I'm BerryBerrySneaky, and I'm digging into the hardware of the Arcade1Up 3/4-scale (~4' tall) arcade cabinets. What I've determined so far:

• AllWinner A13 SoC (used in many low-end Android tablets) running Linux-3.4.43.
• 128MB of DDR3 memory, 128MB of SPI-connected flash storage.
• Easily-accessed UART (serial) port. (login is default:mimebox, use 115200,8,N,1)
• Easily-accessed pads for USB host port. Successfully mounted & accessed USB flash drive via the serial console! Plug in a standard USB keyboard to access MAME keyboard menus/settings, including spinner sensitivity.
• The development platform was very likely this Olimex board.
• 2 4 different emulators are used: MAME v0.139u1 on Atari 12-in-1, Asteroids, Centipede), MAME v0.139u1 & RetroArch LibRetro FinalBurnAlpha or RetroArch LibRetro MAME2003 (Different versions of Rampage board. Retroarch used only for Gauntlet), and the commercial "MOO" emulator on SF2, Galaga, Pacman, MK2, etc.
• Here's /root/mame.ini from Atari 12-in-1 board: https://pastebin.com/pbvqH1ay
• Here's the file structure for the main (2nd) partition on 12-in-1: https://pastebin.com/F2TUgNgJ
• The output of the Linux terminal at boot on 12-in-1: https://pastebin.com/9MB5i6r0

See this thread for more info - mainly focused on the hardware side currently, but hopefully will get moving towards emulation stuff soon. Any help you can offer is greatly appreciated!

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BIG UPDATE: Add USB port to access MAME options (controller sensitivity, etc.) on 12-in-1/Rampage/Centipede/Asteroids.