Please any reference on drivers for QNS and FREERTOS for XTend® 900MHz RF Modems?

The xtend does not require drivers. It is an RS232 or CMOS level device. Simply connect it to the devices RS232, RS485, RS4222 or 5V CMOS port and send your data just like you would if there was a serial cable.

Sorry, I was talking about the USB version. Any reference in this case?

I am sorry but the only drivers offered are Windows. You might want to check on FTDI web site as that is who’s USB chip Digi uses and the driver comes from.