Sync polarity(both H and V) is resolution dependent as it was used to tell the monitor which timings to switch to in the EGA and VGA era, before SVGA came into being. Most multisync monitors don't care about sync polarity and adapt automatically.

You can build a circuit using XOR gates + RC time constant to make a circuit that adapts polarity automatically for both line and frame sync signals so that the generators will work with anything tossed at them, or add an inverter and switch for manual operation.
The RC time delay determines how fast the circuit reacts to resolution changes. Circuits online tend to use really big value capacitors which makes them quite slow.

Thanks. When the Windows driver is loaded the card appears to output TTL level vsync and averages to 3.66v according to my multimeter. When in DOS, its output appears to switch to line level vsync and averages to 0.38v. I don't have an oscilloscope to verify the exact peak-to-peak signal levels, but TTL sync should be 5v.

Is there an interrupt or register that can be changed in MS-DOS mode to switch vsync to TTL level, maybe via a TSR?

https://imgur.com/a/q3wIPRS

If you have the ability to make simple devices yourself, you can use the option offered here - Simple DIY scanliner . This device supports any resolutions and scanning frequencies automatically.

Titan91 wrote on 2024-12-25, 03:09:

Thanks. When the Windows driver is loaded the card appears to output TTL level vsync and averages to 3.66v according to my multimeter. When in DOS, its output appears to switch to line level vsync and averages to 0.38v. I don't have an oscilloscope to verify the exact peak-to-peak signal levels, but TTL sync should be 5v.

Is there an interrupt or register that can be changed in MS-DOS mode to switch vsync to TTL level, maybe via a TSR?

https://imgur.com/a/q3wIPRS

the 0.38V shows that the sync is positive, where sync pulses go to 5V. Since the pulses are relatively narrow, that is why the average voltage is so low. In windows the sync polarity is negative instead with 0V going pulses. Multimeter will also struggle as freq increases since it is meant to show you DC level or 50/60Hz AC voltage. You'll only be able to read 5V with negative sync at very narrow duty cycles. Anyway the figures seem to be ok.

To change sync polarity, one of the VGA registers will have to be reprogrammed on resolution changes : http://www.osdever.net/FreeVGA/vga/extreg.htm#3CCR3C2W
A TSR can do it when a game uses BIOS function to change resolution, but it won't work for all games since there are games that will program stuff themselves.
Only bulletproof solution is at hardware level, either as an automatic sync polarity adaptation circuit or manual sync polarity selection.

Thank you both for the input. For anyone wanting to get the intended scanline look themselves, I was finally able to figure this out. I used the LCD640 TSR from this thread to invert the vertical sync polarity.

wbcbz7's MS-DOS VGA TSR goodies

https://imgur.com/a/4IlCSdz

With vsync now set to negative, the same as in Windows, the SLG3000 v2 scanline generator now works properly in 320x200 VGA mode DOS games. The results are great! I just created a BAT file and called LCD640.COM before starting the game. It also uninstalls the TSR when I quit the game.

https://imgur.com/a/8720TyS