Last Updated: 4/17/2024
Article Credit: ShootTheCore, Furrtech, cpsystem3,Eduardo Cruz, ExodusEmulator, Caius, Frank_fjs, twistedsymphony, Jotego

Main Board

The Sega System 16B main board contains all the processing logic to run the game. The Sega part number for it is 171-5347.

Note that these boards are NOT JAMMA pinout and need an adapter to be properly connected to a JAMMA harness.

Depending on the protection scheme used for the particular game, the Main CPU may utilize encryption, the Sound CPU may utilize encryption, and an MCU may or may not be present – see Protection Measures below for more details.

Notable ICs

IC	Function	Location
Hitachi FD1094 CPU (encrypted) Signetics SCN68000C8N64 68000 CPU (unencrypted)	10.0 Mhz Main CPU	C9
NEC MC-8123B CPU (encrypted) Zilog Z8040004PSC Z80 CPU (unencrypted)	4.0 Mhz Audio CPU	D15
Yamaha YM2151	FM Audio	D12
Yamaha YM3012	Audio DAC for the YM2151	D18
NEC D7759C	Audio Speech Synth	D16
Sega 315-5195	Memory Mapping	D4
Sega 315-5196	Sprite Generation	I4
Sega 315-5197	Tilemap Generation	I18
Intel i8751	MCU (Copy Protection)	B2

ROM Board (credit: ExodusEmulator)

There are four variants of the ROM board used with System 16B:

• 171-3538: ROMs only. Support for 32K/64K ROMs
• 171-5521: Banking register added to the middle. Support for 512K/1M/2M ROMs
• 171-5704: Jumper pins added to control banking. 512K/1M ROMs
• 171-5797: Adds Sega 315-5250 68000/Z80 mapping interface and an additional banking register. 512K/1M ROMs

Source: ExodusEmulator

Daughter Boards (credit: ExodusEmulator)

There are two daughter boards used with some System 16B games for additional control IO:

• 171-5437 / 834-6180: Trackball input
• 834-6523: 4 Player input

Source: ExodusEmulator

Board Variants

The Sega System 16B board was identical between regions for the full lifecycle of the hardware. However, it was heavily bootlegged and thus there is a wide variety of cloned boards. Compared to the original games, cloned boards may have differences in graphics, sound, gameplay and speed. The bootlegs look dramatically different from the Sega originals and are easy to distinguish from the real thing.

Common Issues and Quirks

System16B boards are NOT JAMMA pinout and need an adapter to be properly connected to a JAMMA harness.

System16B boards draw more power than average, so adjust your arcade PSU accordingly when swapping one into your cabinet or SuperGun.

It is common for the battery in the Main CPU or Sound CPU to die if they are encrypted, causing the game to either not boot or not have sound respectively. See “Copy Protection” below for details and resolution.

It is common for the TL084 OpAmp located at D19 to fail, resulting in faint or garbled audio output. Replacement parts are easy to source.

Capacitor List (credit: Frank_fjs)

Location	Capacitance	Voltage	Part No.
C2	470 uF	16 V	ECA-1CM471B
C3	470 uF	16 V	ECA-1CM471B
C4	470 uF	16 V	ECA-1CM471B
C6	1 uF	50 V	ECA-2VM010B
C7	1 uF	50 V	ECA-2VM010B
C8	1 uF	50 V	ECA-2VM010B
C9	1 uF	50 V	ECA-2VM010B
C10	470 uF	16 V	ECA-1CM471B
C16	470 uF	16 V	ECA-1CM471B
C11	10 uF	16 V	ECA-1CM100B
C12	10 uF	16 V	ECA-1CM100B
C13	220 uF	16 V	647-URS1C221MPD1TA
C14	100 uF	16 V	ECA-1CM101B
C15	100 uF	16 V	ECA-1CM101B
C19	47 uF	16 V	ECA-1CM470B
C62	2.2 uF	50 V	ECA-2AM2R2B
C63	2.2 uF	50 V	ECA-2AM2R2B
C21	2.2 uF	50 V	ECA-2AM2R2B
C22	2.2 uF	50 V	ECA-2AM2R2B
C34	220 uF	16 V	647-URS1C221MPD1TA
C35	0.1 F	5.5 V	80-FYD0H104ZF or 80-FG0H104ZF

ROM Details (credit: MAME)

Due to the variety of ROM boards used across the System16B games, consult the MAME Driver for game-specific ROM assignments.

