Modern video game consoles rarely have expansion ports, but in the 80s and 90s it was practically guaranteed. With the speed that hardware was advancing it made sense to build in some way to expand a system’s capabilities throughout its lifespan, like the memory port in the Nintendo 64 or the Sega CD and 32X attachments for the Sega Genesis. Some were ultimately unused as well, like the port under the Super Nintendo or, arguably, the interesting way that [decrazyo] figured out how to add graphics capabilities to the original Nintendo Entertainment System.
The basis of this upgrade is the fact that the Picture Processing Unit (PPU) on the NES has four pins that are grounded. These four pins tell the NES to display the background color if the pixel is transparent. Since they’re normally grounded, this means the NES can only display a limited background image, but there’s no reason these pins must be grounded. By using a second PPU configured to output graphics information and wiring it to these four pins on the first PPU, the NES can be given all kinds of new abilities, such as adding parallax effects to backgrounds, rendering more sprites, and showing more colors in the backgrounds.
Of course, the hardware requirements for this will require a donor NES to get the second PPU as well as the necessary memory chip for it, and we don’t recommend tearing apart perfectly good retro consoles for experimentation if it can be avoided. Presumably, you could use this open-source NES hardware alternative instead. But for those with the parts and the gumption, creating a demo or adding graphics features to homebrew games using this second graphics chip is within reach.
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