Thread: just pulled the trigger on a pocket pro
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Old 12-02-2009, 07:51 PM   #65
jusmee
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Join Date: Oct 2009
Device: Libre 2
Quote:
Originally Posted by Dave_S View Post
It seems that you just described a Catch-22 situation for devices that are solely powered by USB, like keyboards and mice? While negotiation with the computer for power requirements is possible, with no negotiation the device will receive the default full power of 500 ma. whether it actually draws that much or not. That is how USB powered lights and fans receive power from a USB port with no microcontroller involved at all.
The are 4 wires. Two for power, 2 for data. It is perfectly possible to use the power wires without any computer connected to the data wires. That's how/why the plugpack can charge the reader via the USB port.

nitpick (sorry): ma is measure of current not power (although there is a direct relationship if the voltage is constant). USB supplies a voltage that is capable of supplying current up to some rated amount. The amount actually drawn can be anything up to that maximum amount.
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