74xx161-based boards were used in many games. Being based on an off-the-shelf general purpose 4-bit counter, these boards have less functionality compared to ASIC mappers specifically designed to take advantage of the Famicom's specifications. When converting such a board to flash memory, certain problems will still exist:
- H/V mirroring is only switchable by hardwiring means.
- If the software uses unsettled data whenever a bus conflict occurs, flash memory won't operate in the same way as a Mask ROM does.
Devices that support A0-A10 command addresses are supported (e.g. AM29F040B). Unfortunately this type of board is unable to allocate A0-A14 command addresses.
So far, HVC-UNROM-10 and HVC-UOROM-01 have been successfully tested. Other 74xx161-based boards not referenced in this document might not comply with the following instructions.
- 1: /RES = H
- 2: CK = CPU /ROMCS
- 7: Enable P = L
- 9: LD = CPU R/W
- 10: Enable T = L
The unreferenced wires will depend on hardware design. However there shouldn't be any problem with these referenced wires.
Add a 74xx00. Using the same 'xx' family of 74xx161 devices that already exists on the board is recommended.
74xx00 +-v--+ CPU R/W -+-|1 14|--- +5V | | | +-|2 13|-+- CPU /ROMCS | | | PRG /OE -+-|3 12|-+ | | | +-|4 11|-+ | | | | +-|5 10|-+ | | NC |6 9|--- CPU PHI2 | | GND ---|7 8|--- 74xx161 CK +----+
- Cut exsiting 74xx161 CK wiring, connect to 74xx00 8 pin.
- Connect PRG /OE to flash memory /OE
- Connect flash memory /WE to CPU R/W
- Connect flash memory /CS to CPU /ROMCS
Since the MaskROM of a UNROM board has only 28 pins, modifying this type of board is not recommended. It may be possible to replace the MaskROM with a 32-pin flash memory chip, but the chip will stick out on the board as a side effect. So if you're wanting to keep a cartridge casing intact, a game containing a UOROM board would be a better choice. In addition, hardwiring any connections would be much more stable.
- Momotarou Dentetsu (first Momo-tetsu, not Den[se]tsu)
- Maniac Mansion (uses a compatible non-standard board, not recommended)
1:A17 32:+5V 2:*1 31:+5V 30:+5V 24:A16 22:CPU /ROMCS
*1 CPU /ROMCS
1:A18 = mask ROM 1pin, UxROM Program A17 (skimps, *2) 2:A16 = mask ROM 24pin, UxROM Program A16 31:/WE = cartridge 14pin, CPU R/W 30:A17 = mask ROM 1pin, +5V (skimps) 24:/OE = 74xx00 3pin 22:/CS = cartridge 44pin, CPU /ROMCS (Unchage wiring)
*2 For perfect compatibility with UNROM, settle High or Low level.