JAMMA is the 56 pin arcade game wiring harness adopted by the Japanese Arcade Machine Manufacturers Association.

The vast majority of arcade games made from 1986 until 1998 use this wiring harness, although some games will have an additional harness as well (they are usually refered to as JAMMA+, although "JAMMA+" is not a standard at all).

The one truly awesome thing about JAMMA is the fact that it is Plug and Play. You can take your Street Fighter 2 machine and plug in a Final Fight board and have a whole new game in seconds. (Just like you would with your Sega or Nintendo console system).

There were literally thousands of JAMMA compatible games produced. The boards that they use can be purchased very cheaply on Ebay. Buying a JAMMA arcade cabinet means that for a couple hundred dollars you can have 20 or 30 games on your arcade machine instead of just one. Not to mention the fact that the NEO GEO motherboard is also a JAMMA board. With the addition of one of those you have access to over a 100 Neo Geo games. (Plus they still produce new Neo Geo games in limited numbers).

A few games break the JAMMA standard while still using it. For example Arlington Horse Racing uses the JAMMA connector, but the controls are completely wrong, so you must rewire your control panel. You should also be on the lookout for games that have a 56 pin harness that isn't JAMMA (like some older Capcom games), plugging one of these into a JAMMA harness will probably damage it.

The Jamma Pinout

   ====================================================================
            Solder Side            |             Parts Side
   ================================|===================================
              GND              | A | 1 |            Ground                
   ----------------------------|---|---|-------------------------------
              GND              | B | 2 |            Ground   
   ----------------------------|---|---|-------------------------------
              +5V              | C | 3 |            +5V   
   ----------------------------|---|---|-------------------------------
              +5V              | D | 4 |            +5V   
   ----------------------------|---|---|-------------------------------
              -5V              | E | 5 |            -5V   
   ----------------------------|---|---|-------------------------------
              +12V             | F | 6 |            +12V    
   ----------------------------|---|---|-------------------------------
              Key              | H | 7 |            Key      
   ----------------------------|---|---|-------------------------------
              Meter 2          | J | 8 |            Meter 1          
   ----------------------------|---|---|-------------------------------
              Lockout 2        | K | 9 |            Lockout 1           
   ----------------------------|---|---|-------------------------------
              Speaker -        | L | 10|            Speaker +          
   ----------------------------|---|---|-------------------------------
              Audio Ground     | M | 11|            Audio +
   ----------------------------|---|---|-------------------------------
              Video Green      | N | 12|            Video Red     
   ----------------------------|---|---|-------------------------------
              Video Sync       | P | 13|            Video Blue        
   ----------------------------|---|---|-------------------------------
              Service Switch   | R | 14|            Video Ground         
   ----------------------------|---|---|-------------------------------
              Tilt Switch      | S | 15|            Test Switch         
   ----------------------------|---|---|-------------------------------
              Coin 2           | T | 16|            Coin 1           
   ----------------------------|---|---|-------------------------------
              2 Player start   | U | 17|            1 Player start       
   ----------------------------|---|---|-------------------------------
              Player2 Up       | V | 18|            Player1 Up 
   ----------------------------|---|---|-------------------------------
              Player2 Down     | W | 19|            Player1 Down         
   ----------------------------|---|---|-------------------------------
              Player2 Left     | X | 20|            Player1 Left           
   ----------------------------|---|---|-------------------------------
              Player2 Right    | Y | 21|            Player1 Right        
   ----------------------------|---|---|-------------------------------
              Player2 Button 1 | Z | 22|            Player1 Button 1
   ----------------------------|---|---|-------------------------------
              Player2 Button 2 |AA | 23|            Player1 Button 2         
   ----------------------------|---|---|-------------------------------
              Player2 Button 3 |AB | 24|            Player1 Button 3           
   ----------------------------|---|---|-------------------------------
             *Player2 Button 4 |AC | 25|           *Player1 Button 4        
   ----------------------------|---|---|-------------------------------
              Not Used         |AD | 26|            Not Used
   ----------------------------|---|---|-------------------------------
              Ground           |AE | 27|            Ground         
   ----------------------------|---|---|-------------------------------
              Ground           |AF | 28|            Ground           
   --------------------------------------------------------------------

* This pin is not officially part of the JAMMA standard, it was 
originally unused. However SNK began using it for button four 
with the Neo Geo system, and others soon followed suit.

A few games will use the unused pins for other functions, and some games (like I mentioned above), will break the standard when it comes to controls. Most driving, light gun, and trackball games break the controls standard as well. The standards call for optical controls to be wired up through a separate connection, but some games wire them through the JAMMA harness.

When you purchase a JAMMA cabinet, you may find that not all of your harness is wired up, button 3, button 4, test switch, tilt switch, key, and service switch are often left unwired. You should try and remedy that, just because the game board that is currently in the cabinet works correctly without being fully wired, does not mean that every title will. Believe me, nothing is more frustrating then being unable to play a game because your service switch isn't wired. Many titles require it to fully boot if the battery on the board dies.

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