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I never have enough USB plugs on my computer. I have 2 usb 2.0 plugs in the front, and 4 in the back. All of them are used. I heard i can buy something to add more usb slots, i am just concerened about putting too much power into a single usb outlet, as i have did this before.

asked Dec 13 '12 at 15:10

jdawglosurdo's gravatar image


With USB, the device tells the PC how much power it needs, and the motherboard will know how much power it has to work with on the USB port, if you request more than it can give, it will just give it's max power and give you an error message letting you know that the device is requesting more power than it can offer.

if you packet sniff a USB device handshake, you will see the computer send a 000b request

then you will commonly see a reply such as

CONTROL_TRANSFER    0000: 09 02 20 00 01 01 00 80 64 

In the above case, the device requested 200ma of power (due to the 64 value near the end meaning 0x64 which is 100 and USB uses 2ma units when specifying power usage

Regardless of the port amperage, the device will only pull as much as it needs but when the device announces it's power needs, the system will know how much how much the device truly needs and how much is left over.

So lets say you have a USB port that can only give 500ma and you plug in a external drive and it wants 450ma, then you connect a flash drive and that drive request 100ma, the system will give you an error as it knows that there is not enough power to go around.

If you run into that issue, the solution is a powered USB hub.

answered Dec 13 '12 at 16:32

Razor512's gravatar image


edited Dec 13 '12 at 16:53

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Asked: Dec 13 '12 at 15:10

Seen: 584 times

Last updated: Dec 13 '12 at 16:53