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How does USB work?

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subzero349

New Member
I was just wondering if anyone could give me an idea of how usb works...

I know it has four wires, 2 for power and 2 for data transfer...

Is 1 wire for serial transfer and the other wire a clock? Like I2C or some sort of shift register?

What kind of controller would be inside of a usb mouse?

Any info would be appreciated :shock:
 

Mosfet

New Member
USB transfers data over a differential pair, D+ & D-.
Differential pairs allow more noise or attenuation while still working.
When D+ is more positive than D- the signal is differential 1.
When D- is more positive than D+ the signal is differential 0.

A 'balanced line' is faster & less error prone than a rigid protocol like RS232.
 

Noggin

Member
Too add to what Mosfet said (and maybe clear it up a bit) when the first data line is "higher" than the second, its a 1, and if the second line is "higher" than the first its a 0, you can almost think of this as just a single data line. There is no dedicated clock line though, its mixed in with the data line and transferred at the same time. Unfortunately I've forgotten the specifics of it though.
 

subzero349

New Member
Thank you....
are these lines multiplexed?
I don't get how the computer knows when to send/receive data to/from a USB device... with no clock...
 

Noggin

Member
If I remember correctly, USB sends out all the data to every device on the chain, and the devices all have an ID number associated with them. They look for the ID number in the data/clock stream and if it is theirs, they take the data.

**broken link removed**
 

linuxking

New Member
USB Info

Jan Alexon's USB Explained is a great book, besides that the USB Specification (PDF Available from usb.org) is a definte read for every USB enthusiast

Cheers
Linux
 
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