PRG RAM circuit

The iNES format implies 8 KiB of PRG RAM at $6000-$7FFF, which may or may not be battery backed, even for discrete boards such as NROM and UxROM that never actually had SRAM there. This inspired some people on the nesdev.org BBS to come up with circuits to add PRG RAM to the original boards, so that games relying on it can run on an NES.

Some gotchas to watch out for include the fact that PRG /CE and M2, used together to decode $6000-$7FFF, don't change at the same time. Writes to a mapper register at $E000-$FFFF can cause spurious writes to PRG RAM, as pointed out by loopy.

kyuusaku's circuit

On the forum, kyuusaku and Bregalad discussed SRAM decoder circuits built from 7400 series parts to approximate this behavior in an NES cartridge board. The first tries took two chips[1] or had possible timing problems.[2][3] They settled on the following circuits:

Using 7410

kyuusaku suggested a circuit based on a 74HC10 (triple three-input NAND) stick a pulldown on CE2 to take advantage of Phi2 going high-impedance during reset in order to "offer some write protection".[4]

           ,-------------- ROM /CE
          |   ____
/ROMSEL --+--|    `-.
             |       \
A14 ---------|        )o-- RAM /CE
             |       /
A13 ---------|____,-'

              ____
+5V ------+--|    `-.
          |  |       \
          `--|        )o-- ROM /OE
             |       /
R/W ------+--|____,-'
          |
          `--------------- RAM /WE

Phi2 ---------+----------- RAM CE2
              |
              <
              < "big R"
              <
              |
GND ----------+----------- RAM /OE

Using 7420

He also suggested a circuit based on a 74HC20 (double 4-input NAND), which appears to be the same one in Family BASIC:

Or you could just use a NAND4 to decode any active low memory, also using the /WE priority method. If this is done with a two gate 7420, the second gate could be used to invert r/w to prevent bus conflicts as in the circuit above. This is probably the *final* best way unless you happen to need the extra AND3 from the 7410 and have a positive CE.

The pinout:

• A = Phi2
• B = /ROMSEL
• C = A14
• D = A13
• Y = WRAM /CE
• WRAM /OE = GND
• WRAM /WE = Vcc or R//W, depending on the Family BASIC cart's write-protect switch

You could also use the other gate to invert R//W and use that for /OE (for /OE on the ROM too to prevent bus conflicts).

Using 74139

If you don't need bus conflict prevention, you can use a 74HC139 (double 2-to-4 decoder), which may be cheaper than a 74HC20. See further suggestions from kyuusaku.