case of V+ suddenly appearing or disappearing. This is usually done 
with a pair of MOSFETs to electrically switch either self-power or bus power 
to the unit. (Some devices can work with either, and others must be self-
powered and do not accept the USB plug's power. Either is fine and within 

The USB specification also states that a self-powered device MUST NOT source 
or sink current on the D+/D- lines if V+ is not present.  That is, if the 
cable is not plugged in or the USB host is not putting power on the USB port, 
the device will maintain the D+/D- lines as high-impedance or essentially 

It's perfectly fine to plug in a powered USB device into the computer. If it's 
not working then the device is probably breaking spec, or is buggy.

It's also safe; USB also specifies that the USB host must current-limit its 
V+. I have, however, run into a few motherboards that will quite happily try 
to pass several Amps worth of current to a port that is supposed to, by 
specification, limit to about half an Amp at the most. :-)

While hotplug memory does exist, comparing unplugging normal system memory and 
unplugging USB devices when powered is not a very useful comparison at all. 
One is designed to be hotplug; the other is not.


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