JAMMA projects

The guts of arcade video games... The term JAMMA refers to the universal connector which connects the PCB to the wiring harness of an arcade cabinet. It was agreed amongst most game manuafaturers that adopting the specification of the JAMMA connector would allow Site Ops to change their games without having to change the cabinet.
When a game is changed the entire PCB is swapped - the JAMMA cabinet, in theory, compatible with the new game.

Moonwalker Board Repair

UPDATED - 11th March 2007

The first of the bunch had obviously been kicked around a warehouse floor for a few years, there seems to be a reasonable amount of physical damage to the board probably because the previous owner had deemed it unrepairable.

The board is a Sega System 18 design with Micheal Jackson's Moonwalker ROMs installed, released in 1990.
The contents of the ROMs are encrypted to prevent bootleg games from being created using Sega's copyright code. It's the CPU's job to decode the ROM data using a key stored in SRAM, however the SRAM backup up by a suicide battery (see section) which is molded to the CPU - it's no doubt died already.

Missing ROMs and busted components

Damaged PCB tracks

The board outputs no sound or video upon power-up, and the main CPU appears to do nothing. I certainly wasn't expecting any sound from it since the sound CPU had been pulled.

With the uncertainty that the suicide battery had died, I ruled it out by replacing all of the ROMs with a new game - Shadow Dancer. This is the only game on System 18 that uses un-encrypted ROMs, so I programmed a replacement set, replaced the main CPU with a known functioning 6800, replaced the missing sound CPU with a new Z80 (thanks to G1SLE) and tidied up the rest of the PCB, replacing a few components.

Shadow Dancer conversion

The game now appears to begin booting, despite there still being no sound or video, a whining can be heard for several seconds through the less than isolated sound section. This certainly suggests the main CPU is initially doing something - most probably performing a checksum of the ROMs, before falling over.

Sunset Riders

Made by Konami in 1991, this is a much simpler single layered board. Like Moonwalker, it uses a 68000 CPU and a Z80 for the sound. There wasn't a great deal wrong with this one, the audio was reported broken which turned out to be poor contact volume pots. They'd obviously never been adjusted.

Sunset Riders PCB

Since this game didn't require -5V (usually required for the sound amplifiers), power was taken from a small ATX power supply. The board was connected directly to the SCART socket on a TV (see JAMMA on a TV)
Having converted my home-brew arcade controls to work with either MAME or JAMMA (see MAME page), this game is fully playable.

            

Various stages of the game running

Mortal Kombat 2

UPDATED - 11th March 2007

Mortal Kombat is considered to be one of the best beat-em-ups of all time, particularly in the Mortal Kombat series the second one is usually rated even over MK4.
I got the game off eBay for 99p, the description was plagued with NOT WORKING. Seems nobody fancied it.
I used a real arcade power supply since, unlike Sunset Riders, the game required -5V.

The board is two-layered - most of the ROMs are on the underside of the upper PCB

When powered up, the screen did indeed complain that most of the ROMs had failed their checksums - peculiarly all on the upper board... not so peculiar once I'd noticed it'd been plugged in with half the pins hanging over the edge of the socket! After I reseated the plug, without the obvious pin offset, the game booted successfully.
It does occasionally complain of bad ROM checksums so I suspect there are some dirty pins somewhere.
The sound section is on a separate PCB because of it's sheer complexity. The manufacturer, Midway, boast about the sound quality of their "DCS" system. It is impressive, especially considering everything is stored in a bunch of EPROMs.

MK2 working

There are twelve 8Mbit graphics ROMs, two 4Mbit game ROMs and six 4Mbit sound ROMs totalling 128Mbits - thats 16MB of data stored in solid state ROMs. Not something that sounds impressive these days, but you really need to put it into context. This is basic, low-level shit; the CPU runs at a steady 10MHz.

It really shows the beauty and efficiency of low level hardware and code design. Look at the hardware: most of it's discrete logic and memory, with a CPU in the middle of it all. The only reason a game like this would fail to boot up would be due to physical damage.
These days, software and hardware is designed from such a high level that things just fail because of bugs and incompatibilities between the hundreds of layers the code it's running on. When was the last time you saw a classic arcade game with a general exception error window floating over the top? High level programming promotes lazy engineering and a complete lack of understanding in the subject because you never actually know what's going on 'under the hood'.
This entire game was written in an assembly language for this exact architecture. Each software instruction controls some part of the hardware directly, rather than compiling it down to something that might do what you expected. There is no substitute.
It's the simplicity which makes it so complex. There will never be anything engineered in the future, in this field, as precisely and as skilled as this.

JAMMA on a TV

Since all JAMMA boards output RGB video signals, they can be easily connected to a standard TV via the SCART input.
Results are impresive, since the graphics are being displayed in their native format and resolution there is no aliasing or buffer tears you find when emulating an arcade board (see MAME)

Suicide batteries

Some manufacturers, including Sega, designed a custom package which surrounded the CPU. The epoxy sealed package contains a small amount of SRAM which holds a key needed to decode the encrypted ROMs.
This prevented anyone changing the ROMs in the board to another game, and also prevented the ROM code being copied to make bootleg games, since the entire content is encrypted.
A small battery is also enclosed within the package to stop the decryption key being lost from SRAM; since SRAM is volatile, it must be constantly powered to retain its data.

Standard 68000 processor

68000 processor sealed inside an epoxy time-bomb...

If any attempt is made to disassemble the package, in the interest of retrieving the decryption key, the battery would become disconnected and the key wiped.
This also means that after 10-15 years, the battery dies, and the key is lost.
The CPU is unable to execute the encrypted code so the game doesn't even boot up.

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