Power Base Converter
From Sega Retro
The Power Base Converter, known as Mega Adaptor (メガアダプタ) in Japan, Master System Converter in Europe and Adaptador Para os Jogos Master System in Brazil, is a peripheral for the Sega Mega Drive allowing it to play Sega Master System cards or cartridges.
One of the key design features of the Mega Drive is its compatibility with its immediate predecessor, the Master System, as the Mega Drive's design is based upon the Master System's design, albeit enhanced and extended in many areas. As the cartridge slot of the Mega Drive is shaped differently than that of the Master System, and because its games could not be played directly through the Mega Drive, Sega released the Power Base Converter, an accessory that is placed between a Master System cartridge and the Mega Drive, allowing the user to play the previous generation of Sega games without the need for an extra console.
The Power Base Converter does not contain any Master System components but acts as a pass-through port. The converter contained 2 slots. The top slot was for cartridge based games. The front slot was for card based games and accessories. The Power Base Converter would be fully compatible with the cost-reduced Mega Drive II, were it not for the different shape of the Mega Drive II's casing.
One can remove or modify the PBC casing, allowing it to work on a Mega Drive II. The alternative is the differently shaped Master System Converter II, though this accessory was only released in Europe. The Master System Converter II is also fully compatible with the original Mega Drive, but lacks the ability to run cards.
In order to achieve backwards compatibility, the original Master System central processor and sound chip (the Z80 and SN76489) are included in the Mega Drive/Genesis and the new Video Display Processor is capable of the Master System VDP's mode 4 (though it cannot run in modes 0, 1, 2, or 3, so cannot run SG-1000 games). Once a Master System game is inserted, the system's bus controller chip (later integrated with the I/O chip into a single multi-purpose ASIC) will put the Z80 in control leaving the 68000 idle.
Both 2-button Master System pads and standard Mega Drive pads can be used to play SMS games. Due to slight differences in how the Mega Drive pads operate, some Master System games may inadvertently cause the wrong set of inputs to be selected in a 3/6-button pad and prevent input from working properly. In this case a Master System controller must be used. Like the Master System, the PAUSE button is not part of the gamepad connector and instead is implemented as a pushbutton switch on the Power Base Converter or similar devices.