How do USBs work

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Joined Mar 22, 2011
I know there are a range of USB devices that use a range of technologies. Im just after an explaination into how data is stored from the computer to a USB, how that information is able to store data, what is storing the data (charge ?) and how the information can be overridden. Just include anything you like on USBs as I would like this post to be a general discussion of the matter!


Joined Feb 24, 2006
It is a serial interface with four wires. There is a power pair that carries +5VDC and Ground. The other pair is a data pair that uses differential signaling with a common ground.

For USB 2.0 there are 3 standard data rates called Low Speed (1.5 MBits/sec), Full Speed (12 MBits/sec), and High Speed (480 MBits/sec). Data is transferred using a connection based protocol with "endpoints". Once the connections are set up, and the devices are "enumerated", data transfers can happen with very low overhead.

In a USB thumbdrive for example there might be a processor and a flash memory chip. The processor implements the USB device profile for a "Mass Storage Device" just as if it were a rotating memory.

For more information get the book by Jan Axelson, now in the forth edition.


Joined Jun 7, 2009
USB as well as many other serial protocols consist of 'packetized' transmissions. The host and device communicate by sending packets of data back and forth. The host sends a command to the device, and the device responds with a reply. It is up to the host/device to assemble the data into the 'packets' according to strict guidelines, outlined in predetermined formats. Embedded in the packets is information utilized on the hardware level to ensure validity of transmissions. My reading recommendation would be Jan Axelson's USB complete.
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