|June 3rd, 2006|
The Neo Geo is one of my favorite consoles of all time. If I were to pick one word to sum up the Neo Geo it would be “hardcore.” It’s the “no expense spared” design attitude, appreciating collector value and of course the unique and high caliber of games that makes it one of the hardest of the hardcore.
- Expensive/Powerful Processor (for the day)
- Ginormous game carts used as fast access storage
- Easy to hack in additional video/audio connectors of varying types
- Extra debug features are easy to gain access to with a simple mod
- Hardware has aged well (after almost 17 years there are still games trickling out)
- Parallel to arcade hardware
- One set of game EPROMs, many game versions
It was these last two that got me interested in a process known as “AES cart conversion” (or cart killing depending on your beliefs in the religion of Neo-Geoism). See, I really love a lot of Neo Geo games and to me (and many other fans) there is just something different about playing the real deal rather than a port or an emulated version. There are little bugs, quirky hardware timing issues and a generally more genuine feel of playing a game that was designed for the system that you just can’t reproduce. The problem is that the games for this system can get expensive, really expensive. Because of its collector status, Neo Geo games are only getting more expensive with time. This cost is also compounded with my situation of needing to buy AES (home version) carts which are considerably more expensive than MVS (arcade version) carts in most cases. A lot of people have just bought MVS machines to use but I like my AES system and don’t plan on hacking together an arcade cabinet anytime soon.
Metal Slug, featuring bosses like Saddam Hussein
The quintessential example that everybody points to when talking about expensive Neo Geo games is an original mint condition US AES version of Metal Slug which goes for around $3000 whenever it goes up on ebay or forums like www.neo-geo.com whereas the MVS version can be found for under $100 (but the price has been going up). Now I like Metal Slug, but not that much. I like it more to the tune of around $60, which brings us to cart conversions.
For the most part a Neo Geo game only has one set of EPROMs as was mentioned above. Different languages as well as both the arcade and home versions of the game are all stored in the same set of silicon. The hardware that the game is plugged into determines what mode and language the game plays in. This means that the only real difference between AES and MVS games is the PCB and controller chips that the data chips are soldered into. Thus the theory is that you can de-solder the EPROMs off of an MVS cart and stick them into an AES cart and run it in an AES machine!
I remember when I first heard the rumblings to that effect. Dreams of cheap AES games for all danced in my head. Things are not quite that simple but the fact is that the theory has been put into practice and it works! Upon learning that, my first objective was to get me a hold of some Metal Slug…
First I needed a Metal Slug MVS cart. A friendly interweb arcade parts shop sold me a US branded one for $40. Now I needed an AES cart to become the new host for my Metal Slug chips. This is where things start to get tricky. See, not just any AES cart will do. You need a cart that can hold all of the chips that you take off of the MVS PCBs. There are different types of data chips in Neo Geo games: C, V, P, M1, S1, etc… You need to make sure that your AES host cartridge (also known as a sacrificial or sac cart) has enough of each type of slot for you to put your MVS chips into. You can find out beforehand how many and what kind of slots your carts have from listings and pictures scattered over various sites on the internet. My sacrificial cart of choice in this case was 真サムライスピリッツ覇王丸地獄変 (Samurai Shodown 2) Japanese branding, which is one of the more popular host carts because it is relatively cheap, prevalent and has a layout condusive to many different conversions.
Now comes the long and grueling part. One by one I soldered off all the connections holding the EPROMS into their spots in both the MVS and AES carts. I then swaped the MVS chips into the corresponding slots in the AES board. Then re-soldered one by one every pin back into the AES board. This process requires a fair chunk of time so if you want to attempt something like this be sure to give yourself an afternoon for it. So now that that was done I had a working copy of Metal Slug right? Not quite. If I put it into my AES machine I got garbage. The problem was that I needed to change around some of the jumpers on the PCB to match the new EPROMS that were now present in the PCB. To find out which ones need to be on or off you can again look around on various internet sites or try to match the settings on the MVS and then try different settings until the game starts looking more like it is running correctly. Ok, so now I had found the correct configuration for Metal Slug running in a Samurai Shodown 2 cart and switched the jumpers accordingly. I popped it into my AES, pressed the on button and lo and behold Metal Slug booted up! I felt very satisfied that my hard work had come to fruition and that I now had an AES version of Metal Slug to play! Oh wait a minute… Everything plays fine and the graphics are perfect but the sound is messed up. Did I get a jumper setting wrong? After switching around a few of the jumpers that affected sound I concluded that any changes only made the sound worse and that I had already found the “best” possible setting. But it can’t be! I was so close to a perfect working copy! I decided to email the expert in the field of Neo Geo cart conversions and ask him if he had any advice. He responded by explaining the problem and giving me a really slick solution which I would have never thought up on my own. The problem was that even though Metal Slug and Samurai Shodown 2 use the same form factor for chips with the same number of pins and such, the sound chips for Metal Slug contain twice as much memory (32Mb vs 16Mb) as the ones in Samurai Shodown 2. Because the SS2 sound chips only contained 16Mb the PCB only had enough traces for the exact number of address lines that it needed. This meant that for Metal Slug’s sound chips, the PCB had exactly one less address line trace than I needed. Without that last address bit, anytime the game referenced data in the upper regions of the two sound chips the addresses would be incorrect and return the wrong data.
