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Re: Help with S-100 VDP board VGA output

The LMH1980 may be a better choice as it outputs Hsync and does't have the 2 x Hsync frequency, equalization pulses that may upset VGA monitors.
But it only comes in surface mount. This may be OK as we are already using one surface mount IC.
Regards Leon

On Monday, 2 April 2012 20:26:10 UTC+10, lynchaj wrote:
Hi Pontus!  Thanks!  Using an LM1881 is a great idea.  It is an 8 pin
DIP and commonly available part -- perfect!

I did not know about that chip but it seems perfect for this

Once we generate VSYNC it can be fed to the VGA interface logic.

The board won't be truly VGA compatible since the dot clock is too
slow (NTSC rates) but for some recent LCD monitors they can sync at
very low rates.

We've had decent success with this on the N8VEM Color VDU project and
it works well.

Thanks and have a nice day!

Andrew Lynch

On Apr 2, 3:29 am, "Pontus Oldberg" <pontus....@invector.se>
> Andrew,
> Couldn't you just use a LM1881 synch separator on the CSYNC signal. CSYNC
> should basically be just the signal without any video related data.
> /Pontus
> lynchaj skrev:
> > 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- Hide quoted text -
> - Show quoted text -