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I understand it is probably more trouble than it is worth, but I am keen on using my 4863 IBM monitor as a second display for my x230. So far I've looked into converting its VGA signal to a CGA output, then using the CGA adapter for the 4863, but not sure how feasible this is as a solution. Any help?

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    It's likely that the 4863 will need very specific timings, so as long as your OS on the x230 can produce these, you should be fine. A VGA to CGA should be unproblematic.
    – dirkt
    Jul 26, 2023 at 4:22

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Not very feasible.

You would first have to capture that VGA signal - how exactly depends on how do you want the result to look like, but either with a real video ADC or just some comparators to turn analog RGB signals into three RGB on/off bits.

The second problem is that the CGA monitor only shows a 60 Hz signal with 200 active lines at 15 kHz. You likely can't make the modern video adapter to output CGA resolutions and timings. So for getting any VGA signal to CGA timings, you need a video processing or a scaling chip.

It is unclear also how this can be useful, as VGA outputs have 16 million colours and the CGA only has 16, so the result would be pretty unusable. Unless you again use video processing magic and use temporal dithering, similarly how 24-bit (8-bit) colours are converted to 18-bit (6-bit) TFT panels.

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  • It's actually pretty easy to force a "modern" video adapter into CGA resolutions and timings (I have done it a few times, and for weirder resolutions), you just need the OS support for it (e.g. modelines on X on Linux). I also would be surprised if the monitor limited the levels and you could see only CGA colors; that's usually not how color monitors worked.
    – dirkt
    Jul 26, 2023 at 15:57
  • @dirkt Yes requesting any weird resolution and pixel clock is easy, but getting what you request may be harder. The problem may be that such low pixel clocks are not achievable. Way back in the Super VGA era, not all chipsets were capable of dividing the pixel clock down to rates required for displaying 320x200 on a 15 kHz TV, which is what CGA resolution and scan rate is. I recall VGA to Scart adapters were a thing.
    – Justme
    Jul 26, 2023 at 16:03
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    @dirkt CGA is a digital interface with 4 bits for color. It's called digital RGBI by some people. It really limits the levels and that's how it works. The "DAC" in the monitor is just a bunch of resistors driven by logic buffer.
    – Justme
    Jul 26, 2023 at 16:04
  • Yes, but the monitor will render these colors, so it's not a problem in any way. No need to use dithering. And the monitor may even exhibit the color artifacts of the original CGA (at least it's worth a try).
    – dirkt
    Jul 26, 2023 at 16:36
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    @dirkt No the monitor won't render any other colours than the 16 predefined CGA colours you can choose over a 4-bit digital interface.
    – Justme
    Jul 26, 2023 at 20:01

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