I have to agree that my analogy is rather poor once I
read your indepth explanation on the device reply to
the host. Looks I can't escape in reading that USB
book from Intel.... Speed == (cars / minute) which is
more appropriate as you said. 

Thanks,
John


--- Olin Lathrop <olin_piclist@embedinc.com> wrote:

> 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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