Soon I am going to update this thread with a bank switching tutorial that allows for you to program multiple games onto a single cartridge and switch between them with the reset button! It only requires one more IC and is relatively simple to complete!
Until I test this myself, attempt at your own risk. The reset button will switch between each 1MB bank, so make sure to program four ROMs that are exactly 1MB each. If the games you want are smaller than 1024KBs, then use a program to expand them.
Requires:
1x 27c322 16bit EPROM
1x 74xx93 4bit ripple type counter IC
1x 100nF cap
1x 1K ohm resistor
B27 is on the cartridge but will not be labeled, so count the pins on the front of your cartridge pcb until you find it.
Right now, it is all theoretical, but if it works I will also write up how to do smaller banks.
EDIT:
Now here is a method to switch between two 512KB banks on a 27c800 (middle picture)
and a method to switch between four 256KB banks on a 27c800 (right picture)
A simple way to expand a ROM image is to add "FF" in a hex editor at the very end of your ROM. Add "FF" until offset 0007FFFF for a 512KB file or offset 000FFFFF for a 1MB file. Then copy your other games starting at 00080000 or 00100000.
Also note that you must not connect the Address lines (A17, A18, A19 or A20) to the board if you plan on hooking them to a switch. By adding the switch, you are deciding for the console which Address Line to activate or to deactivate and connecting them to the cartridge pcb would be counter productive.
