Yamaha P80, US-122, Cubase Lite
I have the same setup and the same issues.
According to Tascam, the US-122's manufacturer, the MIDI IN light should NOT be constantly on -- I've followed every step in the P80's manual to the letter when it comes to setting up for MIDI, but haven't been able to discover what is sending that signal. I've also tried to contact Yamaha, but so far, no response.
As for Cubase, here's what I have discovered. The first thing is,
the Yamaha P80 is NOT included on the list of MIDI devices in the MIDI Device Manager window. What this basically means is that you are going to have to add a New Device and program that device in order to hear the native sounds of the P80. All that wonderful audio that you see listed in the other devices? You're not going to be able to hear them even when you get the P80 to work.
If you've set up the US-122 correctly, you'll probably notice that the IN/OUT displays on Cubase's transport window light up with every keystroke on the Yamaha. This is good.
In order to hear SOMETHING before you figure out how to add your keyboard (something I'm working on right now), you're going to have to assign a VST Instrument also. In the DEVICES menu, click on VST Instrument, click on one of the panes that pops up in the window, scroll down to synths and click on Universal Sound Module. Back in the Project window, make sure that the IN: on the MIDI track is assigned to the US-122 MIDI, and change the OUT: to Universal Sound Module.
Just below that, select a program PRG: -- and you should be able to hear what you're playing in one of a really, REALLY lousy selection of samples. You won't be able to record what you hear, though, because no MIDI signal is being sent out -- it's a catch 22 -- because you won't be able to hear what you CAN record with a MIDI Device selected either.
Yamaha's manual for the P80 MIDI set-up pp. 41-48 (right at the back) was evidently written BY software engineers FOR software engineers -- and very little in it corresponds to anything in Cubase's setup. It's difficult to connect the virtual dots.
I've tried Cubase out for a bit so far -- and frankly, I'm not enthralled by its layout or its operation -- I'm thinking seriously of plunking down some more money to pick up a program like Logic Express -- provided it has the P80's presets pre-programmed -- because aside from learning some interesting things about how MIDI operates, I'm not convinced the hassle will be worth it if, as I suspect, I'll opt to throw Cubase in the trash in a couple of weeks, anyway.
Hope that helps -- if you find anything else out that might help me with this same problem -- please pass it along.
fjd