Irem M92

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

IC	Function	Silkscreen Label	Location
NEC D71036L V33	Main CPU		H8
Nanao GA20	Sound FX Samples	IC32 GA20	B6
Nanao GA21	SRAM DMA and Address Generation for GA22	IC42 GA21	M7
Nanao GA22	Graphics Rendering	IC63 GA22	M10
YM2151	FM Sound Chip	IC33	C6
YM3014	Sound D/A Converter	IC208 YM3014	C5

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.

Game	Board Model	Sound CPU Model	Notes
Blade Master / Cross Blades	M92-B-A M92-D-A
Dream Soccer 94
GunForce	M92-B-A
GunForce II / GeoStorm
Hook	M92-D-A
In The Hunt / Kaitei Daisensou	M92-E-B
Lethal Thunder / Thunder Blaster	M92-C-B
Major Title 2 / The Irem Skins Game	M92-B-F		Only M92 game to utilize an EPROM for saving scores.
Mystic Riders / Gun Hoki	M92-B-B
Ninja Baseball Bat Man / Yakyuu Kakutou League-Man	M92-F-A M92-Z-C
Perfect Soldiers / Superior Soldiers
R-Type Leo	M92-C-N
Undercover Cops	M92-E-B

Notable ICs

IC	Function	Silkscreen Label
Nanao 08J27261A1 V35	Sound CPU	IC10
Nanao GA23	ROM 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

Location	Capacitance	Voltage	Part No.
C201	1 uF	50 V	ECA-2EM010
C202	470 uF	25 V	ECA-1EM471
C203	220 uF	10 V	ECA-1AM221B
C204	22 uF	25 V	ECA-1EM220I
C209	100 uF	25 V	ECA-1EM101
C210	1 uF	50 V	ECA-2EM010
C213	47 uF	16 V	ECA-1CM470I
C215	1 uF	50 V	ECA-2EM010
C216	1 uF	50 V	ECA-2EM010
C217	1 uF	50 V	ECA-2EM010
C218	1 uF	50 V	ECA-2EM010
C219	1 uF	50 V	ECA-2EM010
C236	100 uF	25 V	ECA-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)

Label	Location	EPROM Part No.	Function
H0	B Board	27C020 (256kb)	Main CPU
L0	B Board	27C020 (256kb)	Main CPU
H1	B Board	27C020 (256kb)	Main CPU
L1	B Board	27C020 (256kb)	Main CPU
SH0	B Board	27C512 (64kb)	Sound CPU
SL0	B Board	27C512 (64kb)	Sound CPU
C0	B Board	27C040 (512kb)	Background Tiles
C1	B Board	27C040 (512kb)	Background Tiles
C2	B Board	27C040 (512kb)	Background Tiles
C3	B Board	27C040 (512kb)	Background Tiles
000	B Board	27C080 (1024kb)	Sprites
010	B Board	27C080 (1024kb)	Sprites
020	B Board	27C080 (1024kb)	Sprites
030	B Board	27C080 (1024kb)	Sprites
DA	B Board	27C040 (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):

• M92 Partial Schematics
• M92 KiCad Schematics

Additional Resources

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