The OS V.1.16 is available at http://www.yamaha.co.jp/english/product/pk/nonflash/products/psr9000/psr9000.htm
It corrects a system fault occuring when modifying the voices.
Here's the info:
Power ON/Custom Voice
A system failure may occur indicated by the LCD displaying "Assertion Failed!" after turning on the power. This occurs when a custom voice is created from a specific voice (see list) and stored, and another custom voice is stored in the next area. The voices that cause this condition are: Organ:RotaryDrive, Guitar:Hi Q Bass, GuitarrySynBass, GuitarX JazzGtr, Guitar:Bass&Cymbal, Strings:Solo Violin, Strings:MarcatoStrs, Trumpet:Sweet Tromb, Trumpet:BaritonHorn, Trumpet:Bariton Hit, Flute:Bagpipe, Choir & Pad:Template, Synthesizer:Under Heim, XG:Bagpipe.
A system failure may occur when the LCD (I) or (J) button is pressed twice quickly when a voice or style select screen is displayed on the regist bank view screen.