Reading data from usb port

[high_flyer]

The QextSerialPort is not what you want.
Search in the forum - someone posted few weeks back a Qt wrapper for USB lib.
But this is really not a Qt issue, you can use any USB lib that will work for you in your Qt project.

[schnitzel]

In case you are using a usb to serial converter like FTDI which features a driver with a virtual comm port (judging by the high com port number), then you could use qextserialport, however, be aware that the new location for that project is on google code:
http://code.google.com/p/qextserialport/

the version you downloaded is old.

Once you get the proper source, look at qextserialport examples and please refer to the forums/mailing list over at qextserialport on how to use this library.

[high_flyer]

In case you are using a usb to serial converter

If schnitzel is correct, then you want to read from a SERIAL port, not a USB port.
The fact the serial port is originally a USB port, plays no role for you.
Your code will only deal with a serial port - in which case the code you posted makes sense.
So what is it that you need?
Do you want to read a USB port or a serial port?

[schnitzel]

I should clarify...

I'm using one of those FTDI chips on a custom board. The chip is supported by a library (win/mac/linux) available for free from ftdi website. The chip isn't actually a usb to serial converter *but* it features the so called VCP (Virtual COM Port) driver. From ftdi web site:

For most of these operating systems two types of driver are available: Virtual COM Port (VCP) drivers and direct (D2XX) drivers. The VCP driver emulates a standard PC serial port such that the USB device may be communicated with as a standard RS232 device. The D2XX driver allows direct access to a USB device via a DLL interface.

I have never used the device with VCP, because the transfer rate is a bit lower compared to using the device with the direct drivers.

Like you said high_flyer, OP needs to clarify what he needs/uses.