Konami Part number: GKE33-PWB, original
The EXT-IO is pretty much a gold bar that reads schmitt triggers from the stage PCBs, swizzles some cables, and poops out Leprechauns, Rainbows and Unicorns over RS232 so that the games can read all sensors of the stage individually and light up the foot panels.
No known hardware uses this part.
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The EXT-IO is only used on Dance Dance Revolution SuperNOVA and up as well as Dancing Stage Fusion (The only DDR game on the Python 1). While DDR X and up up as well as Dancing Stage Fusion REQUIRE the EXT-IO in order to boot and function, Dance Dance Revolution SuperNOVA 1 & 2 can bypass this requirement by jumping pins 1 & 6 on the stage light connectors for both players and flipping DIP 2 on the Python 2 IO board to place the game into MANUFACTURER mode. In the mode, on boot the games will show a FOOT PANEL error but will still boot. The stage IO check and foot panel lights will be nonfunctional in this configuration.