[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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