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I have a Commodore PC 30-III with a Cirrus ISA VGA card in it.

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I wanted to connect to an HDMI display by using a mini VGA-to-HDMI converter

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The converter doesn't generate any output. The machine works when directly connecting to a VGA monitor. The adapter also works when I use with an i3 laptop. The manual says that the smallest resolution it can handle is 800×600. Can it be that this card has smaller resolution? Is there a way to see what is my screen resolution in DOS? What else can cause this?

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    This doesn't really look like a retro related issue if the adaptor (VGA card) used works fine with a contemporary VGA capable screen, but rather all about the converter used. Doesn't it? It should be more constructive to ask its manufacturer/distributor, shouldn't it? (P.S.: since you know that it can only do 800x600 and up, did you make sure that a fitting resolution is used?)
    – Raffzahn
    Sep 5, 2021 at 22:56
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    It seems to me this is similar to questions we get about modern hardware to connect floppy drives to modern PCs. They combine multiple aspects: knowledge of the hardware in question, but also knowledge of old video standards and requirements; so I reckon there’s some value in having them here. I’ve seen various adapters and monitors whose designers assumed that VGA was only 640×480 at 60Hz; their manufacturers wouldn’t be able to help someone trying to use them with a real VGA... Sep 6, 2021 at 7:11
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    No need to find out the resolution in DOS; when in text mode, DOS on VGA will invariably use 720x400 pixels of resolution (9x16 pixels per character, and 80x25 characters per screen) at 70Hz vertical scan rate, with a horizontal scan rate of 31.469 kHz, exactly twice that of NTSC TV. Unless you use some very unusual driver or mode.com command in your config.sys or autoexec.bat files that is.
    – TeaRex
    Sep 6, 2021 at 9:04
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    @Raffzahn “no issue with the real thing”, yes; but I still think it’s useful to understand why the adapter doesn’t work. Monitors don’t last forever, and there are already more people interested in running old computers than have monitors to hook them up to. I think it’s fair to assume that “no issue with the real thing” is true in most cases involving practical hardware use here — after all, if the hardware has survived until now, it probably worked back in the day! Sep 6, 2021 at 11:26
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    @PeterSzanto if you really want to get this system on an HDMI display, something like this might be more suitable. It's specifically designed to handle older graphic modes. As it's designed to be embedded in an arcade cabinet/etc though, you'll need a separate 5V 3A power supply and an enclosure if you want it to look pretty. Be aware there are some negative reviews of this particular product... but try searching "CGA EGA VGA to HDMI converter" and you might find something better.
    – Doktor J
    Sep 8, 2021 at 13:07

3 Answers 3

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The manual says that the smallest resolution it can handle is 800x600.

That is almost certainly the key. Original VGA supported 640x480 16-colors and 720x400 (essentially a small step up from the 720x350 monochrome (MDA) text, but with color). While VGA has come to mean "any video card and monitor that uses a blue 15-pin connector" that's not what it originally meant. Even when VGA (e.g., SVGA) started to support higher resolution, that was not, initially, directly supported by BIOS, DOS, etc. but rather extra modes with special drivers in Windows, some games, etc. I would not expect a vintage card to produce 800x600 (or higher) without loading some extra software first, and it might not do it at all.

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    I wonder how could I check what is the screen resolution in dos? Sep 6, 2021 at 5:05
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    @Peter that wouldn’t necessarily be all that useful, because DOS won’t know what the graphics adapter is actually sending to the monitor. This Q&A has links to a number of tools which might help you produce output which your adapter can use; try SVGATextMode in particular. Sep 6, 2021 at 7:18
  • @StephenKitt thanks it looks promising, I will try to research if any of those tools would work Sep 6, 2021 at 10:36
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    800x600 was specifically called "Super VGA" back in the day! Sep 6, 2021 at 19:28
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    Note that the number of colours should be irrelevant for this; VGA is analogue so number of colours is just how fine the steps between different levels are for the R, G, and B signal lines. (Or for a palette-based mode, how many different combinations of levels). That's totally irrelevant when the analogue signals are just digitized into 24bpp RGB. Same for whether those scan lines came from text mode or graphics mode. Not saying you shouldn't mention those factors in the answer, just adding detail for anyone that didn't find this obvious. Sep 8, 2021 at 13:26
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DOS is almost surely not outputting 800x600 or up, and is probably in the ballpark of 720x350 or thereabouts. Further, it's likely 70Hz, which this adapter may not like.

I'd recommend you get a display that can use the signal from card natively -- any old multiscan CRT and lots of early LCDs can do this -- but if you're determined to use this adapter, you could get a scan converter like a Sony DSC-1024G and scale the output of the card to 800x600 or up, and then feed that to this adapter. Or, you may well be able to find another VGA-to-HDMI doodad that will do the trick, but I don't believe in digital video, so I wouldn't know.

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    Unless you're using a CRT, digital video definitely believes in you!
    – Graham
    Sep 6, 2021 at 12:28
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The standard text mode for VGA DOS was 80 by 25 characters, which translated to a resolution of 720 by 400 (characters of 9 by 16 pixels) per the table on VGA text mode page in Wikipedia.

One option would be to change the mode and see if you can get the resolution high enough for your converter, but that would depend on the capabilities of the graphics adapter: Standard VGA didn't support 600 pixels vertically and the highest vertical resolution you can rely on working would be (640 by) 480.

However, most adapters could do better. You can try putting SVGATextMode in your autoexec.bat to see if that enables image passing through the converter. But caution! While the program lists Cirrus Logic compatibility, it doesn't list the CL-GD5401 (read from the chip).

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    On VGA, 80×25 text mode was usually 720×400 (9×16 pixel characters); that’s modes 2 and 3 in the table on Wikipedia. Sep 7, 2021 at 15:18
  • Thanks for the correction, will edit the answer. For some reason, I recalled that mode 10h would have been the default mode. Sep 7, 2021 at 15:28

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