It’s about time for an update for Hackaday’s latest project – a modern retrocomputer based on the Motorola 68000 CPU. In this update, we’ll be taking a look at the enclosure, the backplane itself, and how we’re going to power this thing.
This is only an update to the project; you can check out the current status over on Hackaday Projects. It’s Hackaday’s new collaborative project hosting site where you (and your friends) can design, build, or document anything you have in mind. Request an invite for the alpha release of Hackaday Projects and you can give this project a skull! Seriously, this project is only the third ‘most skulled’ one on Hackaday Projects.
Now that the completely transparent pitch for Hackaday Projects is over with, we can get on to the update for the Hackaday 68k. Click that ‘Read More…’ link.
You don’t build a house by starting with the kitchen cabinets, and like any project this computer needs a good foundation. This means picking out a nice enclosure, figuring out some way to power the thing, and constructing the backplane that will connect all the different cards I’ll be designing and building.
Having a good-looking enclosure shouldn’t be a primary goal for what is really simply a prototype, hacked-together homebrew project, but I have a few special considerations for this project. Firstly, I’ll eventually be lugging this around to hackerspaces, meetups, and Maker Faires. It needs to be secure. Secondly, a lot of people are already following this project on Hackaday and on Hackaday Projects. It needs to look good.
The enclosure I’ve picked out is a beautiful steel instrument enclosure from Hammond. Specifically, the 1458VD4B enclosure, measuring 8x8x4 inches. I love Hammond enclosures, and when I put the top on and screw everything together, it really looks like something from the early 80s.
In the world of backplanes and retrocomputers, power supplies are a bit of a problem. There are very old S-100 bus systems that used huge linear power supplies, with transformers and caps big enough to kill an elephant. That’s not something I want – or could even fit – in an eight-inch square case. Linear supplies are old tech, though. The Apple II and the Digital VT100 terminal – contemporaries of the 68000 – had switching power supplies, but unfortunately they were also huge.
A much better option for providing power to the backplane would be to take a normal PC power supply, add a 24-pin ATX header, and plug everything in. It’s a great idea, but even the smallest computer power supply would eat up a ton of volume in my enclosure.
Here is the solution. It’s called the picoPSU, and it’s barely larger than a standard 20-pin ATX power supply connector. It’ll provide 6A of 5V, and also has 12V, 3.3V, and -12V should I need that in the future. It’ll also power a hard drive, all without requiring any load. This is, by far, the easiest and cheapest way for me to power this computer.
Now we come to the important part of this update. The backplane. The board I’m going to plug the CPU, ROM, RAM, Video, and Ethernet cards into. If the CPU is the brain of a computer, the backplane is the brainstem. If this doesn’t work, nothing will.
Next up are seven Eurocard connectors. These are 64-pin Eurocard connectors, despite there being 96 pins. Only rows A and C (row B is the middle) have pins soldered to the backplane. There’s no logic here, just simple solder traces from one pin to another. If you’re extremely clever, you might be asking yourself why not use 96-pin sockets. I’m going to answer that with another question.
That’s the problem I faced when trying to design a two-layer 96-pin backplane. Why a two layer backplane? Because it’s cheaper. I’m also pretty sure it’s mathematically impossible. Proofs are welcome.
Either way, I don’t need 96 pins on this backplane. 64 pins are enough, once you know what you can safely ignore when designing something with the 68000. I’ll get to that in the next update.
With only seven card connectors, this isn’t a very large backplane. When you consider I’m doing the CPU on one card, RAM on another, ROM on a third, two more for Ethernet and video output, I don’t have much room to work with. From everything you’ve seen so far, there’s not even space for a cool ‘switches and blinkenlights’ front panel
But Wait, There’s More
There’s the front and the back of the backplane showing off a neat little feature I snuck in. I call it a ‘frontplane’, but basically all it does is break out all the signals to a female 0.1″ header socket graciously supplied by Samtec‘s amazing sample request order form. They’re low-profile 0.1″ headers, meaning I can barely squeeze in an additional board between the backplane and the front panel of the enclosure.
What’s that green board, you ask? That’s my first attempt at making an external power switch, power LED, and reset button. Mechanically, this design did not work.The space between the backplane and the front panel of the enclosure is just too tight. This isn’t really a necessary part of the build – I have power and reset switches and buttons on the backplane and CPU board – but it does make it look nice. Right now, the ‘external’ controls for this project have been pushed back to the very end of this project. Or when I get stuck on something. I don’t know.
Other Electronic Considerations
In the first post for this project, a few people asked me how I would be terminating the backplane. Until I get the RAM and ROM working, I don’t know if I need to. I found this app note for RC terminator networks saying the 68000 usually doesn’t need termination on the data, address, or control busses. Don’t get me wrong, I’m going to be taking a scope to this when everything is wired up, but even if I do need termination, I’ll only need a few one dollar parts. It’ll also be a great use for the frontplane.
That’s All For Now
This is the most boring part of the project, I know. Still, it needed to be documented. The next update will more than make up for it. I’ll be going over the 68000 CPU itself, showing off what you can safely ignore, and telling you why designing a computer around the 68k isn’t much more difficult than designing a computer around the old 8-bit CPUs like the 6502. Really, there’s not much to it. Also, blinking LEDs. Yeah!
Here’s the link to the entire project on Hackaday Projects. Rate, comment and subscribe, or something like that.
And yeah, that’s a silkscreen of BMO.