Good news, bad news for Apple IIgs fans: the Sync-inator does work on the GS when you have a monitor that can handle 15 kHz, but it doesn't work the way I'd expected. Composite sync splitting of the 15 kHz GS signal doesn't work very well, but proves to be mostly unnecessary anyway. Here's the GS/OS desktop in all its glory:
Chapter 1: I bought a Samsung SyncMaster 512N specifically because it was reported to work at 15 kHz with the IIgs, in this page at Manila Gear:
https://manilagear.com/vga-monitors-for-the-apple-iigs/ Unfortunately it didn't work. The 512N's activity LED would turn green and the backlight would turn on, but no picture would appear. The monitor's front panel buttons were also unresponsive, as if the monitor firmware were frozen. I tried this in composite sync splitting mode, as well as sync-on-green and csync-to-hsync modes, with the same behavior every time. The failure in sync splitting mode could be a flaw with the Sync-inator, but the other modes should definitely work since this monitor supports 15 kHz as well as SOG and csync-to-hsync (I confirmed the sync support with some other video sources).
So I started doing some digging. In the manual for this monitor, it lists 30 kHz as the lowest supported refresh rate, not 15 kHz:
https://www.manua.ls/samsung/syncmaster-512n/manual?p=67 Then I found a handy web page with a big list of older monitors and their 15 kHz support:
http://15khz.wikidot.com/ This page lists the 510N's 15 kHz capabilities as "partial". It doesn't have an entry for the 512N, but I think they're the same thing with and without audio speakers. The other SyncMasters in the same product series were also listed as "partial" or "no".
After doing more reading, this leads me to believe that 15 kHz is an undocumented feature that may only work on some 512N samples. Either Samsung changed what parts are used during the product's lifetime without changing the model number, or it's a question of engineering tolerances and manufacturing variability whether 15 kHz works or not. In my case, the answer seems to be "not".
Chapter 2: But hang on, that handy web page also listed the Asus VE228H as having full 15 kHz support, and that just happens to be one of the eleven monitors in my growing fleet of test equipment. So I hooked up the VE228H in sync-on-green mode, and it worked! I also tried the mode where csync is sent to the monitor's hsync input, and that worked too, even when I physically cut the green video line to be certain it wasn't sneakily using SOG.
Sadly the Sync-inator's csync splitting didn't work on the VE228H - mostly it just showed a blank screen or "out of range". That's OK because csync splitting isn't needed in this case anyway, since you can use one of the other sync modes instead. But it meant that something wasn't quite right with my 15 kHz sync-split signal, which might eventually be a problem in other settings with other monitors. I spent a long time fiddling with parameters and attempting to hand-tune to resulting sync signals to be as good as possible, and while I did succeed in getting the IIgs video to appear for brief moments, I never found a combination that worked reliably.
Just for grins, I reverted all my Sync-inator code changes back to December 20, when I had completely changed the way the sync splitting is performed, and that old December 20 version actually worked!
Chapter 3: So now what? I can't change all the code back to how it was on December 20, because the recent changes were made for good reasons for compatibility with many other Mac video sources and monitors. And I can't include both versions as a user-selectable option with a switch, because there's not enough room in the microcontroller flash and also because there are no spare DIP switches unless I do another hardware update.
But maybe I don't need to. Looking at that 15 kHz web page again, most of the comments for monitors that worked say that they were tested with a simple pass-through adapter, and not any fancy csync splitting. How common would it be to find a monitor to supports 15 kHz on its separate hsync/vsync inputs but that can't support that same signal as SOG or csync-to-hsync? Those comments would suggest it's rare.
I think I'll probably remove explicit IIgs support from the csync-splitting code, since it doesn't work in its current incarnation, and since most of the time you should be able to use another sync mode with the IIgs (assuming your monitor is up to the task). Removing the IIgs code would slightly simplify the main loop for csync splitting, enabling it to run a bit faster, which might also help in other areas. So the answer to "Does Sync-inator support the Apple IIgs" will be "yes it's supported with two different passive sync translation modes, if your monitor can handle it.
Chapter 4: With that, I think I've nearly reached the end of compatibility testing. I've added a feature that dynamically changes the microcontroller's system clock speed depending on the supply voltage it detects, which allows for slightly more precise csync splitting behavior when power is available. This has no affect on the other sync translation modes.
Current status:
- "Normal" Mac-to-VGA adapter behaviors are 100% working, including setting the desired video resolution through DIP switches, and choosing a passive sync translation mode. This matches the capabilities of other adapters like Belkin, etc.
- For troubleshooting help,. the csync and hsync/vsync activity LEDs are working, and the serial port output is working. The default DIP switch setting for sync mode is also "automatic", which aims to reduce guesswork and reconfiguration.
- For the Mac IIci and IIsi, composite sync splitting is 100% working on every monitor that I've tested. That was my main goal, and it's met. This enables the IIci and IIsi to work on a wider selection of monitors than before.
- For the Macintosh Monochrome Card, composite sync splitting is working out-of-the-box with >90% of monitors tested, and the only holdout works if you attach an external 5V power supply.
- I could only find two other Mac video cards that don't output separate hsync/vsync signals and that would benefit from csync splitting. The Toby Nubus video card works with >90% of the monitors tested, although two monitors worked by ignoring the csync splitting and using sync-on-green. One stubborn monitor would not work with the Toby. I would rate csync splitting support for Toby as 4.5 out of 5 stars.
- The SuperMac ColorCard SE/30 fared similar to the Toby, with csync splitting working with > 90% of monitors tested but with a couple of instances where the monitor reverted to using sync-on-green. It worked best at 640x480 resolution, and still worked well at 1024x768 although two monitors showed a slight shimmer at this resolution when the adapter is self-powered. The 1024x768 image was stable on those monitors if you attach an external 5V power supply. I would rate csync splitting support for this SuperMac card as 4 out of 5 stars.
- Most of the time you won't need csync splitting mode, because you're not using a crusty old video source with composite sync or because your monitor supports SOG or csync-to-hsync. Those cases are completely working, 5 out of 5 stars.
I still need to clean up a few loose ends here, but it's good progress and I'm finally starting to see the light at the end of the tunnel.