So your system will work with driver?
I guess you can use USB communications device class (or USB CDC)
There is another way. The major OSs support the CDC (Communication Device Class) protocol on their USB stack to use RS-232C communication. It enables PC to access the CDC device through virtual COM port, without developing own driver.
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The CDC specification has been developed to answer the need of telecommunication services (devices that have traditionally terminated an analog or digital telephone line), and medium speed networking services ("Always Connected" LAN/WAN media types) and is ideal for devices such as, Mobile Phones, Analog/ADSL/Cable Modems, analog/digital telephones Wireless Terminals, Ethernet Adaptors as well as Legacy and other network devices.
The CDC USB Driver support the following subclasses: ECM, NCM, ACM, OBEX, Device Management.
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Another method without driver
is HID class
http://www.lvr.com/hidpage.htm
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http://www.microchipc.com/sourcecode/