I've had big problems using my HP6515 with Tom Tom 6. Basically I cannot get a signal with the onboard GPS. I think the software does not have the option of selecting Comm 7 where the GPS must be connected too.
However I have just sourced a good link to check if the GPS is working.
http://www.visualgps.net/VisualGPSce/
look for a free program called visualGPSce. Set it to COM7 and the baud to 56000 or 57600 and see if it picks up any sats.
Please let me know if you are having problems with this device or TT6
Cheers