Re: Help with S-100 VDP board VGA output

[lynchaj <lyn...@yahoo.com>]

Hi!  Thanks Pontus and Leon!  I appreciate your help!  This is great
progress!

I like to use http://findchips.com to help assess whether a part is
"commonly available".  Although there are some exceptions most of the
parts we use on the S-100 boards are DIP/PLCC PTH which helps with the
hobbyist construction.  Personally I have no problem with SMT but I
know some hobbyists have issues with the tiny feature sizes.  However
the small SOIC-8 parts aren't too bad since there are so few pins.  My
personal goal with SMT parts is to restrict them to only the optional
components like FMS6141 video amplifier.

The V9938 already generates an HSYNC signal so my plan at the moment
is to use the LM1881.  It is an older part and available in DIP-8,
very common, and relatively cheap.  All we need is the VSYNC signal so
the rest of the outputs can be left NC.

The way the S-100 VDP board is right now there are two paths to video
output.  Either use the V9938 generated composite video or the pseudo-
VGA that the BT478 RAMDAC will produce.  One of the major goals of
this prototype board is to determine if the BT478 will work with the
V9938.  The BT478 worked like a champ with the uPD7220 board we did
earlier and produces beautiful VGA compatible video.  Hopefully we can
get something similar with the V9938.

Leon, I think the way ahead on the prototype is to build the VGA
interface from the new schematic and splice in the LM1881 to produce a
VSYNC signal.  With some luck we should see some nice display graphics
and text!

Thanks and have a nice day!

Andrew Lynch

On Apr 2, 8:05 am, "Pontus Oldberg" <pontus....@invector.se>
wrote:
> There is also the GS4981 which is similar to LM1881 (similar in pinout as
> well) but it has the horizontal sync output instead of the field signal,
> and I could find it on ebay in DIP package.
>
> /Pontus
>
> Elsid skrev:
>
>
>
> > 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
> >> application.
>
> >> 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>
> >> wrote:
> >> > 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* =
> >> > > CSYNC* XNOR HSYNC*
>
> >> > > 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 -- Hide quoted text -
>
> - Show quoted text -