Hopefully you guys can save me from wasting time

I have downloaded some trial piano sample software to my laptop computer. I have a Yamaha P-80, and I want to use it to check out the piano sounds of this software. The P-80 docs say to use the "To Host" port to my Windows computer using an 8 pin mini din to 9 pin RS232 cable, and download a driver from Yamaha. I don't have one of those cables handy, but I do have a Tascom US800 USB interface unit and a 5 pin midi cable. Can I just connect the p-80 midi out to the US800 with the 5-pin midi cable, and go from there. I have the Tascam driver installed on the computer. Will the Truepianos, Pianoteq, and Pianissimo software recognize the p-80?

Yes, the software will operate using the Tascam MIDI interface. It won't recognise your P-80 as such, because it is just receiving MIDI note data. Your software needs to recognise your Tascam interface. It doesn't are what sort of keyboard is actually sending them.

Thanks. That's what I was expecting and hoping for. Hypothetically, if I was to come across a "To Host" cable (8 pin mini din and db9 rs232 cable), and install the Yamaha interface software on my computer, would the piano sample software recognize the Yamaha interface software? Any experience with that?

Yes, it would work like that. But since you already have the Tascam, and presumably a 5-pin MIDI cable, I wouldn't bother. You already have what you need to starting playing.