I made a cable that has all the available signals from the SNES output on it connected. I was fortunate that the SNES RGB connector cable I bought had all the pins populated, so I just had to hack it up and solder some wires to it. I terminated the other video signals with an RJ-45 connector. Here is the description of what I have currently:
EDIT: Removed no longer used.
I also got myself a scope and tested the RGB output channels. I made a basic test program to test maximum output levels. It looks like max of 170 mV for full 5 bits set color, which seems pretty low (I see 660mV online for RGB signals) (EDIT: I had attenuation settings wrong, its actually ~1.5 volts.). For real world testing I captured some waveforms and the results are kind of interesting. Rise times are pretty slow compared to pixel clocking rate. One pixel lasts ~200 ns and rise times can be approximately that long. Obviously this still works as the RGB output channel is used regularly but this something to keep in mind when comparing pixel color emulator. Here is a sample:
I still have the option to tap the digital RGB on the ppu, but I would really prefer if I didn't have to touch the console so I will press on with this for now.
Now I need a clock source, split the csync signal, and to convert to digital color.
It turns out I made the most basic of basic mistakes and didn't have my probe attenuation matching my scope setting, so the actual RGB voltage peak to peak is ~1.5 volts not ~0.15 volts, oops!
So now instead of attenuating to ~75% and being able to use differential input I have to attenuate to only ~33% to fit in the 500 mV peak to peak limit of the MAX1003, disaster! I tested this using basic potentiometers to divide the voltage down and got the above picture. The voltage signal is still technically DC so nothing weird happens, but the potentiometers must have some small internal capacitance because the voltage drop takes longer than for the original signal. I think this is still in time for pixel to pixel transitions, but it's pretty close.
Aside from that the signals are still pretty clean after attenuating so I will continue with my original plan.
I successfully managed to setup an lm1981 on a bread board and extract hsync and ysnc signals from the SNES. This is a very basic operation but was good practice for my first attempt at bread boarding stuff.
Obviously I can use hsync and vsync to count scanlines, which is half the information I need. Now I need a clock source for generating ppu clocks, which are not available from the display port. If I sync the clocks (and clock generation) using hsync, Then sending this to the ADC I should be able to get pixel level resolution.
It's slow going but so far so good.
