SegaMaster System (SMS) and MegaDrive joystick

Each button (left, right, button 1 ect.) is in fact a voltage(very small), this voltage is grounded to result in movement, fire, jump ect. For standard master system controllers this is directly feed to the system via wires only (some other controllers use a micro chip) they also are not connected to voltage, while some others do.

The Sega Genesis (in Europe sold as Sega MegaDrive) pads are an extension to the Sega Master System pads. They use more buttons (3+1, 5+1, 6+1). They use the one usnused pin as a select pin for reading more buttons.