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NES Controller to Master System/Atari 7800


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I created a schematic for a 2-in1 joystick, but due to the fact the 7800 fire buttons share a separate ground plane from the joystick, and the SMS fire buttons share the same ground plane with the joystick, a third button is required. I don't think it would work well as a pad hack, but here is my design:




This uses a DPDT switch to change the function. The 7800 buttons act as dual fire on 2600 and both function as button 1 on SMS, necessitating adding a third button for button 2 in SMS mode. In addition the schematic adds a diode and pullup resistor which holds pins 5 and 9 high when connected to Atari in SMS mode, enabling 2600 homebrew that detects Genny controllers to utilise the extra button.


But sadly I don't see cramming this into a controller pad conversion, although a 3-button Genesis would be the perfect candidate. It may be easier to do separate controller mods for NES-to-7800 and NES-to-SMS and use a stock Genesis controller for the 2600 homebrews.


The diode and pullup resistor from my 2-in-1 design could be incorporated into the NES-to-SMS mod to allow 2600 homebrews to read the second button (the homebrews that read Genesis will not work with stock SMS due to the lack of pullup resistors), although I have not created a wiring diagram for this nor tested the design.


Unless you loathe using a Genesis controller to play SMS/Atari, it can be made compatible with the few SMS games that don't respond well to Genesis by disconnecting SELECT pin 7. You could wire a SPST switch inside the 3-button Genesis controller to restore pin 7 when playing Genesis games so you don't lose use of the controller. Alternately you could sever this connection using an extension plug. Due to a programming bug, certain SMS games pull the Select line low which switches B/C inputs to Start/A and disables half of the Dpad. Select pin 7 is not connected on SMS controllers by default so the state of this line should in theory have no effect on gameplay, but in practice it will screw up proper operation when a Genesis controller is connected.


Pin 7 needs to be connected for the Genesis controller to work on a 2600 because that is Atari VCC and the logic chip needs a power source to function. Pin 7 Select is connected to Sega VCC pin 5 via a pullup resistor which somehow passes enough current for the controller to operate.

Edited by stardust4ever
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  • 4 months later...

That's not going to work at all since the NES controller is multiplexed and you'd need to build a decoding circuit so the button inputs will actually register. This page describes how to use an NES controller on a PC gameport, but the principles are pretty much the same: http://www.tredonox.com/temp/nescontrollerdecoder/NEStodiscreetadapter.htm

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  • 2 years later...

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