RFU

   Here are the schematics I made for the RFU. I actually built mine right into the Sega Master System itself. Using this information you can build one internally or for external use. I decided to use variable resistors so I could adjust the speed of the rapid fire. in some games like Rocky the rapid fire can be to fast or to slow. This way you can set it to the optimal speed for you. The best way to test this is to use a led between the ground and the output before hooking it up to the controller. This way you can get a visual of how fast the button is repeating. Good Luck!

The 555 Timer is still sold at Radioshack and the catalog number is 276-1723