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Thread: Mega CD all memory ok but corrupted graphics

  1. #1
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    Sega CD Mega CD all memory ok but corrupted graphics

    I have a model 1 japanese mega cd. It boots fine, but when I enter the options screen I get a completely black screen, and pressing start I get some blue rectangles.
    Similarly, when I boot a CD I get completely garbled graphics.
    I assumed I had some of the ram (either program ram or one of the two word ram) broken or with traces disconnected.
    To identify which, I wrote this test program
    https://github.com/fabiodl/megacdTest/
    which I start using an everdrive. The program sets fills the memories with a known pattern,and then reads them back.
    To my surprise, all the memories pass the test.
    The mega cd 68K seems fine as well. As by program, it is able to communicate back to the genesis 68K and to blink the two LEDs.
    I also tried to start the bios from the everdrive to see if there were problems with the ROM but the behavior is the same. Using the US bios I get no spinning SEGA logo at all.

    What could be possibly wrong?
    Last edited by kamillebidan; 09-20-2019 at 12:55 PM.

  2. #2
    Hedgehog-in-Training Hedgehog-in-TrainingNameless One pyroesp's Avatar
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    Which ram did you try? Just the RAM or VRAM too?
    If VRAM is fine then your issue might be the graphics chip, but I'll leave that to the experts here.

    My guess would be VRAM.

  3. #3
    Comrade as in friend. Master of Shinobi ComradeOj's Avatar
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    How well does the Megadrive work alone playing MD games and the MegaCD disconnected? Are the graphics still messed up?

    Does your program test MD memory, or only MEGA CD memory, or both? I also wrote a similar program, and it looks at all of the MD's memory, but not the Mega CD. Maybe it can detect something for you?
    http://mode5.net/download/GenTestV3.0.zip
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    Visit my web site at www.mode5.net
    Or my collection of homebrew Genesis games, programs, and music on SEGA-16!

  4. #4
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    pyroesp: I tried all mega cd's RAMS (prog / word / backup). VRAM's is internal to the megadrive, and it should be ok, because all cartridge games work fine.
    ComradeOj. Thank you very much. As reported above, I think the MD is fine, but I will test your program as well.
    I think the fault lies in the connection board between the MD and the MEGA CD,in detail a chip called 315-5540, which is present only in the Japanese model, second revision.

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