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Question on special video out feasibility with Parallax — Parallax Forums

Question on special video out feasibility with Parallax

I am working on a project and want to know if any of the Parallax processors can generate the video out signals I need. I've been looking at samples and forums and see the ease with which VGA, Composite and even Broadcast signals can be generated, but haven't found anything too close to what I need yet.

I am trying to interface to an older monochrome monitor which takes video, hsync, and vsync as inputs. In the original system this was driven by a 6845 display chip (super common back in the day). The frequencies, in this case, are close to composite, but not quite and of course the separate h/v sync lines is also a difference.

HSYNC is 15.863 kHz (63.04 µs, with a 5.12 µs positive pulse)
VSYNC is 60 Hz (measured at 59.973 Hz - 16.67 ms with a 1 ms negative pulse)

Is this something I could achieve with a Parallax product? Can anyone point me to a project where someone has done something similar? I know it can be done with an AVR, but I wouldn't call it easy.



  • I suspect starting with the VGA.spin code will get me moving in the right direction, but want to avoid heading down a dead-end.

  • Yes, the VGA.spin code will get you text or bitmap at those specs no problems, but not a lot of text (due to the rather high resolution of the built-in font).

    In general, any VGA driver should be able to be tweaked for 15kHz scanning.

  • Great. That's what I suspected. I would be using bitmap as the font (8x10) on the board I'm replicating has a lot of unique "graphics" characters (Heathkit H19/H89 terminal board). I ordered a devkit. Propeller is about the only [modern] architecture I haven't worked with before, so might as well check that box!

  • If it's just a normal monospace raster font, you probably want to render it on the fly to conserve memory, but it's a bit tricky, since you'll need to mess with the guts of the VGA code.

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