clb wrote on 2021-07-25, 06:45:

I measure no resistance between pins 10 and 12, so I connected them with a 10kOhm resistor in between. […]
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I measure no resistance between pins 10 and 12, so I connected them with a 10kOhm resistor in between.

Doing that, and booting with nothing connected to the VGA port except the pins 10-12 via the resistor, the system boots with a yet third refresh rate:

hsync=31.46kHz,
vsync=58.92hz,
ossc detected mode=534-p
H. samplerate: 800, H. s.rate frac: .00, H. synclen: 96, H.backporch: 48, H. active: 640, V.synclen: 2, V. backporch: 33, V. active: 480, Sampling phase: 180 deg

This is the default VGA 640x480 resolution and timing. Your 87Hz timing also was at 640x480. On the other hand, VGA text mode is expected to run at 720x400. The sync frequencies do not tell you how many pixels you have horizontally, so I can't tell you whether this actually is 640x480, or it is 720x480, but the x480 is strange. A VGA card should run at 480 lines only in its highest resolution graphics mode. It should run at 400 lines in standard VGA text mode, and in CGA-resolution graphics modes (they are double-scanned from 200 lines). It should run at 350 lines when emulating EGA-compatible modes. Both the 400- and 350-line mode use the same vertical timing of 70 Hz (it's just that 350 line modes are stretched vertically to make the 350 lines fill the screen. The video signal of the 350-line mode just adds enough black lines to make it identical to the 400-line mode). On the other hand, the 480-line mode has a vertical timing of just 60Hz. There are a lot of VGA monitors that produce noticable flicker at 60Hz, so the 480-line mode is avoided unless needed.

I'm confused why your Paradise VGA emits a 480-line signal. Possibly the POST runs in 640x480 graphics mode or an extended 80x28 text mode that uses around 480 scanlines. But as soon as DOS is booted, the system should enter a 400-line mode. If you start DOSSHELL or Windows, you might be back to 480 lines. At what stage of the boot process did you take the timings?

The real reason that everyone is dancing around the "original" question is that video digitizers that can cope with high refresh rates (>70Hz) are pretty rare

maxtherabbit wrote on 2021-07-26, 02:05:

The real reason that everyone is dancing around the "original" question is that video digitizers that can cope with high refresh rates (>70Hz) are pretty rare

There is also the fact that running certain DOS games and demos at non standard refresh rates will cause unpleasant timing issues . For example, running Future Crew's Second Reality at anything other than the 70Hz rate it expects will slow down or accelerate the music . This is one of the initial reasons why I really wanted to (and luckily succeeded in) getting 70Hz (instead of 60Hz) working under DOS over DVI on my Geforce FX 5900 .