The USB port on the C302 is a hastily implemented hack. Its not at all sufficiently documented, supported or supportable by Cambridge. Its no more than a third party USB to serial adapter board stuck inside the C302. Its not providing "real" USB connectivity, just the USB to serial conversion. You have to deal with drivers for it being properly installed on your computer. Whether this happens via plug and pray on Windows computers or not properly, ... um... well good luck.

If you have a Cambridge 302 and want to connect to it from a laptop use a good quality third party USB to serial adapter from a company that provides good driver updates etc. The Keyspan USA-19HS Hi-Speed USB Serial Adapter being my recommendation. Only $28 on Amazon and is likely to save you significant hassles.

The C302 uses an special Cambridge bus (actually its just an electronics' industry standard I2C "eye squared C" serial bus) for communicating with the C303. The I2C serial bus and the RS-232 communication to the PDA are entirely separate, use different connectors, wire signals and protocols etc. Whether you have a C303 present or not makes no difference to how you connect a PDA/PNA to the C302.

Your PDA/PNA should use a serial port. Forget USB anything unless you have specific need to connect a laptop that does not have a serial port. In that case use the Keyspan adapter I mentioned above.

If you have a PDA/PNA without a any serial port then look at a Bluetooth adapter like the K6-BT. Although a very nice adapter it can have issues with the baud rate switching involved with the C302 IGC file download.

Some PNAs have serial ports that are logic level and need a level shifter to drive at RS-232 signal levels (well most vendors don't strictly bother to get that right but they work).

Some PDA/PDA devices are capable of being USB bus masters. Especially via USB OTG (on-the-go) switching. But that's getting off topic...

Darryl