Sunday, February 26, 2017

Building a 68000 Single Board Computer - Revision 2.1

I've now updated the design with some changes that I call revision 2.1. It is in a different github directory from revision 2.0:

In revision 2.1, the large and hard to obtain MC14411 baud rate generator is replaced by a smaller, simpler circuit using the more easily obtained 74HC4060. The difficult to obtain 25LS2548 chip is replaced by more commonly available parts.

Both circuit changes were tested with the wirewrap prototype and a breadboard.

I've done a new PCB layout and checked it reasonably carefully. I'm confident enough in it that I have ordered some boards from These will take some time to arrive. I plan to build up one board using the parts from the wirewrap prototype.

Friday, February 10, 2017

Building a 68000 Single Board Computer - PCB Layout

Plot of PCB Layout from Kicad

I spent some time learning how to use Kicad's PCB layout software and created an experimental board layout. I had never laid out a circuit before using CAD software before, so there was some learning curve and I may have made some rookie mistakes. The routing was all done using the FreeRouting software. The resulting layout is quite complex but looks pretty good. It is a double-sized board, and based on the board size, it looks like it would cost about US$200 - US$250 to have three high-quality boards made from a supplier like OSH Park, and less from some manufacturers in China. Before I did that I would probably carefully review the layout a little more.

All files can be found at github.

3D Rendering of PCB Layout from Kicad

Expensive? Well, I just saw a Motorola MEX68KCB MC68000 Educational Computer Board show up on eBay. The minimum bid is US$295. It will be interesting to see if it sells.