I finally figured it out! And it's way simpler that I thought Just needs some glue logic on the /CS line and it works great. Big thanks to whicker on NESDev for pointing me in the right direction (I'd tried glue logic before, but apparently not the correct combination. Here's the working cart, kludgey diode-based glue logic and all:
Well, I'm happy to say that I've gotten PCB's in, and it works great! Rather than using the diodes, I actually ended up using a 74HCT00 quad NAND gate IC. For anyone looking to do this for themselves, you can hook the F-RAM chip directly to the SRAM signals EXCEPT FOR /CS. The logic to drive /CS is:
CS_IN = CS_OUT | (RD & WR)
Where CS_IN is the /CS pin on the chip and CS_OUT is the /CS signal coming from the cart. To achieve this with a single quad-NAND gate IC, the logic has to be converted to:
CS_IN = (CS_OUT NAND CS_OUT) NAND (RD NAND WR)
If anybody wants one, I have a full run of 10 available that I'd be willing to sell (I'd have to check chip prices before I could say how much I'd sell them for). I also have the ability to dump carts, so if anybody is interested, I would able to back up your save, mod the cart, and restore the save to the new chip...
Just needs some glue logic on the /CS line and it works great. Big thanks to whicker on NESDev for pointing me in the right direction (I'd tried glue logic before, but apparently not the correct combination. Here's the working cart, kludgey diode-based glue logic and all:
