Retro Computer Update

I’m writing regular logs on about the m68k-based retro computer I mentioned I was building a while back, but I thought I’d post a quick update here too. If you’re interested in the project, be sure to follow it on hackaday as that’s where the regular updates are going.

The main progress so far has been:

  • Get the CPU free-running
  • Decide on a memory layout, and design an address decoder to support that layout
  • Build the address decoder
  • Decide how I want to lay the buses out on the board for easier interconnection
  • Actually wire up the buses accordingly

I actually designed the address decoder a while ago, but gathering the parts and then wiring it up has taken some time – I finished it yesterday. Since then, I’ve been breaking out the CPU signals onto control, data and address buses, which I’m doing on an otherwise-bare breadboard in the middle of the main boards.

Right now, the rosco_m68k looks like this:

It’s still free-running at the moment (I’ve not wired the memory to the bus yet as you can see) but all the control pins I care about (basically everything except the legacy 6800 peripheral pins and the function code) are now on the control bus. The jumpers you can see there are configuring those control pins for free-run mode and will be replaced by connections to the RAM and ROM.

I did come across a couple of design errors while building the address decoder, some of which I fixed and others I’ve decided I can live with for now (e.g. an issue with the expansion select line I only discovered once I’d completely built the decoder and hooked up the LEDs at bottom left).

Anyway, that’s where it’s at right now. I expect that after another day’s wiring, it’ll be ready to run real code, and I can’t wait.