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

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: ... 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