[sane-devel] USB speed

[Lauri Pirttiaho]

Pierre Willenbrock  on Sat May 6 16:50:46 UTC 2006 wrote:
> i am trying to make the scanning speed of my scanner
> depend on the usb speed available. I first attempted
> to use the detection from the cs3200f backend, but 
> found that it only detects if the device is high 
> speed capable, not if it is currently communicating 
> with high speed.

Hello,

That way of USB speed detection in cs3200f is very
experimental... :)

However, that is what the TWAIN driver seems to rely on.
Now, CanoScan 3200F has this peculiarity in the firmware
and HW that it actually detects whether it is connected to
high speed (480 Mbps) or full speed (12 Mbps) bus and
gives out the descriptor based on that. So even though
this scanner is high speed capable, it reports back
only full speed (USB version 1.10) if it is not connected
to high speed bus.

So, this is a property of this particular scanner and
may not generalize to other HW.

Detecting the speed more accurately (i.e. more than just
knowing whether both host and the device support high 
speed), would actually need dynamic speed measurement 
(varies by the USB bus load) and adjustment of scanning
parameters continuously. I don't think any current 
scanner HW supports that; the general solution 
therefore is to use proper back tracking. 
That is, indeed, what happens in TWAIN driver.

With best regards,

Lauri Pirttiaho
Oulu
Finland

ps. As to the status of cs3200f, I have currently about
zero excess time for that but the project has not been
abandoned. I will continue it as soon as I can allocate
time for it. The 2006Q2 schedule will not hold...


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