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Old 07-10-2010, 05:49 AM   #1
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one GSM/GPRS/3G usb stick modem creates 3 usb serial devices


I'm running a Slackware 13 box, and after a bit of struggle I managed to make my mobile internet usb stick to work with linux. After enabling a few kernel modules (symbols: USB_SERIAL, USB_SERIAL_GENERIC and USB_SERIAL_OPTION) and made a small udev rule which literally ejects the stick which gets firstly detected and mounted by hal as a scsi cdrom, three usb serial devices are created: /dev/ttyUSB{0,1,2}. By trial and error I managed to successfully use /dev/ttyUSB2 to dial in and connect.
What puzzles me is why the 3 devices created? Are there 3 physical devices contended within the stick which get detected and may serve other purposes, from which only one is the plain modem or is it a driver issue/redundancy?
I can provide syslog messages and udev information (events, environment, etc) if needed.

Any thoughts are greatly appreciated.
Old 07-15-2010, 03:29 PM   #2
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Similar to how plugging in a device that can both play and capture audio results in (at least) two devices being created under /dev, it looks like the driver handling your usb modem does the same. The modem may actually have 3 separate chipsets that you can now directly access from user space. If you're interested in knowing more about how devices are loaded (and how to play with those devices directly), then I'd highly recommend Essential Linux Devices Drivers.
Old 07-16-2010, 02:32 AM   #3
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Thanks. I'll check it out.


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