# Q: Help with 8 bit system video

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

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

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.

2. ### DonaldGuest

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

3. ### joseph2kGuest

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)

4. ### Sjouke BurryGuest

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