6809 single board computer (was: "New" PDP-8)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sun Mar 31 13:06:48 2002

see below, plz.


----- Original Message -----
From: "John Honniball" <coredump_at_gifford.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Sunday, March 31, 2002 7:18 AM
Subject: Re: 6809 single board computer (was: "New" PDP-8)

> tim lindner wrote:
> > > I am interested in building a computer like this, but this seems a
> > > too much for me to tackle. Are there any other Classiccomputers one
> > > build which are far simpler and don't involve 600 wires and stuff?
> I keep thinking about putting together a 6809-based machine, just
> for the fun of it.
> > http://home.netcom.com/~tlindner/Download/Free6809.zip
> >
> > I also converted the included postscript file into a PDF.
> Something like the CMU Free6809 board would be great, but I'd
> prefer to add a CRT coltroller to get a video display. I'd
> also want a keyboard interface so that the machine could
> be independent of a terminal. Maybe even a disk interface
> of some sort, and the AY-3-8910 sound chip, and a parallel
> port, and... Oh dear, now I'm making it too complicated!
> What do other people think about a small-scale 6809 board?
What amazes me is that I frequently run into oxymorons such as this one.
People want a "simple" system, yet they want a fancy video interface and
sound. Now, I'm convinced that at least 75% of the problems I've encountered
(not on my own systems, since I always omit/remove/disable it) on PC's relate
to sound. Sound consume huge amounts of bandwidth and, moreover system

The thing that made our computers of yesteryear simple and manageably small
was the fact that the console functions were provided by a terminal. If you
want to build that functionality, into your "simple" circuit, keep in mind
that the board in a typical terminal was larger than the one on which the
computer lived. Further, keep in mind that the video that simple computers
e.g. the Commodore types, (not because they're poor, though they are, or
simple, because they're not) were considerably less capable than many
terminals, and certainly much less capable than what one is accustomed to
seeing even in a cheap PC.

I built a VERY capable system last week. One with 128KB+ of memory and
considerable I/O. It does, of course, rely on a serial terminal, but it is
supported by a wide range of FREE compilers, assemblers, interpreters, etc.
The wire count was on the order of 200, and the entire design, wiring,
checkout, and software installation took under three hours. Of course, the
software's simple, since there's no mass-storage, and no file-system. Mass
storage is provided by the host system to which it's tethered by means of a
serial link. This system runs VERY fast, and is capable of even more speed,
but, since it's small, that wouldn't interest game-freaks.

Another thing. 600 wires is NOT a big, complicated circuit. 600 IC's might
be, but 600 wires is a 2-hour wiring/checkout job. Once I get set up, even
with my failing eyesight and aging memory, I still wire and continuity-check
(for both missing and undesired connections) a 600-wire circuit in less than
two hours.

If you want a small and simple circuit, supported with a decent OS and a wide
range of software, whip up a modern version of one of the classic CP/M
systems. Use SRAM, extracted from an old '486 motherboard's cache, a 20 MHz
Z80, (still avaiable at DigiKey) and substitute standard I/O peripherals for
the overpriced, underpowered devices in the Z80 family. Use an 82C55 in place
of a PIO, and an SCC2681 in place of a DART. Use a CPLD in place of the
multi-IC video circuit, or, if you're scared of PLD's, use a 6845-based
approach for the video display.

You'll have a lot more trouble getting a decent keyboard and NTSC-compabible
monitor than you will in building up a high-speed copy of, say, a Xerox 820 or
a Ferguson BigBoard-II, particularly if you leave out the stuff you don't
need. So long as you leave the mass-storage interface alone, you'll have
little trouble generating a working system.

Now, if you want a 6809-based system, you CAN build an even simpler one, but
unless you like the garbage-quality video of the 6847 or similar chips, you're
still going to be better off with a terminal. You can easily hit the 200-wire
mark with a 6809-based system. What you'd do with it is another issue, of
> --
Received on Sun Mar 31 2002 - 13:06:48 BST

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