You would think it should have a Serial Port mode if it has GPS. It sounds like not a hardware limitation but rather firmware.
Curious if you can setup the serial port in the Menu -> Set Your Location -> Set Up GPS -> Set Serial Port?
Cheers!
Can confirm it has a GPS Port above the USB port, just like the HP model.
I'm too also inclined to think that it's a firmware limitation. In lieu of 'Serial' control, we have 'PC Programming Mode'. This mimics the function of Mass Storage on the HP version. Unfortunately, the PT version doesn't give you the option to run it as either Mass Storage or Serial when a USB cable is plugged in; it defaults to Mass Storage.
"Menu -> Set Your Location -> Set Up GPS -> Set Serial Port?" is there in the Menu. When in this menu, I get the option of 6 bps rates and an option for 'Off'.
On a side note - I purchased ProScan from you many many years ago, to use on my 396XT. Boy, do I miss using ProScan (I no longer have the 396XT, only this 436-PT). It was the best to use.