JAMMAJamma stands for 'Japanese Amusement Manufacturers Association' but you don't really need to know that. Back in the early 1980's there were lots of videogames being made by lots of different companies. The progression away from dedicated cabinets and the manufacture of 'kit' games caused real headaches for everybody because there were so many pinouts and standards. The Japanese had a very good idea and gathered up the main companies involved and decided to come up with a standard connector that would interface the circuit boards with the cabinets. This way, if everyone stuck to the standard, it would be a lot easier to sell these new kits and games to operators becasue the operator knew that they could change the game over in minutes instead of spending a couple of days re-wiring the cabinets. The standard was agreed upon in 1985, but the first game did not appear made to the jamma standard until 1987 i believe (Double Dragon).
Standardization was a good idea, but its shortcomings today are apparent. Its not really fair to blame the JAMMA people for not being able to see into the future at what technology would bring, and what it would mean to a universal standard such as the one they came up with. In actual fact, i bet the JAMMA people thought 'well, if we make a standard, maybe it'll help everyone for the next 5 years or so' which in economic terms is a good idea, if the idea lasts that long, then its worth doing. 20 years later, and there are still games being made to the JAMMA standard, which is pretty good going, along the way though, the standard has been re-interperated and mis-interperated and its a good idea to know a few basics that are accepted as 'the norm' in collecting circles from games people collect.
The JAMMA standard uses a 56pin 0.1" pitch connector, 28 pins on the top, 28 pins on the bottom. So, JAMMA is a standard connector, this means if you buy or own a JAMMA cabinet, you can buy any JAMMA board, plug it in and it will work, and you can play it. Well, thats the theory anyway, 98% of the time this is true, but as time progressed, the games got a bit more advanced, and they needed more things to interface with parts of the cabinet. These mainly being an extra speaker for audio to give stereo, and the advent of 6 button games. Companies tackled these additions in different ways unfortunately, or they fudged the 'standard' jamma in such a way that with only a small mod to a cabinet you could get it working properly. It seems thatthe JAMMA people never re-convened to think about how to tackle these issues, so once again, it was up to individual companies to make up their own 'standards'. For the extra buttons, they usually supplied a 'kick harness' for operators to hook to the extra buttons on the control panel (these were usually the Kick buttons on capcom games, and hence the name sticks today). This would then run those inputs to a little connector that plugged onto the game pcb in addition to the jamma connector. The JAMMA standard did allow for a few extra buttons on top of the standard 3, they left some blank pins on the connector, this means that if your cabinet had buttons wired to these pins, it gives a maximum 5 buttons per player while still conforming to the jamma standard. This was ok for Mortal Kombat for example, and bootleggers of this game ran the buttons there (the originals used their own kick harness). SNK brought out their Neo Geo system which used 4 buttons, they simply used one of the unused pins for their 4th button, making integration into a jamma cabinet fairly easy.
Audio is a funny one. While the original jamma standard dictated 2 pins on the connector for audio, this only allowed for one speaker which was ok back in the day of '85. Simply hook the two terminals of a speaker to these two pins and you'll hear sound. When stereo games came along though, most manufacturers still wanted to stick to the jamma standard, so the extra audio was available on yet another additional connector on the game pcb itself, while still running mono to the jamma connector. Later on, some people thought why not put the two positive speaker signals to those two pins on the jamma connector, and instead instruct the operator to run each of these to the two speakers, with the other side of both speakers hooked to ground. et voila, stereo sound through the jamma edge. Problem is, plug one of these pcbs into a regular jamma cabinet and you wont hear a great deal apart from fuzz as what you are hearing is the difference between waveforms on the two channels, audibly this is just 'noise' and nothing else. Bootleggers are probably more responsible for bastardizing the standard, or for mis-interperating it than anybody, an example of this is where the positive speaker signal on the jamma connector should be on the top pin as the standard dictates, but some bootlegger swapped this round. Not usually a problem if you just have a speaker wired to both pins, but if you have a cabinet that runs just the audio signal from the top pin to a speaker and the other side of your speaker runs to ground, you wont hear anything.
So manufacturers and cabinet makers alike did things that they thought wouldn't matter too much, but can easily catch you out if you collect pcbs and don't understand the different ways in which they messed with things.
In the 1990's it got a little silly, and manufacturers figured that they might as well base their pcbs on JAMMA as that is the type of cabinet that the kit game they were making was likely to be put into, so the connector might say JAMMA, it might fit right into a JAMMA cabinet, but it certainly isn't JAMMA standard. I'm talking about games that used controls other than regular joysticks, like trakballs for example... Even later on, the standard was retained for games that used a different frequency of video display (medium resolution) the rest of the controls and power and stuff works, but the video runs at a higher rate, it was again, a way of retaining semi-compatability with existing cabs, and forcedan upgrade in monitor...
Anyhow, there's much more you'll find out about how jamma isn't quite so standard if you get into collecting a bit more, i hope i've covered the basics enough for you to be a little more informed.
Written by Andy Welburn on a rainy morning 13/07/06