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Re: Help with S-100 VDP board VGA output

Hi Andrew,
The V9938 CSYNC and HSYNC are Tri-Level I/O - not tri-state
On the CRO the minimum for the pusle train is about 3V and the max is 5V.
Verticle scale is 2V/div, top trace 0Vdc is centre, bottom trace 0Vdc is first division up.
Also HSYNC is a positive pulse train that is not present during the blanking period.

The horizonal sync pulses on the CSYNC and HSYNC waveforms are slightly displaced from each other. ie the HSYNC positive pulse starts at the trailing edge of the CSYNC pulse. So a simple XNOR won't work.
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 - it may be with the S0, S1 which change the transparency of the video.(S0 and S1 are 0 in the test program)
Leon Byles

On Monday, 2 April 2012 11:22:59 UTC+10, lynchaj wrote:
Hi, it is possible that if CSYNC* = HSYNC* AND VSYNC* then VSYNC* =

Leon, would you please confirm that CSYNC* and HSYNC* pins on the
V9938 are TTL level outputs?

If they are TTL outputs and you happen to have 74LS86 XOR and 74LS04
inverter chips available would you please test to see if we can
reconstitute VSYNC* signal?

Thanks and have a nice day!

Andrew Lynch

On Apr 1, 8:41 pm, "Andrew Lynch" <LYN...@YAHOO.COM> wrote:
> Hi
> 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
>  Printing S100_VDP-sch.pdf
> 356KViewDownload