Backplane & card cage construction
In the fall of 2002 I'd been working seriously on this project for a year and half, without anything physical to show for it. All of the work was design and simulation. I was a bit tired of that, and decided to go ahead and build a backplane - even though I wasn't anywhere near being able to begin the real construction.
The pictures below show the process, and at the time I was pretty pleased with how it came together. However, since then I've realized that my power and ground distribution is wholly inadequate. I've thought a bit about how to fix this, and I think the route I need to go is to have some custom PCBs made. However, to do it right I would need a 5 layer board - which would cost a bit more than I'd like to spend. One fix possibility is to do a two-layer board that just does power & ground. I could then just sandwitch it on top of my existing backplace. The big problem with that is it would close things off. If there was a single wiring mistake, I would be unable to correct it, and would have to trash the whole thing. Another option is to trash what I've done, get the two-layer board and use it as a base. Then, I'd redo all of my wire-wrapping to connect the signals together.
As of now (1/3/2004), my inclination is to go with the sandwitched two-layer appoach - but I'll put off that decision for awhile.
Anyway, here are the photos. Click on a picture to get the hi-res version.
Took the cover off of the card cage to let me see the LED display on the EPROM daughter board, and after a while the posts were getting a little wobbly. So, I cut a piece of plexiglas and installed it to make the cage more sturdy. Here's a picture:
The backplane is complete, and I've been playing around with how the signals look.
I've completed the left half of the backplane. Lots of wires done, lots of wires to go....
I'm running power and ground into the left half. The right half will pick up ground through the chassis (as well as some dedicated traces).
Here's the back side. A bit of overkill, but that will be a theme throughout construction.
The first real construction began this weekend. I bought a nice card cage on eBay which will fit 5 of my wire-wrap prototype boards. The cage was set up to take 7 boards, but I spaced it out to allow for the wire-wrap pins. To create a backplane, I'll be wire-wrapping along the spine. I'm backing the backplace with a copper-clad perfboard to reduce noise.
The screws were an odd size - 2.5mm, but I found some at HSG in Santa Clara. The white strips are wire-wrap tags. I'll be using them throughout.
Pin side of the wire-wrap cards. I got them from Excess Solutions in San Jose, at $15 each.
The component side.
I had to do a little sanding to get things to fit, but it looks pretty good. Next step is to find some more machine screws of the right side, and I also need to decide what kind of socket arrangement I'll use to connect to the power supply. After that's done, the wire-wrapping begins.
Here they are after covering them with copper tape, and the cutting out holes for the mounting screws and wire-wrap sockets.
I didn't have perf-board big enough, but had some pad-per-hole prototype boards on hand. It's a waste, but I cut two of them down to fit the backplane opening. Unfortunately, 96-pin wire-wrap sockets had wide portions of the pins at the bottom, presumably to give them a tight fit. To make them work with my boards, I had to use a Dremel tool to widen the holes. It was a bit messy doing it free-hand, but it worked.
The cage after re-spacing the guides.