I am reverse-engineering a daughter card which uses a ADV7125KSTZ140 VGA video DAC driven by a FPGA. The FPGA is a Xilinx Spartan 3E programmed by an off-board processor using slave-serial mode, meaning the daughter card does not require any firmware whatsoever. It also means I do not have any access to the code.
After probing around the board for a couple of weeks, I believe I traced out the entire schematic which can be found in PDF format here. Unfortunately, the board does not work. When plugged into the main device, this is the video output I am getting:
The "static" shown on the screen does not change, it is perfectly still, suggesting it's not simply noise. Unfortunately I have zero experience with VGA, so I am not sure what might cause this - bad data? Bad sync? Missing connections? I am hoping that someone with more experience in this area would be able to check my schematic and see if there are any blatant problems with the design. It is quite possible that I missed some connections during my probing, though all of the connections in the schematic seem logical to me.
If further information is required, please let me know and I will update the question.
After reading Dave's comments I took another look at the original board again this afternoon. I did some more probing, this time with a meter which had sharper probes. I was able to find several missing/mislabeled connections, which I have now updated. The new schematic can be found here. As I mentioned earlier, my /CE1 was not connected correctly. It should have been connected to /CE3 (mislabeled in the original as /OE3) and also to the MODE and ZZ pins of the SRAM. The connection between /OE and the input of the inverter was correct, but I had left out something very important - the driving source. This should have been connected to Pin 143 of the FPGA, which was originally incorrectly labeled as FPGA_HSWAP.
I do seem to be missing a driving source for the /BW pins though. /BWA, /BWB, and /BWE are all tied together, and pulled high through a 4.53k resistor. I cannot seem to find a connection to the FPGA though, or anything else on the board for that matter. I can't imagine the Byte Write pins can be held high permanently, can they? It seems to me in order to do much of anything with the SRAM you'd have to be able to write to it....
I am wide open to ideas and suggestions of what's going on here.