[gwbas1c]

What I'd like to know is: Why did Nintendo allow the PPU to pass along another pixel color, but didn't take advantage of it in a shipping product?

Is this a case of "you ain't gonna need it" overengineering; or was the PPU used in other products. (And thus these pins were used elsewhere?)

[ndiddy]

It’s a feature that doesn’t take much die space to implement, so if they didn’t need those pins otherwise I don’t see why not to add it. If it turned out that the PPU wasn’t good enough for what developers expected, this would have let them make a quick follow-up console as a stop-gap with little R&D expense required. If you want an actual example of overengineering on the NES, they put a giant custom connector on the bottom of every system that never ended up getting used for anything. They probably wasted at least a couple bucks per unit on that.

[abbeyj]

They might have been inspired by a similar feature in previous chips, like the external video support in the TMS9918: https://en.wikipedia.org/wiki/TMS9918#External_video.

If they had extra pins that they had no use for, I'm sure this would have seemed like a very easy and cheap addition. You take 4 unused pins and add 4 pulldown resistors. Then when you go to draw the background, instead of using index 0, you take the value for the index from those pins.

Maybe they planned to use this in arcade hardware, where you'd have a bigger budget than a home console and could afford two PPUs. Then you'd get more colors, and you could scroll the background layer independently from the foreground layer. I believe they later added support for independent layers on the SNES hardware so this type of thing was probably already in demand from game designers.

[NobodyNada]

The PPU (and variants of it) was used in quite a few arcade machines, in addition to the NES.

I don't know if there were any actual machines that used dual PPUs, but the functionality was likely intended for creating an arcade machine with dual-layer background graphics.

[ndiddy]

The RGB PPUs used in arcade machines didn’t have the master/slave functionality, they instead used those pins for the analog RGB output.

[erik]

There is one obscure product that actually did use the feature. The Sharp Famicom Titler (or Famicom Editor) was a full Famicom that could show an external video input behind the Famicom graphics.

I found this video that shows a Playstation running in the background of Super Mario Bros: https://youtu.be/TCsle-J9YzY?si=oyj_zZCKGionnzLu&t=423

[ndiddy]

The EXT pins only allow setting the backdrop color to one of the palette values in the master PPU's color palette, so the Titler can't use that functionality to superimpose graphics over arbitrary video. It actually uses an RGB PPU which doesn't even have the EXT pins. Instead, it uses a separate video encoder chip for the video superimposing feature. https://www.nesdev.org/wiki/X0858CE

[erik]

Oh, interesting. I assumed the PPU could pass through video the way the TMS9918 can. My mistake!