Re: Com port to USB failure
When you say you installed the driver that came with the USB cables, are you sure you're using it? Newer versions of Windows allow multiple drivers to be installed and you select which one is to be used. You'll want to either disable or REMOVE the Microsoft ones to be sure you're using your driver. The Miklor site will take you through all that.
If you've done all that, it was worth a try. WIndows XP is very much dead, doesn't get updates, etc. so my speculation was that it had the old drivers that work with all the knockoff Prolific chips. I would generally agree that new FTDI based adapters from reputable vendors should work. FTDI chips were also counterfited but not to the level like Prolific ones.
Can you connect pins 2 & 3 together, start a terminal program (Putty has serial port support) and see if you can see what you type? Maybe connect the serial port to anything else like maybe an old dial up modem (Hayes, USR, etc) and get output? Maybe you can test the serial port on the D700 like how this HAM did and see if you can get any output?