Thursday, November 24, 2016

Building a 68000 Single Board Computer - Introduction

I recently started a new hardware project that I plan to document here as it progresses.

I've built several retro-computers from kits that use the 6502 and 8080 processors, including the Briel Replica 1Altair 8800, and Superboard III. While most of my work on these systems was with software, for the Replica 1 I did a little hardware design and built a 16K RAM/ROM memory board as well as a 65816 CPU adaptor. In the back of my mind I eventually wanted to do a more ambitious hardware design project.

Another classic CPU is the 68000. Years ago I worked with some 68000-based systems, but mostly programmed them in high-level languages. I always felt it had a very elegant architecture, with the later CPUs in the 68xxx series possibly being the pinnacle of CISC design, before RISC architectures took over.

I bought an excellent book on the 68000: Microprocessor Systems Design 68000 Hardware, Software, and Interfacing by Alan Clements. This is one of the best books on the 68000 in my opinion, covering both software and hardware in some detail.

The book describes a 68000 single board computer called the TS2. It was used for training at the University of Teesside where the author taught. It was similar to, and mostly software compatible with, the Motorola 68000 Educational Computer Board.

I realized that there was enough detail in the book, including schematics and theory of operation, that the board design could be replicated. Unlike some designs in textbooks, this one had actually been built and tested. The book also included on CD-ROM the code for a monitor program for the board so I would not need to write one from scratch.

I felt that designing a 68000 based single board computer from scratch myself was too risky a project, but using an existing design would have a good chance of success. I did see a number of challenges:

  1. While the design was known to work, there were likely some errors or omissions in the schematics in the book as they were hand-drawn and didn't come directly from a CAD system. My copy of the book was the third edition so I thought it likely that there were not too many errors in it.
  2. There was no board layout or even pictures of the assembled board. so I would need to figure out a suitable layout myself. I have not been able to find a picture of a Teesside TS2 anywhere on the Internet.
  3. I would need to determine if I could use the monitor program software on the CD, possibly finding a cross-assembler to build it, and program it in to EPROMs.
  4. I would need to find all of the required parts, many of which are no longer being manufactured.

These challenges all looked surmountable, and even if it got stalled or failed, the project would be a good learning experience, so I decided to go ahead with it.


VinVBR900 said...

Have you seen

An upgrade would be 52pin 68008 for more memory space, and CS8900 ethernet

Unknown said...

The "65816 CPU adaptor" link is broken.

Unknown said...

@Vincent Crabtree:

68008 is obsolete ( well, more than others anyway). 68000(S)EC can be persuaded to work in 8-bit mode if needed, with extra address space.

WRT to schematics, there is not much to it. Connecting periphery to 68000 is mostly kidstuff.

Unknown said...

I have a manual, circuit diagram, and GAL logic for a commercial 68008 board.
And many peripheral boards it can talk to. It runs EHBASIC. Let me know if you are interested.