I follow Natalie the Nerd on Bluesky, who does the most amazing things with Gameboy hardware recreations, including (I kid you not) making the lego gameboy actually work. Recently she posted about her shenanigans with 3D printing cartridge cases for a retro console, and it got me to thinking that cartridges would be cool for FreeBee too. They could be a neat way to share code between machines, a bit more accessible than the Compact Flash stuff has been (can just copy them on an EPROM programmer or similar without regard to card format), and (this was what really got me going) they could be an easy way to extend FreeBee, both for memory _and_ I/O.
So I went looking for edge connectors to base my cartridge design off, and found the EDAC 0.1" pitch 34 way connector. This would have been just the thing to plug your 5.25" floppy drive in with back in the day. Including it's ears, it's 63.5mm long, and with 34 pins it has enough to support a 32 pin 39SF040 512K Flash memory, with a couple of pins left over for I/O addressing.
So memory wise four cart slots fit across the back of the board. I can do another four 32 pin Flash chips on the board, plus one chip for Basic, plus a RAM. Each of the Flash chips is a 512K 39SF040, giving me the possibility of 2MB on the board and 2MB plugged in via carts.
I departed a little way from previous coreboards and dropped Microbee compatibility. The Freebee main board does reset, and includes 32K of RAM, so let's use that to simplify the RAM decode and overall logic on this board. The RAM is 128K, and any 32K bank may be dropped in to the upper 32K address space, pushing aside ROMs (Basic and PAK) and video memory when it does so. This gives us 160kB of RAM total, which is more than you'll ever need.
The carts get a chip select for their PAK ROM space, and for I/O they get a decoded address space of 16 I/O positions, wich each one on a separate range. This means that I/O will be in different places depending on where it's plugged in, but is the only way to avoid conflict.
