Good sleuthing on the VG900b. My sample certainly doesn't appear to accept SOG, so maybe it's a firmware issue. I had two more thoughts about the VG900b troubles, based around the idea it might be a signal quality or signal timing problem. Remember how I found a glitch at the beginning of most HSYNC pulses in the LM1881's CSYNC output? Well, there's actually no reason for me to use the LM1881 CSYNC output at all, since it should be identical to the Mac's CSYNC output. I can just pass the Mac's CSYNC directly to the VGA connector, and if that signal's a little cleaner, maybe it will help. I also thought about why the glitch might exist. I am feeding a 5V CSYNC signal into the LM1881 input that expects a 1.5V peak-to-peak composite video signal, and that has an absolute maximum rating of 3V. Maybe that's causing some glitching (and possibly some damage). I'll try using a voltage divider to reduce the CSYNC signal from the Mac.
I'm feeling under the gun to figure out this stuff asap, since all three of these fussy monitors are slated to be given away or sold in 9 days at Mactoberfest. I won't have anything to test against after that. It's not very likely that I can design a PCB and have it assembled and tested in 9 days.
For troubleshooting purposes, it would be nice to have LEDs that show if there's activity on CSYNC, or on HSYNC/VSYNC, or neither. I need a way to detect if those signals are changing, and not simply stuck high or stuck low. I'm thinking of using a monostable multivibrator 74HC123 for this, configured to output a short high pulse every time there's a rising edge on one of the sync signals. As long as the sync signal keeps changing, the '123 will keep getting triggered and will produce a continuous high output that can power an LED.
Then I started thinking about how else this activity detector might be used. If I have a way of detecting activity on the Mac's sync pins, then I could also use that info to auto-configure some of the VGA connections instead of requiring DIP switches. If there's activity on the Mac's HSYNC/VSYNC, then for sure we want to connect those signals to the VGA HSYNC/VSYNC. If there's no activity on the Mac's HSYNC/VSYNC, but there is activity on CSYNC, then for sure we want to connect the Mac's CSYNC to VGA HSYNC. This would eliminate the need for two or three DIP switches, but at the cost of more circuit complexity. Not sure it's worth it.