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Are you sure, I can’t seem to see any inputs on the pin outs given at for example at https://md.railgun.works/index.php?title=VDP#Pinout https://md.railgun.work

Are you sure, I can’t seem to see any inputs on the pin outs given at for example at https://md.railgun.works/index.php?title=VDP#Pinout https://md.railgun.works/index.php?title=VDP#Pinout - there’s definitely an ROUT, GOUT and BOUT, but no RIN etc. They could be using the digital pixel bus that the Model C uses to get more colours from the VDP than a Megadrive, and merging that with other data streams from other video chips ala Supergrafx??? Its interesting though that the Megadrive VDP has features like the pixel bus and extra VRAM support to let them use the chip in arcade machines without some of the limitations the home system has because of cost.

mikepavone 4y ago
To be precise, the SMS and Gen/MD VDP versions of this feature work slightly differently from the TMS9918A version of it. The CSYNC output pin becomes an input and is used to synchronize video generation, but you need external hardware to mix the video together. This is also true of TMS9918A variants that output separate luminance and color difference signals like the TMS9928A. Generally this mixing is done with the analog video signals (RGB in the case of the SMS and Gen/MD, luminance/color difference for the TMS9928A), but you could use the digital pixel bus. The downside to that is you need external CRAM and video DAC. edit: Technically, the TMS9928A doesn't have the CSYNC as input part and just relies on the two VDPs running in lockstep.