SRHuston Dot Net

Old days, Good times I remember. Fun days, filled with simple pleasure.

–Chicago, “Old Days”

As mentioned last month, I picked up some kits to put together recently and had a blast doing so. I’ve already detailed the Altair 8800 kit that I assembled, so now it’s time to talk about the next one – the PDP-11/70 replica called a “PiDP-11“. It was every bit as much fun as the previous kit to assemble, though a bit more challenging!

[Note, in order to see captions on the photos, you’ll have to click through to the gallery on Google Photos]

This kit was shipped to me from Panama, so it had a bit different packaging in case it had to be opened by customs (mine wasn’t). Sorting through all the parts I found everything was in good order and re-read the instructions for assembly (much like the last kit, I’d already read through them quite a bit before things arrived). The only thing which is not part of the kit is a Raspberry Pi itself, and while I had one at home that I could use I wanted to be able to use wireless to talk to it; when I realized the wireless adapter would be about half the cost of a new Pi 4, I decided to just get the new Pi with the built-in abilities. The extra power wouldn’t hurt, even if it’s emulating an old machine!

Since this time I wasn’t starting until after dinner, I knew I wasn’t going to have a lot of time to work on it before it would be time for bed, but I figured I’d get some of it done at least. I got as far as seating the LEDs into the board before I called it a night. By then I had all of the diodes and resistors installed as well as a couple other components, and had come to my first conclusion about this kit: the pads for all the components are extremely tiny. I use a fairly small soldering tip on my iron with a wedge shape that works well on through-hole work, but for this I really should have swapped to the pinpoint tip that I have for doing SMD work. Unfortunately I didn’t come to this conclusion early enough to make it a worthwhile change, because it would have taken too long to wait for the iron to cool off to swap tips the first night, and on the second night I was soldering things that were landed on larger pads anyway so it wasn’t a problem anymore (and in fact, I was going to be doing the switches the second night and definitely wanted the larger tip for those).

Testing the kit before the switches went into place was nice, and involved seating the Pi on the back panel. One nice thing is that it’s mentioned multiple times in the instructions that you need to put something to insulate the Pi from the back of the board to avoid shorting out any of the LEDs, however the socket has been redesigned to be extra tall now so when the Pi is seated on the back it sits high enough off the board that there’s no danger of shorting things unless you leave large tails on the components and don’t trim them. Since I don’t do that, everything was fine, and the test succeeded. Time to move on to the switches, which was definitely the hardest part of things. By the time I was finished, my fingers were in a bit of pain (not just from arthritis) and I found myself thinking, “this is good enough.” Now that’s not a way to say I was giving up on it, and things do look good, but maybe they aren’t aligned perfectly side-to-side or up-and-down. As a replica of a PDP-11/70, maybe it’s not exact in that sense, but I’m perfectly happy with it – there’s no part where I look at it and wish I’d done it better.

So how do those switches get installed differently than the Altair? Well with the Altair you set all the switches in place and then rest the front panel over top to line them all up. On this, however, there’s two stencils that come into play; one stencil goes just on top of the circuit board and the switches go through it, and it also contains the information on it for which direction each switch goes as well as the color and type (momentary vs. toggle), and the other template goes over top of the switches to hold them into place and keep their spacing up-and-down and side-to-side. Once everything is plugged in, you are supposed to “wiggle” the lower stencil up over the sides of the switches and then use some zip ties to hold the two stencils together which in theory keeps everything where it’s supposed to be. In practice, this was still an extremely difficult endeavor and left me wondering a few times if I was going to get it right or be horribly disappointed. In the end when I started soldering, I realized that I was chasing perfection at the expense of a good result, and I was going to either hurt myself or damage something if I continued at this rate. And you can see yourself in the photos linked above, some of the switches may be a little closer to each other than others, and some may be a little higher or lower than others, but nothing looks like it was slapped together without a care and assembled poorly. It may not meet the rigorous standards of DEC in the 1970s with a manufacturing plant at their disposal, but I think it’s perfect for me.

After a few more touches everything was done, and it was time to close up. I did leave the back panel off for a bit because I wanted a different power cord than the one I’d bought, and I installed an oversized grommet and a couple zip ties to create a strain relief as well. At some point I may want to install panel mount USB or Ethernet ports, but for now I’m perfectly fine using SSH over the wireless network, and I can use VNC to display the rPi’s console as well if I want to get more out of it or do other things. And now I have two retro computers with immense amounts of history to explore and learn more about since they were far enough before my time to have been gone when I became aware of the digital world. I’ve already started with a few things, running CP/M or 2.11 BSD, various flavors of BASIC, and of course entering in small programs via the front panel which is part of the whole purpose! Oh, and if you’re curious about the title of the post, the Digital Equipment Corporation PDP-11/70 is the machine that helped UNIX gain a foothold in the world, having been ported to it from the earlier PDP-8 where it started at Bell Labs. So aside from the blinkenlight factor, I really wanted a PDP because it’s kinda the reason why I do what I do in the end.