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  1. I've been mulling over the problem for a while now and I think I've figured out a solution. My proposed joystick uses two fire buttons, and a third button for SMS and 2600 homebrew compatability. Inspired by a post I made some time ago when someone posed the question if it would be possible to add a third button to the 7800 joystick for SMS support. I have devised a solution that would allow switching between 7800 and SMS function with the use of a third button. A toggle switch is required to switch modes. Most 2600 games will function in either position. Games which use paddles as an alternate input method will need to use 7800 mode. Homebrews that use the "C" button on a Genesis controller will need to use SMS mode. Read on for a technical explanation. Here we have 3 buttons A, B, and C. In 7800 mode, the controller functions like a standard 7800 controller. A is Left Fire and B is Right Fire. In 2600 mode, both fire buttons work simultaneously like a normal 2600 controller. In SMS mode, buttons A and B still activate input I or Fire as they would on a standard VCS joystick. However unlike real SMS controllers, both paddle lines are held high by the diode connected to pin 7 (Atari VCC) of the VCS, allowing homebrew games to detect the presence of a "Genesis" controller. Real SMS controllers are not connected on pins 5 and 7, and pressing II will only pull paddle pin 9 to ground, which will have zero effect because it is already held low by the Atari. Furthermore, certain SMS games are incompatible with Genesis controllers because they pull the select line (pin 7) low instead of high. This shifts buttons I and II to the Start and A button on the Genesis controller, and additionally causes some inputs on the Dpad not to respond, breaking the game. If used on a real Sega Master System, incompatible games should work normally. Genesis/SMS VCC (pin 5) will pull the II button high via the 10k resistor until it is pressed. Because the diode effectively prevents reverse current to pin 7 (select), the SMS joystick will have no effect on the state of Pin 7 as if it weren't connected to begin with. Because the 7800 buttons do not share a common ground with the directional controls or the SMS I and II buttons, they cannot be made to work as SMS buttons without extensive rewiring. As it stands, only a DPDT switch needs to be added to the circuit to switch operating modes. SMS setting and 7800 setting both function as a normal 2600 joystick on 2600 games that ignore the paddle lines. Using the wrong setting should not break anything. 7800 setting on an SMS game will function like a normal 2600 joystick and button II will not work. SMS setting on a two button 7800 game will behave like a Genesis controller with both buttons pressed continuously. Useful for Xevious if you are lazy and just want non-stop autofire; will probably not work for most other games. Comments? Thoughts? Any potential issues? I'll definitely have to try this design out if I ever decide to build a sequel to this baby: Now if only I could magically extend the box to the right by 1.5 inches...
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