USB data architecture ?

[Externet]

Is there a standard for USB data transmission? How many data bits, start bits, stop bits, spacing between stop and start bits ? Synchrony ? I believe the speed part depends on the

Are those the working speeds or the maximum ? Backwards compatible means USB 3 can work at 12MBps ? What if USB2 runs at -say 66 Mbps; will it work ?

 

[rjenkinsgb]

The data rates are fixed, as far as I know, as the receiving end has to synchronise to the data rate.

USB3 has nine connections (or possibly more with USB C connectors rather than USB A).
The extra five connections only connect with a USB3 plug and socket pair. Otherwise, just the four USB2 contacts are used.

The extra five connections are two data pairs for tx and rx, plus an extra ground / screen connection.

For USB1 or 2, Each data transmission uses a run-in or synchronising pattern 32 bits long, alternating for 30 bits then the last two the same, with the two data lines complementary to each other.
Then the data, then both data lines low to indicate end of data.

See here:

USB communications - Wikipedia

en.wikipedia.org

 

[Papabravo]

USB is a synchronous protocol so there are no start or stop bits just data bits. An entire frame will have a preamble and a frame check sequence. For more information get the USB specification or Jan Axelson's book.