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Q: Help with 8 bit system video

Discussion in 'Electronic Design' started by [email protected], Sep 14, 2006.

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  1. Guest

    I could use a favor in helping me understand some video display logic.
    I'm a software developer by trade so what little I know about
    electronics is self taught. (This is a self-education project btw)

    I'm trying to understand how video is displayed from an old video
    arcade board. It's basically an 8 bit computer system. It uses a
    Hitachi 46505 (crt controller) which is the same as the Motorola 6845.
    I've looked through the datasheet for a 6845 and understand it's
    purpose and how it goes about displaying characters. What I'm having
    trouble understanding is how colors are added to the mix.

    The schematic I'm looking at is on Page 16 (in the PDF file) at the
    link below:
    playfield. The characters are generated via the CRT controller and are
    ouput serially, one character row at a time (through a color PROM) to
    the RGB lines. The game keeps both a character map and a color map in
    video RAM that's shared by a z80 and the controller.

    Based on old posts I've been reading, I'm sure that someone here has
    experience with a similar setup. If you could answer some questions
    for me I would really appreciate it.

    How is color for the character determined? How did this work in old 8
    bit systems? I know the color PROM feeds out 8 bits that are run
    though resistors to get 256 colors but I don't see get how color RAM
    assists in doing that. I'm also trying to understand how sprites are
    drawn. Were sprites displayed in an alternate refresh cycle or are
    they somehow combined during the drawing of the character map?

    Again, if you can give any advice I would really appreciate it.

  2. Donald

    Donald Guest

    Please study the 6845 docs.

    You actually answered your own question.

    The resistors act as an analog sum and the color is based on their
    output value. 3-bits RED, 3-bits BLUE, 2-bits GREEN.

    These voltages go directly to the monitor on page 14.

    Good Luck

  3. joseph2k

    joseph2k Guest

    Um, you really need the docs for the Hitachi 46505. While it is based on
    the Motorola 6845 it is by no means the same; the 6845 is pretty strictly
    monochrome and character prom oriented. It was also next to impossible to
    make it do adequate interlaced graphics. (well back in the day, ~20 years
  4. Sjouke Burry

    Sjouke Burry Guest

    Checkout the BBC-B computer schematics,with
    rgb. bw. and palvideo modulator.
  5. Sjouke Burry

    Sjouke Burry Guest

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