The Cardboard/3S expansion interface plugged into the cartridge port of the Commodore VIC-20 and added 3 additional cartridge slots and a reset switch. Each of the slots had it's own switch-selectable memory address, allowing memory expansion to be used at the same time as program cartridges, as well as other combinations. The cartridges sat vertically off of the card, vice horizontally as they would from the VIC-20.

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