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RE: [N8VEM-S100:763] S100 VDP success
Hi Leon! Thanks! Wow! That's fantastic news! Woo Hoo!
Sorry for the late reply. Frankly I think I've forgotten who is even
working on the S-100 VDP but I *think* it is yourself, Neil, and Dave. That
makes sense since I still have two of the five prototype boards left. If
I've missed anyone I apologize. My mind has been elsewhere for the last
three months it seems...
I will make the corrections on the S-100 VDP board. This has a lot of
potential to be a really neat project. Having a working demonstration
really helps as well. Are you interested in building up a AY-3-8910 sound
generator chip circuit? I think that this board with a V9938 and an
AY-3-8910 would have a reasonable chance of some amount of MSX
We can clone the AY-3-8910 and joystick ports from the N8 design and make
adjustments to the IO decode logic.
Would the rest of the S-100 VDP team please give a quick update as to where
you are with the project?
Thanks and have a nice day!
> -----Original Message-----
> From: n8vem...@googlegroups.com [mailto:n8vem-
> s1...@googlegroups.com] On Behalf Of Elsid
> Sent: Monday, March 19, 2012 7:57 PM
> To: N8VEM-S100
> Subject: [N8VEM-S100:763] S100 VDP success
> I've finally had some success getting video out of the prototype S100- VDP
> The first thing I found was the data bus on the V9938 swapped around.
> That is, D7 was wired to CD0 not CD7, D6 was wired to CD1 not CD6 etc down
> to D0. I think this may have been an early error on one of the prototype
> N8VEM VDU boards also.
> I correct the Data bus swap around but still no video output on p21
> COMP_VIDEO or P22,23or 24 (RGB). I should say there was actually a
> composite video signal complete with color burst and synchs but there was
> no video component on the top half of the waveform. I tried another V9938
> - same result. I pulled out the BT478 to just try and get the V9938 part
> on its own. Still no luck.
> Finally last night Wayne posted that he'd found some problems were caused
> on the N8VEM Color VDU board when the VT82C42 is not reset. I looked at
> the VDP reset circuit and found that the V9938 is not receiving a reset
> because BRD-RESET* is being gated by SYNTMS* at U27C. SYNTMS* is reset
> at turn on by BRD-RESET*. - Thanks for the inspiration Wayne.
> I'm not sure what the function of SYNTMS* is. If it's to enable a
> programmable reset for the V9938 we may be better off inverting it and
> eliminating the inverter U11F after U27C. This should give us a reset from
> either SYNTMS or BRD-RESET.
> I was using a modified version of HW9918.COM to test the VDP board so I
> added a couple of lines to toggle the SYNTMS* signal to reset the
> V9938 in the initialization section and Bingo - "Hello World" popped up on
> I've put some photos and the test program on the N8VEM site at:
> for anyone who is interested.
> I set the address base to 90H but maybe it would be nice to change this to
> 98H (start of the V9938 registers) for MSX2 compatibility. I think it will
> some rewiring in the address decoding.
> Next step is checking out the BT478 and maybe VGA output.
> How's everyone else going with their VDPs
> Leon Byles