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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?

Stephen Kitt
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Peter Szanto
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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 05 '21 at 22:56
  • The datasheet suggests that it does support an 800x600x16 video mode. But that's probably not the default mode, so you'll have to find a way to get it into that mode. – Nate Eldredge Sep 06 '21 at 03:22
  • I tried to read that datasheet (and others) but couldn't find what is the screen resolution. Do you know how could I change the default screen resolution in DOS? – Peter Szanto Sep 06 '21 at 05:09
  • By default DOS uses the 80x25 text mode, which is a 720 by 400 format at 70 Hz. It might be possible to run some TSR to modify video format timings but then many progams like games won't work as they expect the default timings or set custom timings. If the adapter does not accept default timings from a VGA card then the adapter is unsuitable and you need a suitable adapter. Even if the adapter is suitable, it is also possible that your monitor is not suitable for the resolutions, they usually don't work with 70 Hz formats at all. – Justme Sep 06 '21 at 07:04
  • @Justme UltraVision was pretty good for this, but I don’t think it would go as far as producing 800×600-style timings for plain text mode. – Stephen Kitt Sep 06 '21 at 07:07
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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... – Stephen Kitt Sep 06 '21 at 07:11
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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 06 '21 at 09:04
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    @TeaRex not invariably; it’s not relevant for this question, but laptops typically use whatever their built-in screen supports, e.g. 640×480 for VGA laptops (which is why I wrote an 80×30 utility). – Stephen Kitt Sep 06 '21 at 09:15
  • @StephenKitt Yes and no. Here the fault is obvious at the new hardware component being non fit, as the configuration does work fine without. So it falls under the 'no issue with the real thing' clause, doesn't it? – Raffzahn Sep 06 '21 at 09:29
  • @StephenKitt I admit I never owned a laptop in the VGA era, so I didn't know that. Thanks for the correction. However I think for desktop VGA PCs - at least almost all of them - my statement is correct, right? – TeaRex Sep 06 '21 at 09:56
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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! – Stephen Kitt Sep 06 '21 at 11:26
  • @TexRex yes, for desktop VGA PCs your statement is correct (hence why my comment isn’t relevant for this question). – Stephen Kitt Sep 06 '21 at 11:27
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    @StephenKitt True. I guess it'S in part how to classify the question in the first place. After all, "if the hardware has survived until now, it probably worked back in the day", does still include the chance of no longer working, which should be quite on topic, vs. "Why does modern device XYZ not work", which is about that modern device, isn't it? More so when the question already states an out of spec usage? I'm not really sure what connection I miss (or jump). – Raffzahn Sep 06 '21 at 12:26
  • @Raffzahn right, perhaps the key to deciding whether this is on-topic or not is determining which device is being asked about. It seems to me that the modern device is a known quantity (as you say, part of its specs are given), the retro device isn’t (to the author of the question): “Can it be that this card has smaller resolution? Is there a way to see what is my screen resolution in DOS?” The answers are obvious to you, and to many others here, but that’s the whole point of this site: allowing people such as the question’s author to benefit from our accumulated knowledge. – Stephen Kitt Sep 06 '21 at 17:36
  • @StephenKitt: On the flip side, I would strongly suspect that if the display registers were configured suitably, the card would be able to generate a video signal that the converter could accept, though perhaps only at some weird resolutions (e.g. 240x600 16-color), but knowledge of how to do that would require somewhat arcane knowledge of how the card works. – supercat Sep 07 '21 at 15:15
  • @supercat yes, which is why I suggested SVGATextMode in a comment on one of the answers ;-). – Stephen Kitt Sep 07 '21 at 15:16
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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 08 '21 at 13:07
  • According to https://www.commodore-info.com/computer/item/pc30iii/en/mobile , it came with an ATI EGA Wonder 800+, which is in fact NOT a VGA card, but an EGA card. You should be able to find a compatible VGA card off ebay, just look for an 8-bit ISA card. Or, since this is just a text interface, pop in a network card and open up telnet. – Drunken Code Monkey Sep 08 '21 at 23:19
  • It really is this simple; the original VGA standard had 320x240 with 256 colours (and a couple of slightly higher resolutions, not commonly used), and you could step up to 640x480 if you were willing to only have 16 colours. Then something called "Super VGA" came about, and you loaded your VESA drivers, and you could get 640x480, 800x600, and 1024x768, all with 256 colours. But your CPU would melt depending on what game / software it was trying to run. – Domarius Sep 09 '21 at 05:45
  • @DrunkenCodeMonkey regardless of what the system originally had, it now has a Cirrus Logic VGA card, no need to get another VGA card. – Stephen Kitt Sep 10 '21 at 05:19
  • Thanks for all the answers. The question is still if there is any converter I could use. The one recommended above ( https://rads.stackoverflow.com/amzn/click/com/B078HH526F ) is a bit pricy but most importantly out of stock – Peter Szanto Sep 10 '21 at 09:48
  • I have an old LCD monitor which can show status information about the current resolution (both DVI and VGA). Sounds like such a device could be very useful here. – Thorbjørn Ravn Andersen May 28 '22 at 21:39

3 Answers3

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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.

manassehkatz-Moving 2 Codidact
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    I wonder how could I check what is the screen resolution in dos? – Peter Szanto Sep 06 '21 at 05:05
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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. – Stephen Kitt Sep 06 '21 at 07:18
  • @StephenKitt thanks it looks promising, I will try to research if any of those tools would work – Peter Szanto Sep 06 '21 at 10:36
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    800x600 was specifically called "Super VGA" back in the day! – Lawnmower Man Sep 06 '21 at 19:28
  • According to https://www.vogons.org/viewtopic.php?t=55520, GD-5401 should support 800x600 with proper drivers. – Razvan Socol Sep 07 '21 at 19:05
  • The 286 is lucky if it can even output 640x480. I would actually expect 320x240 or lower. – Nelson Sep 08 '21 at 01:55
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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. – Peter Cordes Sep 08 '21 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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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).

Geenimetsuri
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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. – Stephen Kitt Sep 07 '21 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. – Geenimetsuri Sep 07 '21 at 15:28