This Yamaha computer is specialised in music and sound production. In fact it is a classic MSX 1, but if you bought the special Yamaha synthesizer and piano keyboard, then it was clearly different. You got 48 pre-recorded sounds with a quality really surprising for that time, and as it is real synth, you could change or create you own sounds.
The YIS-503 is in fact the same computer as the Yamaha CX5M, but it has not got the SFG-01 Sound Module and MIDI interface integrated.
We need more info about this computer ! If you designed, used, or have more info about this system,
please send us pictures or anything you might find useful.