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RE: [N8VEM-S100:1301] Re: An

Hi Leon!  I really wish I could answer the question on the RAMDAC.  The true guru of the RAMDAC is John Coffman.  He did some serious miracle working on the ECB uPD7220 GDC board that uses the identical BT478 RAMDAC.  However in that case it was connected to a uPD7220 GDC generating VGA frequencies.


Is there some way we can rewire the board or are you saying the logic in the V9938 color bus is just different than the uPD7220 color bus?  It may be configurable with registers inside the V9938.  I can’t even say for sure they’ll work together. It was more of an educated guess from the previous boards.

Honestly Leon, I don’t recall seeing your previous message on the RAMDAC producing video.  When did that happen?  I can’t believe I missed it!  Can you take a photo of the screen?  Is it recognizable?  Either way, if the RAMDAC is working, even in an insane mode, that’s huge news!  Woo Hoo!

Thanks and have a nice day!

Andrew Lynch


From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of Elsid
Sent: Sunday, January 13, 2013 6:05 PM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:1301] Re: An


Hi Andrew,


I'm sorry I haven't given the S-100 VDP board much attention lately.

In my last effort working with John I saw video coming out of the RAMDAC. It was garbled so I need to sort out what is going on there. If you can delay the next prototype for a few weeks I'll do a bit of debugging to see if I can get correct video out of the RAMDAC.



See below for email re last effort:


I’ve managed to get video out of the RAMDAC. For real this time.

I was looking at the colour bus data going into the RAMDAC along with the CLOCK and BLANK signals.

The BLANK signal seemed to be running at about half the CLOCK frequency. This would mean all of the color bus data was not being accepted as valid.

The BLANK signal is p7 BLEO on the VPU. I bent the pin out and soldered a 1k pull up to the BLANK signal going to the RAMDAC so it was -continuously  inactive.-Now I have Video out p25,26,27 of the RAMDAC but it’s not exactly correct.-I suspect the colour bus data is not correct. I think in the video mode we are using, there is only 4 bits of valid data that are repeated on -the top 4 bits. Also the colour palette order in the VDP is G,R,B whereas it’s R,G,B in the RAMDAC. We may be able to correct these problems -using the Pixel mask register and a reordered RAMDAC colour palette, what do you think?


Leon Byles


On Monday, January 14, 2013 7:08:03 AM UTC+10, lynchaj wrote:

Hi!  The S-100 VDP V3 prototype is almost ready to go.  Are there any builders who would like to build and test this new version?

Were there any more changes from the last round of build and test (S-100 VDP V2)?


This board is a V9938 video display processor and AY-3-8910 sound generator.  It is roughly an MSX2 class color graphics and sound board.

I’d like to get some prototype boards if anyone would like to chip a bit for some. 


Thanks and have a nice day!

Andrew Lynch