I've uploaded some new pictures of it here:
Sorry for the quality, but I've only got a cheap webcam. I'll probably make some in-action pics later.
The 32MBit RAM cart is a general purpose one (like a SIMM module) which has 22 address lines (2^22=4MBytes) and 8 data lines, therefore you can use it for anything else. I'm planning to make a modification so that you can also use it with the Genesis as well (which uses 16 data lines). The second small board which is connected to the RAM cart at the bottom (see chn_snes_rom.jpg) is the power supply for the RAM ICs. The switch selects the DC connector (directly above the switch) or the 9v battery as the power supply. The two ICs store two additional bits - for the SNES, that is the memory scheme (LOROM/HIROM) and the presence of SRAM. These two bits are set by the transfer program. The cart uses 8x 4MBit SRAM ICs of the type KM684000(=628512) and a SN74LS138N as an address decoder. A previous version used 4MBit flashroms of the type 29F040... I was thinking about using DRAM, but trust me, it is a pain in the ass to use :-(
The BRAM cart (chn_snes_bram.jpg) is very straightforward. It uses a 1MBit SRAM IC of the type 621024 and just provides its address and data lines at the connector. The additional components are the power supervisor circuit which provides the SRAM ICs with the data retention power from the battery when the SNES is powered down.
The HIROM/LOROM board (chn_snes_hilorom.jpg) maps the BRAM and the game ROM into the SNES' memory space according to the HIROM/LOROM bit on this small board (see above). It is the only SNES-specific component of the whole system. For each console, an own address mapper like this would have to be built. The connector on the left side is for the RAM cart, on the right for the BRAM cart and the third one for the actual SNES.
In order to write and read the RAM and BRAM carts with your PC, you use a simple 8255 based ISA I/O card. I used this one to write the flashroms as well. AFAIK they are pretty common for such a type of DIY stuff...
You can buy one from Conrad:
Or build one yourself:
Whereby the latter is of course much more fun ;)
If you want, I can also scan my version of the IO cart which has 96 in/outputs, as opposed to 48 or 56 of the other two. However, you only need 48 for this particular project.
Because I have 2 spare flashrom ICs, I'll probably also make a flashcart for the Game Gear, which is very easy to do. With a simple 1:1 connector adaptor one could use it with the Master System as well.
Ok I think that was a brief introduction, but you asked for it ;) I hope that wasn't too much at once... :)