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Thread: Who says you can't display MDA on a CGA monitor!?

  1. #1

    Default Who says you can't display MDA on a CGA monitor!?

    I just installed a Sound Blaster Pro card in my Tandy 1000SL and I was dismayed to discover that the GLX MOD file player doesn't support CGA. I could hear the music playing just fine, but all I got was a blank screen. But before I installed a VGA card, I wondered what would happen if I switched the Tandy's onboard video into MDA/Hercules mode by pressing Ctrl-Alt-Shift-V, while still using my Tandy CM-11 CGA RGB monitor. Mind you, it's not supposed to work, because MDA outputs its video on pin 7, while CGA monitors ignore this pin and use pins 3, 4, and 5 for Red, Green, and Blue, respectively.

    But to my surprise, after adjusting the monitor's controls, I got a steady, clear image! The top line and left side were cut off a bit, but the majority of the screen was perfectly readable. It was even showing on the CM-11 in the obligatory green phosphor color! (click for hi-res photos):



    The odd thing is that some of the vertical bar characters appeared to be gray, instead of green. It didn't photograph well, but here's a close-up:



    And here's the kicker: the CM-11 monitor doesn't even have pin 7 connected to anything -- it's missing from its DE9 connector. So, clearly the Tandy is outputing MDA/Hercules video through the RGB pins... and perhaps could even be emulating the early IBM MDA cards with the "color MDA" RGB output? I know it's also outputting video on pin 7, because I've connected a real MDA monitor to it and it worked fine.

    Just to prove there was no trickery involved, here's the same CM-11 monitor showing proper color CGA video -- it's squished down to the center of the screen because that's how far off I had to adjust the monitor's controls to get the MDA video to display properly (or at least as properly as it could manage):



    I had the CM-11 running with the MDA signal for nearly an hour with no apparent ill effect.

  2. #2

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    Because this monitor ist multisync. Try IBM 5153 monitor with MDA and you will see....nothing

  3. #3
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    Interesting... I wonder what signal it is sending to the monitor, timing-wise.

    On a slightly related note... My ATi Small Wonder can display MDA/Hercules on a CGA monitor by showing it as an interlaced 400-line screen at 60 Hz, with green (real MDA/CGA is 50 Hz).
    It can also do the opposite: CGA on an MDA/Hercules monitor (that is without using any tools such as SIMCGA).

  4. #4

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    Quote Originally Posted by romanon View Post
    Because this monitor ist multisync. Try IBM 5153 monitor with MDA and you will see....nothing
    I wouldn't suggest even trying, as it may kill the monitor. It's not supposed to handle 18 Khz.

  5. #5

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    Quote Originally Posted by Scali View Post
    My ATi Small Wonder can display MDA/Hercules on a CGA monitor by showing it as an interlaced 400-line screen at 60 Hz, with green (real MDA/CGA is 50 Hz).
    It can also do the opposite: CGA on an MDA/Hercules monitor (that is without using any tools such as SIMCGA).
    Well that card is really a small wonder I have one too..

  6. #6

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    I thought there was some combination of multisync and original MDA's that would actually show 8 color text modes? Am I remembering that right or not?
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  7. #7

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    Quote Originally Posted by romanon View Post
    Because this monitor ist multisync. Try IBM 5153 monitor with MDA and you will see....nothing
    The Tandy CM-11 is not a multi-sync monitor; it just happens to have a wide enough range of its V-Hold and V-Size controls to be able to lock onto an MDA/Hercules video signal. Here are the CM-11's official specs:

    http://support.radioshack.com/suppor.../doc6/6012.htm

    I did some testing with my multimeter, and in MDA mode the 1000SL is putting out video signals on both pin 7 (the normal monochrome pin) and pin 4 (the CGA green pin). There are no signals on pins 3 or 5 (red or blue) in MDA, so what appears to be "white" on the monitor in MDA mode is just an artifact of the color CRT pixels.

    And in CGA mode, in addition to the normal RGB signals on pins 3, 4, and 5, it is also putting out a signal on pin 7, the MDA video pin.

    I wonder if this is normal behavior for most switchable CGA/MDA video chips -- to keep both the RGB and monochrome video outputs active in either mode, and only change the scanning frequencies? That certainly simplifies the design, and causes no problems in normal use, since CGA monitors ignore pin 7 and MDA monitors ignore pins 3-5.

  8. #8
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    Default

    There weren't many CGA/MDA adapters made, so it's hard to forumulate a general rule.

  9. #9
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    I'm a little skeptical about hearing the 1000SL outputs MDA video. In any event, the difference between the horizontal scan rates of the CGA and MDA is not that large. 15.75khz vs. 18.5khz, roughly. Back in the day they would somehow use CGA monitors to display EGA. The colors would be off, I don't feel like getting into it. But there's a much bigger difference.
    Some monitors *can* do this, but it isn't always advisable. You *can* also hobble the x-ray protection circuitry I'm told. Be careful . . . and you may also be putting strains on the monitor it wasn't designed to take.

  10. #10

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    Quote Originally Posted by tipc View Post
    I'm a little skeptical about hearing the 1000SL outputs MDA video.
    It's listed in the specifications -- "MDA or CGA compatible":

    https://magisterrex.files.wordpress....1000slrear.png

    Some monitors *can* do this, but it isn't always advisable. You *can* also hobble the x-ray protection circuitry I'm told. Be careful . . . and you may also be putting strains on the monitor it wasn't designed to take.
    There was no audible whine from the monitor's oscillator, like what you get when trying to display a video signal a monitor or TV can't handle, such as trying to display Super VGA video on a regular VGA monitor.

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