The Casio DG-20 was an "entirely new type of digital musical instrument which features the latest in state-of-the-art technology". Played with untuned nylon strings, the notes on the fretboard correspond to that of a guitar in standard tuning so it's a breeze to play. These are the MIDI functional models! 
Tested and working on power supply and batteries (neither included) Appears all original and is very clean other than the missing battery door that's been taped over.