On Mon, 12 Apr 2004, Francisco Fernandez Velasco wrote:
> Hi there,
>
> I've a doubt related to a scanner device I'm accessing to through the
> javax-usb API. To send to device the bytes that order it starts image
> scanning works fine. I use syncSubmit in its OUT PIPE and the scanner
> works fine!!
> The matter is that, when retrieving the image, I use different UsbIrps
> (one per image chunk I retrieve), but the problem is that I have NO way
> to know when to stop requesting bytes. Is there any way to resolve when
> the device has no data to send? Is it correct to assume that an UsbIrp
> with actualLenght set as zero would mean that there's nothing else to
> send?
Do you have the specs for the scanner? Is it a standard protocol? There is
really no way of guessing how the scanner will behave. It probably sends
the data in a format such as you can actually know how many packets to
expect.
If it does send packets of size 0 a reasonable guess would be that it is
"done".
> > Is this piece of code correct?
> do {
> //IN PIPE (device to host)
> UsbIrp usbIrp = inPipe.createUsbIrp();
> usbIrp.setAcceptShortPacket( true );
> usbIrp.setLength( 1024 );
> usbIrp.setOffset( 0 );
> usbIrp.setData ( new byte[ 1024 ] );
>
> try {
> //SYNC REQUEST: Locked till device response!!
> inPipe.syncSubmit( usbIrp );
> } catch ( Exception e ) {
> printStackTrace( e );
> }
> //Storing image data for later use!!
> buffer.append ( usbIrp.getData() );
> } while ( usbIrp.getActualLength () != 0);
Looks ok (assuming you actually get a packet of length 0). However, you
can reuse your UsbIrp.
To get better throughput, create a list of UsbIrps, say 20 or so, and
submit that. That will probably both lower the CPU usage and increase the
bandwith you can use a lot.
//Roger Lindsjö
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