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S100 VDP success

I’ve finally had some success getting video out of the prototype S100-
VDP board.

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 going 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 the screen.
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 need 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