PC Engine Duo is the Japanese version of the Turbo Duo which is a 2-in-1 Turbo Grafx-16 and CD console similar to the rare JVC X'Eye and Sega CDX Genesis/Sega CD systems.
All CD games will work on any system but not so with Turbo Chip/HuCard games which are oddly region locked.
I was lucky enough to have someone from the PCEngineFX forums to install a mod switch on my system so I can play every HuCard game from both US and Japan excluding the 5 Super Grafx games which will only work on that expensive Super Grafx system.
