At the beginning I simply wanted to re-build an Ocean game cartridge. I didn't have an EPROM programmer and no UV lamp and I didn't want to buy this kind of stuff in the 21st century. That's why the cartridge had to use flash memory. I aimed at building a simple large game cartridge, sticking a nice label on it and give it to somebody as a christmas present.
First, with some help by HofMar, we analysed how an Ocean cartridge worked and how we could add write access to it. The first drafts of the hardware were quite simple. At that time I did not have much knowledge about the expansion port and its timing. We got more wishes after a while: In addition to Ocean cartridges, it should be possible to write 8k, 16k and Ultimax cartridges to it. We wanted to link several of these cartridges to a multi-cartridge. One should be able to deactivate (“kill”) the cartridge easily by keeping <Stop>, <C=> or <Q> pressed when switching on the C64. This all should work with no jumpers if possible. We thought that it should also be possible to write several programs, like tools and games, to the cartridge.
These wishes and the required compatibility to C128 (C64 mode) and to the Ultimax mode made the schematic a bit more complex after a while. Nevertheless we managed to build the control part with only 5 discrete logic ICs worth only a few cents. Alex and Enthusi noted that it would be very useful to have some RAM, for example to be able to backup parts of the zero page temporarily. That's why we added 256 bytes of RAM to the I/O area at $DF00.
One important point for me at this project was to use parts which can be bought easily all over the world. It should be easy to build such an cartridge. It should fit into a standard case and be as cheap as possible.
We never wanted to build a freezer cartridge or a replacement for a disk drive. So it didn't get a competitor to any existing hardware but more or less something new.
Another important point has been to release everything as open source. We wanted to provide good tools for users and for developers and useful documentation. This task is still not complete, but we are working on it. What we reached already: It's running stable, the parts are no more than about 10 Euro, the PCB only about 5 Euro and the case about 5 Euro.