The DX7 has internal memory for 32 voices, which even for the 80s was relatively little. It has an expansion slot, however, where cartridges can be plugged in to provide direct access to further 32 voices. The DX7 shipped with two ROM cartridges, each containing 2 preset voice banks (2 x 32 voices) selectable by a switch on the cartridge. At the time, Yamaha also sold RAM cartridges onto which single voices or the full contents of the internal memory could be stored, sort of an external RAM extension. Unfortunately, the cartridges are very hard to find these days, and even the used ones sold on eBay are very expensive.
The publicly available circuit diagrams [PDF] of the DX7, e.g. on Dave Benson's DX7 page, reveal that the cartridge design is very simple. It is really only an external memory chip, and the pins on the cartridge slot are just the address, data and signal buses to control the reading and/or writing of data from or to an EEPROM.
another story, but there are prototype PCB manufacturers like OSH Park that do them for USD 10 apiece or so. Don't even ask about the enclosure.
As you can guess, I'm going to try to design and build such a cartridge. In the following weeks I will be reporting on the progress of this project. Peek preview: The prototype boards have already been sent away for manufacture.
Update [19.08.2014 08:42 CEST]: I just got a notice from OSH Park that the PCBs have been shipped!
So stay tuned!