A Long, Long Overdue Update

A project log for MC68000 Computer

BenchoffBenchoff 04/25/2015 at 06:072 Comments

Quick story. I brought this computer to the Vintage Computer Festival 9.1 last year to show Bil, Dave, and all the other cool people at the event. At the time, I was freerunning the processor, watching the blinkenlight count up. Great stuff, and proof that the CPU works.

A few hours into the show, the CPU board stopped working. It just wouldn't free run. No idea why, as I was able to get the board working by wacking it against a table once I got home.

Then the Hackaday Prize happened, and then the summer con season happened. I've been working on this off and on in the meantime, but no *real* progress. I made a better RAM card (more on that in a bit), but it's still a bit away from sending characters out over the serial port.

Since then, I got the wire-wrapped CPU card free running again, and I decided to take it to the Vintage Computer Festival X last weekend. When I pulled it out of the box and turned it on.... nothing. This computer hates VCF for some reason.

Therefore, I have decided to build a proper CPU PCB. It's just buffers, the CPU, a clock, and some blinkenlights:

And I have to route that before I leave my house for a month for con season. Great.


I had a front panel milled out of a big ass piece of aluminum. Here are the videos of the milling:

And a picture of the front panel:

That panel was made by the SeeMeCNC guys when I was up there for the Midwest RepRap Festival. I traded a gigantic (5 foot x 8 foot) hackaday flag - made by me - for the CNC work. All the relevant files are up in the Git.

Front panel not included. I'm moving the reset circuitry to the front panel, btw. Another board to build.

Four megabytes of RAM

I hated the wirewrapping on my RAM card. The initial plan for all of these cards was to prototype them with wire wrap, then build a board. I met myself halfway on this one.

That's the layout for the RAM chips, eight 4-Megabit chips. You'll notice there is no control circuitry on this one; there's a reason for that. I might want to change the control circuitry to a PAL or something down the line. Easiest solution is to make a proper layout for the RAM chips themselves, break out the data, address, and control lines, and leave everything else up to wirewrapping. Easy enough.


Since I'm doing all of this from scratch, It would be nice to have a development tool.

That's the Motorola Educational Computer Board, the official 68k trainer from 1981. It works, I have it plugged into a terminal, and it has a great monitor in ROM. I'll be using this heavily once I get a few characters spitting out of the serial port of my project.

Picked that up on eBay for $40, btw.

Also on the eBay Front...

I have most of DTACK Grounded.

DTACK Grounded. the journal of simple 68000 systems, was a newsletter put out by [Hal Hardenberg] on how to design a simple 68000 system.

Most of DTACK grounded covers the development of a 68000 add-on card for the Apple II and a BASIC interpreter. It's good writing, and the issue ever 68000 homebrew wants to read - number 6 - tells you exactly what you need to leave out, what you need to leave in, and what pins to connect to where.

If you want to read these for yourself, here you go.

I did pick something up at VCF...

That's a 12MHz 68000. Five dollars. It's an actual Motorola part, so this is what I'll be using from now on. I don't know if I'll be running it at 12MHz; I specced all my decoding logic for 8MHz. I'll try it - it's just changing a crystal - but I don't expect this computer to run at 12MHz.

That's it for this update.

I'm leaving for NYC next weekend, and LA a few days after that. I won't be home for a month and for some reason that means no Windows, and no Eagle. Don't read too much into that last sentence.

I need to get this CPU card done and sent off to fab. It's exactly like the earlier wire-wrapped CPU card, only this one hopefully won't fail all the time.

So... yeah... it's been embarrasingly long between this update and the last, but hey, I've been busy, and technically, I have been working on it.

This thing is going to be running a monitor program by Christmas. Then it's on to putting uCLinux on it.


Benchoff wrote 04/26/2015 at 15:41 point

I'll bring the RAM board, at least. I think I may be spending the LA event laying out the CPU PCB...

  Are you sure? yes | no

Jac Goudsmit wrote 04/26/2015 at 05:05 point

Thanks for posting! Now I know what the board was that you were waving around in Fran's video :-)
Will you be bringing this project to the LA event?

  Are you sure? yes | no