Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!ames!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!hpfcdc!hpfclm!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Video Circuits
Message-ID: <16750021@hpfcdj.HP.COM>
Date: 14 Apr 89 18:54:25 GMT
References: <2414@lll-lcc.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 37

>Does Anybody have a reference to complete schematics for a
>video interface that uses the 6845 or similar chip??  Is there
>an "application note" on the 6845 that shows some video
>circuits??  I'd like to build a Hercules clone type circuit,
>but not on the PC bus -- perhaps as a serial device??  I think
>that I saw plans for a custom card (for the Apple??) that uses
>the 7220?? chip in R-E a while back.  Are there others??

The 6845 is pretty up in years now, but information - including an application
note - should still be available from your friendly neighborhood Motorola
rep/distributor.  You might also try Hitachi - they are/were a second source
for the 6845.

The 6845 is, by current standards, a pretty "dumb" video controller.  It is
intended to provide memory control and sync signals for a "character generator"
type of video display - i.e., one where you toss ASCII into a small RAM, and
the characters are generated by running this code through a character ROM.
The chip can be used, though, to control a simple dumb frame buffer for 
a graphics or bit-mapped alpha display.

The 7220 is a step up; it was introduced by NEC around, oh, 1983 or so (as my
fuzzy memory recalls), and includes some ability to manipulate the frame
buffer data (drawing lines, circles, etc.) in addition to providing the video
timing.  It's probably mor widely available at this time, and will give you
considerably better functionality.

If you'd really like to get closer to the "state-of-the-art", get in touch
with your local Texas Instruments rep/dealer/whatever and look into the
34010 and related chips (such as the 34070 color map).  Many current and
reasonably slick display cards for the PC market are based on this chip.
Be prepared to spend a good deal more for it, though, as compared with the
"dumb" controllers such as the 6845.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
{the known universe}!hplabs!hpfcla!myers |  sentient life-form on this planet.