When replacing a mask ROM with an EPROM, consult this reference chart by twistedsymphony:

Protection Mechanisms

Sega implemented four different protection mechanisms to prevent arcade operators from swapping one System 16B game to another by installing a new set of ROM chips. Games may implement one or more of these protection mechanisms:

• 68000 Main CPU Encryption
• Z80 Sound CPU Encryption
• Intel i875 Memory Control Unit
• ROM Board variants

Main CPU Encryption (credit: Eduardo Cruz)

Some games encrypt the 68000 code executed by the Main CPU. Those games use a Hitachi FD1094 CPU, which is a Motorolla 68000 clone with a RAM-based decryption key that matches up with the encryption key used on Main CPU ROMs. If the battery that maintains the RAM storing the decryption key dies then the key is lost and the main CPU will crash at boot – the game will sit at a black screen at boot (“suicide”).

If the battery hasn’t died yet, it can be replaced while maintaining the onboard decryption key if power is maintained to the supply pins with a bench PSU while the old battery is desoldered and a new battery is soldered in.

If the battery has died, you have two options:

• Replace the battery on the FD1094 CPU and reprogram it using an Arduino and the procedure documented by Eduardo Cruz:
  http://arcadehacker.blogspot.com/2018/12/sega-system16-security-programming-guide.html
  https://github.com/ArcadeHacker/ArcadeHacker_Sega_Hitachi
• Swap the Hitachi FD1094 CPU with a standard 10 Mhz Motorolla 68000 CPU and swap the Main CPU ROMs with decrypted equivalents on the ROM board.

Source: Eduardo Cruz

Sound CPU Encryption

Similar to the Main CPU, some games utilize an NEC MC-8123B CPU with an onboard decryption key maintained by battery-backed RAM, paired with encrypted Sound CPU code in the ROMs. If the battery inside the CPU dies, the decryption key is lost, the Sound CPU will crash at boot, and the game won’t have sound.

Solutions are identical to an encrypted Main CPU – if the battery hasn’t died yet, it can be replaced by removing the cover, powering the battery leads from a bench PSU, and soldering in a fresh battery. If the battery has died, swap the NEC CPU out for a standard 4.0 Mhz Z80 CPU, swap the Sound CPU ROMs out for decrypted equivalent EPROMs, and the game’s sound should then function again.

Source: Eduardo Cruz

Intel i875 Memory Control Unit (MCU) Protection

Some games utilized an Intel i875 MCU (or clone) that sits between the CPUs and specifically encrypted ROMs that “scrambles” the memory addressing such that a game won’t function without a matching ROM set and MCU.

Decrypted ROM sets for every System 16B game are available so that the MCU socket can be left empty on the board, and the encrypted mask ROMs can be swapped with decrypted EPROMs to get a fully-functional game.

ROM Board Variants

As mentioned above, Sega utilized four different ROM board variants with the System 16B hardware across the game library. Assuming that no other protection mechanisms were in place, an arcade operator still wouldn’t be able to change one System 16B game over to another one by swapping ROMs unless the same ROM board was used with both games.

MAME documents the ROM board and configuration settings (if applicable) on the ROM board for each game.

Schematics & Datasheets

Original Sega schematics are available for System 16B, but there seem to be discrepancies between them and the real hardware.

Sega System 16B Schematics

Charles MacDonald wrote up extensive documentation on System 16B’s operations.

Charles MacDonald System 16B Hardware Notes

Jotego authored an FPGA core for System 16B and created a set of schematics for the 315-5195 custom IC in the process.

Jotego 315-5195 Schematics (Link)
Jotego 315-5195 Schematics (PDF)

Additional Resources

MAME Sega System 16B Driver	https://github.com/mamedev/mame/blob/master/src/mame/sega/segas16b.cpp
Eduardo Cruz: Deconstructing Sega’s System 16 Security	https://arcadehacker.blogspot.com/2019/10/deconstructing-sega-system16-security-part1.html https://arcadehacker.blogspot.com/2020/04/deconstructing-segas-system-16-security-part2.html
Eduardo Cruz: Sega System 16 / 18 / 24 / X Security Programming Guide	http://arcadehacker.blogspot.com/2018/12/sega-system16-security-programming-guide.html https://github.com/ArcadeHacker/ArcadeHacker_Sega_Hitachi
Exodus Emulator	http://techdocs.exodusemulator.com/Arcade/SegaSystem16/index.html
SOLID-orange – Twistedsymphony’s Projects	http://solid-orange.com/
Jotego System 16B FPGA Core	https://github.com/jotego/jtcores/tree/master/cores/s16b

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