As a kind of ultimate mod or challenge, I think it is quite possible today to make a NES version of the FDS RAM Adapter given the skills in the collective community. You need to put much of this inside a NES cartridge shell :
The board will have to be custom designed and will probably take up 2/3rds of the cartridge shell, maybe more. However, if you do it the way I suggest that Nintendo would have done it, you can save a bit on the complexity.
First, you do not need to implement the expansion port shown in the top right of the above picture.
Second, the NES provides a system clock signal on pin 37, so you can save some space and routing there. This signal is not on the expansion connector and I believe Nintendo put it on the cartridge connector specifically for this purpose.
Third, you don't need the right-angle cartridge connector, saving space and complexity.
Fourth, you don't need to use a cable, you use the expansion pins. You have 10 pins and you need 9 signals : Audio Out, Serial In, Serial Out, $4025W.0, $4025W.1, $4025W.2, $4032R.0, $4032R.1, $4032R.2. You tie EXT7/BATT to ground to tell the adapter that battery power is always good.
Fifth, mixing happens inside the NES, so you can simplify the audio mixing circuit a bit.
The schematic for the FDS RAM Adapter is here : file:///D:/FC_HVC-FMR-01-04_05.pdf
The chief difficulty is desoldering that 64-pin RP2C33 and soldering it to a board without destroying the chip. There must be the customary space for the lockout chip. You will also need an ENIO board to route the signals from the RAM Adapter to either a disk drive or an FDSStick or similar. Take the +5v and GND from the expansion connector.
This design works for a front loader but not a top loader. For a top loader, you will need to route the cable signals to a cable, which requires a hole somewhere in the cartridge shell. You will probably need to swipe the crystal from the donor FDS RAM Adapter as well. Of course, given that you can use the FDS RAM Adapter with top loader and a 60-to-72 pin adapter and a resistor mod (vertical support strongly suggested), this would only have any real benefit for front loaders.
The board will have to be custom designed and will probably take up 2/3rds of the cartridge shell, maybe more. However, if you do it the way I suggest that Nintendo would have done it, you can save a bit on the complexity.
First, you do not need to implement the expansion port shown in the top right of the above picture.
Second, the NES provides a system clock signal on pin 37, so you can save some space and routing there. This signal is not on the expansion connector and I believe Nintendo put it on the cartridge connector specifically for this purpose.
Third, you don't need the right-angle cartridge connector, saving space and complexity.
Fourth, you don't need to use a cable, you use the expansion pins. You have 10 pins and you need 9 signals : Audio Out, Serial In, Serial Out, $4025W.0, $4025W.1, $4025W.2, $4032R.0, $4032R.1, $4032R.2. You tie EXT7/BATT to ground to tell the adapter that battery power is always good.
Fifth, mixing happens inside the NES, so you can simplify the audio mixing circuit a bit.
The schematic for the FDS RAM Adapter is here : file:///D:/FC_HVC-FMR-01-04_05.pdf
The chief difficulty is desoldering that 64-pin RP2C33 and soldering it to a board without destroying the chip. There must be the customary space for the lockout chip. You will also need an ENIO board to route the signals from the RAM Adapter to either a disk drive or an FDSStick or similar. Take the +5v and GND from the expansion connector.
This design works for a front loader but not a top loader. For a top loader, you will need to route the cable signals to a cable, which requires a hole somewhere in the cartridge shell. You will probably need to swipe the crystal from the donor FDS RAM Adapter as well. Of course, given that you can use the FDS RAM Adapter with top loader and a 60-to-72 pin adapter and a resistor mod (vertical support strongly suggested), this would only have any real benefit for front loaders.