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

From: Andreas Freiherr <Andreas.Freiherr_at_Vishay.com>
Date: Sun Mar 31 16:49:21 2002

Wow, thanks to all for these highly interesting answers, hints and
links! - I created several bookmarks while browsing the mails and will
have to follow up during the next few weeks.

John Honniball wrote:
...
> 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?

My "self-made 6809 computer" project addresses the problem of complexity
vs. functionality by composing the machine from modules. This way, you
can add another low complexity module whenever you want additional
functionality.

The core is a board housing the processor, 60k of RAM and 4k of ROM,
with 256 bytes masked out of the address space in a decoder. I developed
this idea and later heard of the I/O space used in PDP-11 architecture.
It is 4k in the PDP-11, but it is the same idea. Why shouldn't it work
for me, too? The address space is layed out in a way that should make it
easy to eventually replace this board with separate boards with CPU and
MMU on one of them, plus one or more memory modules.

A separate module has two 6850 ACIAs to provide two serial interfaces,
complete with 1488/1489 drivers/receivers and COM8116 (or something like
that) programmable baud rate generators, the board even supports
split-speed operation for applications like BTX (50 baud send, 1200 baud
receive).

To provide an easy way of configuring the hardware, I created a board
with 8*8 DIP switches. The idea is to put these switches as 8 readable
bytes at a fixed address and use them to provide initialization
parameters to the ROM based monitor program. This board also came in
handy in initial testing of the processor board: I managed to squeeze a
9 byte loop into these 64 switches that made a LED on the processor
board (address decoder output) blink when running properly.

Another board implemnents a cassette tape interface using a vanilla
analog tape recorder with a format called "Supertape" (published with
source code for numerous micros in an 80s magazine in Germany, including
the C64, Sharp PC1500, and Eltec's 6809 based Eurocom). It connects to
the MIC and EAR jacks, and it has a relay to control motor start/stop.

A video board (with capabilities slightly above a VT100) exists on paper
only, an EPROMmer on another board was partly built when time ran out.
All that stuff is still around, and I have a 19" rack mount frame that
contains the self-made power supply (with 24V ready to support 8" floppy
drives) and up to ten of my wrapped boards.

All this is based on standard components, but intentionally incompatible
to existing buses: I wanted to force myself to build everything on my
own. Once I dig down to those pieces, I may be able to provide some
pictures. Stay tuned ;-)

Andreas
Received on Sun Mar 31 2002 - 16:49:21 BST