John Chung wrote:
>  Means that the turnaround time for each new request
> send over the line. Just imagine a big highway but the
> the space between each car is big. Speed == the number
> of cars that you can fit into the highway lanes at
> that moment of time.

That's not really a good analogy.  Speed is simply how many cars/minute can
go past a particular point.  USB is much faster by this measure than RS-232.
The problem comes when you need to ask the remote device a question and you
can't do anything until you get the answer.  The main difference in this
regard between USB and RS-232 is that on the USB all communication is
initiated by the host at the low level.  A device "sending" data is a
abstraction presented by the low level USB bus driver software in the host
operating system.

Sending data from the host to a USB device is low latency, because the host
can initiate the transfer when it wants to.  However, the device can't reply
until the host asks it for data.  Hosts do periodically poll devices, but
the latency is still significantly higher for the first byte than for RS-232
at 115.2Kbaud.  On a unloaded USB, the host generally uses otherwise unused
bus time to poll devices with bulk endpoints whether they have any data, but
this still takes time and I don't know if most hosts actually do back to
back polls at maximum rate when they've got nothing else to do on the USB.
Once a transfer starts, it proceeds much faster than equivalent of
115.2Kbaud.


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