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Re: Help with S-100 VDP board VGA output

There may be a problem with HSYNC from the V9938.
The sync pulses are missing during the blanking period.

The data sheet shows a negative HSYNC mode on page 131 that continues through the blacking period, but I can't see how to get into this mode (my scanned copy is to low in resolution to see the writing below the HSYNC label on the left of the waveform) Can anyone help?


Top trace is CSYNC, bottom trace is HSYNC. Verticle scale is 2 V/Div.


On Monday, 2 April 2012 10:41:16 UTC+10, lynchaj wrote:


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