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74xx161 en

Introduction

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.

Supported Devices

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.

Wiring Confirmation

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.

Wiring

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

UNROM/UOROM

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.

Games which contain an HVC-UOROM board

  • Momotarou Dentetsu (first Momo-tetsu, not Den[se]tsu)
  • Seireigari
  • Maniac Mansion (uses a compatible non-standard board, not recommended)

Board Pin Assignment

1:A17 32:+5V
2:*1  31:+5V
      30:+5V
      24:A16
      22:CPU /ROMCS
*1 CPU /ROMCS

Flash Memory Pin Assignment

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.
Last modified:2009/11/30 23:22:52
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