The solution came about from the fact that the SS2 PCB has four sound chip slots even though it only uses two. Only one of the four slots should be read from at any one time. It accomplishes this by setting the chip enable bits on each slot accordingly to turn off all but a certain slot when it wants to read data from the chip in that slot. Normally you would just put the two sound chips in the first two slots because that is what is normally done, but in the case of Metal Slug I put the first one into the first slot and the second one into the third slot. What we are essentially doing is hijacking the address space of the second and forth slots for our own usage of the chips in the first and third slots. When the slot enable bits for either the first or second slots we make sure that the enable bit for the first chip is on. Likewise when the enable bits for the third or forth slots are on we make sure that the chip enable for the second chip (which is in the third slot) is on. This accomplished by making use of some unused AND logic on one of the chips there in the center (74LS08). We then separately wire the second slot enable to the last address pin of the first chip and wire the fourth slot enable to the last address pin of the second chip. Now things should be good. I popped it in my AES and now I had a perfectly working copy of Metal Slug AES for the approximate amount of $40 (MVS Cart) + $10 (AES sac cart) + $10 (materials/tools).
(Note: If you are going to attempt this yourself and use my pictures for a guide, there is wiring on both sides of the PROGTOP2 board to accomplish the address space hijacking. Apologies for the crappy quality of both the pictures and the wiring 🙂 )
I am not a collector and really just like to buy games that have really awesome gameplay. But I must admit that I have an attraction to the rare and exotic. A game like Metal Slug for the Neo Geo has both allures. Also, the experience and satisfaction from building something makes it one of my favorite items in my collection.
Other than being able to get playable AES versions of games for cheap, another reason that motivates people to do cart conversions is when there is a game that never had an AES release. Some people like to add games like Puzzle Bobble (which only came out in the arcades) to their AES collections for the sake of having something really rare and special.
There are also other factors to consider in doing cart conversions. At one point SNK started using chips that had much smaller lead width and pitch which made them a lot harder to solder off. Around 2000 (with Metal Slug 3?) SNK also started encrypting the contents of their EPROMS and included hardware decryption chips on the boards. Both of these changes made finding suitable host boards and doing the conversions a lot harder.
I have completed one more cart conversion other than Metal Slug. I have also converted サムライスピリッツ天草降臨 (Samurai Shodown 4) MVS (Japanese) using a Fatal Fury 3 AES cartridge (Japanese). SS4 is my favorite fighting game of all time and while not quite as expensive as Metal Slug, it still costs a fair chunk of change. Knowing the board layouts, chip sizes, jumper settings and other issues beforehand made that conversion go pretty smoothly.
NOT an original Metal Slug cart
It should also be explained that in some circles cart conversion is frowned upon. One reason is that some unscrupulous people (more so in the early days of cart hacking) do cart conversions and then try to sell them with the impression that they are originals for the full collectors price. Nowadays when people sell converted carts they mention the fact that they are conversions upfront and price them more reasonably most of the time. Another reason that it is sometimes not looked on kindly is that in many cases when a cart conversion is done you come out with one unusable cart and game set. Sometimes you might be able to do conversions the other way (AES to MVS) but not always. This means that you have taken another Neo Geo cartridge out of the wild (cart killing) and carts are only getting scarcer as time goes on. Thus, some people have a feeling of wanting to preserve the endangered carts and keep them in circulation.
Cart conversions have proved to be a very satisfying and rewarding hobby for me and although I probably will not do any more, I am glad that I can now play two of my favorite games in their original versions as they were always meant to be played without killing my wallet to do so.