I have an on-again, off-again hobby of building small retro computers. I built this piece of crap, but the design process of making a small, single-board computer is wretchedly slow. First you need to design the circuit, then you need to do the schematic capture, make a PCB, send that off to a board house, wait a month, and finally solder everything together. If you get something wrong in the first step, start over from the beginning. It’s a tremendous amount of work for something that’s not guaranteed to work when you power it up, but that’s a hobby for you.
One alternate solution to this process is building everything on a breadboard. It’s much, much faster, but building circuits that are faster than a few megahertz is just simply impossible, and breadboards go bad. I’m not for debugging something where the thing you’re building your computer on can just go bad. That’s just not cool.
There is one solution to both these problems: A backplane design with wire-wrapped boards. Classic, and perfect for these retro circuits I’m putting together. There are hundreds of old computers that use a backplane design – just look at the S100 bus. It’s fast enough to keep up with anything faster than a Z80, and the modular design means I can build the CPU on one card, the RAM on another, a serial interface on a third, and some sort of storage on another.
The processor I’d like to use for this computer (The Motorola 68000 or 68010) aren’t really well suited for existing backplane designs. The S100 bus has 8 data lines and 16 address lines, while the 68k is a 16/24 machine. I’ll need to roll by own backplane design.
Yes, figuring out what case I’m going to use for this computer before drawing a single schematic is very stupid. However, without a case I currently have no limitations on what my design will be. I don’t like working without any limitations. There’s too many options.
I’ve decided to go with a Hammond enclosure, specifically the 1458VD4B. That will give me a 7″ x 3 3/4″ front panel to attach the backplane, and space enough for eight inch long cards. Also, this case looks cool. And it’s blue. It could have jumped right out of the late 70s, so it’s perfect.
With this limitation, I can figure out my backplane will be 7.26 x 3.9 inches. Here’s the layout of of the backplane:
Those mounting holes you see line up with the front panel of the Hammond enclosure.
There are two things a backplane needs: connectors, and some sort of power input. For the power I’ll need 5 V and probably 12 V. The supply also needs to be tiny. Instead of rolling my own, I’ve chosen a PicoPSU, a freakin’ tiny ATX power supply that can provide 80 Watts of power for my computer. It’s overkill, but it’s also tiny and twenty five dollars. Can’t beat that.
I also need a way to turn the computer on and off. With an ATX supply, that’s easy. Just connect the PS_ON line on the power supply to ground. A simple switch (and a LED attached to the PS_OK line) takes care of that.
Lastly, I need some sort of connector. You might think the easiest solution would be to get a board-to-board connector with as many pins as possible. This works in theory, but falls apart when you need to design a board to hold all these connectors. Anything over two layers on a PCB gets expensive very quickly, especially considering I’m only ever going to need one of these backplanes.
I’ve chosen regular ‘ol Eurocard connectors (for the backplane and for the individual cards) each with 64 connections. They’re just about the right length to be mounted vertically on my backplane, and with the power supply I can manage to fit seven connectors in this computer.
The designs for the backplane, including the schematic, board files, and gerbers, are all up on my Gits. Next post will be about designing and building a the electronics necessary for this to be a user-friendly device. Namely, the on/off and reset circuits.