6502 Cartridge Port
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The 6502 Cartridge Port interfaces to the 6502's address and data bus, as opposed to the CP-1610's bus. This cartridge port allows adding 6502-specific software such as Microsoft BASIC.
This cartridge port offers a 44-pin card edge, and requires the cartridge to have a female card-edge connector on it. This is the opposite of the regular Master Component Cartridge Port, where the unit offers a female card-edge connector and the cartridge provides only a card edge.
This cartridge port is found on the rear side of the unit behind a plastic door. The complexity of construction associated with the 6502-specific cartridges indicates that Mattel likely did not intend for many packages to use this interface.