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Q: Help with 8 bit system video

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:

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From what I understand, this game uses a character map to draw the

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.

Thanks.

Reply to
todd1814
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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

donald

Reply to
Donald

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 ago)

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller
Reply to
joseph2k

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

Reply to
Sjouke Burry

Sorry, the link kind of dissapeared.

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Reply to
Sjouke Burry

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