Apr 132024
Last Updated: 4/19/2024
Article Credit: ShootTheCore, wickerwaka, ekorz, System11, Apocalypse

Main Board

The Main Board is largely identical across all Irem M92 games. It is labeled either M92-A-A or M92-A-B.

Early boards have less Object RAM (16K) installed at locations IC43 & IC44 than later boards (64K) – selectable by jumper J2 – but all M92 games are confirmed to function correctly with the 16K board. Thus, the difference in RAM was likely determined by parts cost at the time of manufacture.

Notable ICs

ICFunctionSilkscreen LabelLocation
NEC D71036L V33Main CPUH8
Nanao GA20Sound FX SamplesIC32 GA20B6
Nanao GA21SRAM DMA and Address Generation for GA22IC42 GA21M7
Nanao GA22Graphics RenderingIC63 GA22M10
YM2151FM Sound ChipIC33C6
YM3014Sound D/A ConverterIC208 YM3014C5

Secondary Board

The Secondary Board layout varies across the M92 games.

The Sound CPU resides on the secondary board and utilizes an encryption mechanism – see “Copy Protection” below.

GameBoard ModelSound CPU ModelNotes
Blade Master / Cross BladesM92-B-A
Dream Soccer 94
GunForce II / GeoStorm
In The Hunt / Kaitei DaisensouM92-E-B
Lethal Thunder / Thunder BlasterM92-C-B
Major Title 2 / The Irem Skins GameM92-B-FOnly M92 game to utilize an EPROM for saving scores.
Mystic Riders / Gun HokiM92-B-B
Ninja Baseball Bat Man / Yakyuu Kakutou League-ManM92-F-A
Perfect Soldiers / Superior Soldiers
R-Type LeoM92-C-N
Undercover CopsM92-E-B

Notable ICs

ICFunctionSilkscreen Label
Nanao 08J27261A1 V35Sound CPUIC10
Nanao GA23ROM Addressing (Assumed)IC23 08J27504A1

Board Variants

M92 boards manufactured for the Korean region have several noticeable differences from boards manufactured for other territories:

  • The green silkscreen is a lighter shade of green.
  • The JAMMA connector does not have a key notch.
  • SRAM ICs on M92 boards for other regions are manufactured by NEC whereas on the Korean boards they are manufactured by Hyundai and Goldstar.
  • The amplifier heatsink is physically smaller, and is colored silver rather than black.
  • One or both boards may be missing markings for the board type on the silkscreen.
  • A metal serial number tag is not present in the upper-right corner of the A Board.

There are also single-board bootlegs of several M92. They are much smaller than original M92 boards and thus are easily distinguished. Compared to the original games, bootlegs often omit animation frames, shorten the music tracks and gameplay slows down more frequently.

Common Issues and Quirks

  • The factory-installed capacitors are colored green or brown. The ELNA brand brown-colored capacitors are notorious for leaking and should be replaced ASAP.
  • Video output is darker than average with this hardware.

Protection Mechanisms

The Nanao 08J27261A1 V35 Sound CPU on the B Board enforces a security scheme where each game’s sound CPU code is encrypted in ROM and then decrypted by a key specific to the model of CPU. Thus, if the game ROMs are swapped to another game without decrypting the sound CPU code of the new game and re-encrypting the code to match the encryption scheme of the old game, the sound CPU will crash and the game won’t have sound.

Changing jumpers J1 and J6 adjacent to the Sound CPU from “S” to “N” disables the decryption mechanism and allows the CPU to run decrypted Sound CPU code directly.

Capacitor List (credit: System11)

Main Board

LocationCapacitanceVoltagePart No.
C2011 uF50 VECA-2EM010
C202470 uF25 VECA-1EM471
C203220 uF10 VECA-1AM221B
C20422 uF25 VECA-1EM220I
C209100 uF25 VECA-1EM101
C2101 uF50 VECA-2EM010
C21347 uF16 VECA-1CM470I
C2151 uF50 VECA-2EM010
C2161 uF50 VECA-2EM010
C2171 uF50 VECA-2EM010
C2181 uF50 VECA-2EM010
C2191 uF50 VECA-2EM010
C236100 uF25 VECA-1EM101

Resistor List (credit: fuzzbuddy)

  • R201 – 1KOhm
  • R203 – 100KOhm
  • R205 – 4.7KOhm
  • R208 – 4.7KOhm
  • R209 – 2.7KOhm
  • R211 – 10KOhm
  • R213 – 10KOhm

ROM Details (credit: MAME Source, ekorz)

LabelLocationEPROM Part No.Function
H0B Board27C020 (256kb)Main CPU
L0B Board27C020 (256kb)Main CPU
H1B Board27C020 (256kb)Main CPU
L1B Board27C020 (256kb)Main CPU
SH0B Board27C512 (64kb)Sound CPU
SL0B Board27C512 (64kb)Sound CPU
C0B Board27C040 (512kb)Background Tiles
C1B Board27C040 (512kb)Background Tiles
C2B Board27C040 (512kb)Background Tiles
C3B Board27C040 (512kb)Background Tiles
000B Board27C080 (1024kb)Sprites
010B Board27C080 (1024kb)Sprites
020B Board27C080 (1024kb)Sprites
030B Board27C080 (1024kb)Sprites
DAB Board27C040 (512kb)Audio Samples

Schematics (credit: wickerwaka)

Original Irem schematics are unavailable for M92.
wickerwaka developed a MiSTer FPGA core for the Irem M92 platform, and generated the following schematics (Source):

Additional Resources

MAME M92 Driver Source Codehttps://github.com/libretro/mame2016-libretro/blob/master/src/mame/drivers/m92.cpp
wickerwaka MiSTer FPGA Sourcehttps://github.com/wickerwaka/Arcade-IremM92_MiSTer/tree/main
System 11 Blog – Irem Games and Leaking CapacitorsWebsite is offline – PDF Archived on August 2017

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