Hi Wayne,
You cannot use the USB Port on the rear of the GP39 to connect to a PC. If this is how you are connecting it please remove the connection. You may cause damage.
To connect the GP39 to the PC you must use a USB converter or an Opto Isolated Serial Port Adapter. Products like the ActiSense USG-2 or OPTO-4 or similar. These connect to the TD (White + Blue -) on the power cable of the GP39. Then, load the driver for the adapter you are using. ActiSense drivers load with Windows Update so with an internet connection its normally fairly automatic.
Once its loaded and visible in Device Manager, check the COM Port number and set CPLOT to look at that.