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Help with S-100 VDP board VGA output


I’ve been working to update the S-100 VDP with the AY-3-8910 sound generator and convert to VGA output.


However, I’ve hit a major roadblock and need some help to move forward on this project.


It turns out the V9938 does not generate a separate VSYNC signal.  It generates HSYNC and a CSYNC signals separately however both CGA and VGA require separate VSYNC and HSYNC signals.


I imagine there is some relationship between CSYNC and HSYNC where you could synthesize VSYNC but after much studying the V9938 datasheet and numerous Google searches I don’t know how to do it.


Please take a look at the V9938 datasheet and let me know if generating VSYNC is possible and how to do it.  Without a separate VSYNC we lose any hope of even partial VGA compatibility!


Thanks and have a nice day!

Andrew Lynch

Attachment: Printing S100_VDP-sch.pdf
Description: Adobe PDF document