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.

Can you dig it? (yes, I can)
And I’ve been waiting such a long time
For today

–Chicago, “Saturday in the Park”

I mentioned previously that I had ordered a couple kits to assemble, and the first one – the Altair-Duino – arrived and got finished a scant few hours later (because I was so looking forward to it, and I stayed up a little too late playing with it, because it’s so freaking awesome).

[Note, in order to see captions on the photos, you’ll have to click through to the gallery on Google Photos; you’re not missing a hell of a lot this time though.]

The kit arrived Saturday early afternoon, and I set about earnestly unpacking it and checking things out. My youngest was in the process of finishing his own soldering project on my workbench so I knew I couldn’t start right away, which was fine – I’d need a bit of time to sort through all the parts anyway and make sure everything was there, and he’d been looking forward to his soldering project for a while and had started it Friday night after dinner. Everything was packed very well, and when pulling the parts from the box my first impression was the weight and quality of the metal frame for the unit, as well as the details on the front panel piece. The back panel is mostly clear acrylic with a small bit of black acrylic where the daughter card and VT100 emulator attach (more on that later). Overall I was already pleased with the kit.

After dinner and having sorted all the pieces, I look at what was before me. The parts list said I needed a 220k? resistor, but I had a 220? resistor. Reading forward in the instructions, and looking at the board, I found the ‘k’ was a typo in the parts list so I was good there. But it also said I should have 8 of the nylon bolts, and I only had four. Reading ahead, I wasn’t sure if I really needed all 8 – there was at least one spot where I saw they were used and then later removed. Either way, that wouldn’t be until the end stages of assembly so I figured I could forge ahead and worry about them later. Everything else checked out, so I started putting components to circuit board and it wasn’t very long at all before I had to warm up the iron.

Everything went on very easily. There was hints like putting the switches in place and then laying the front panel over them before soldering so that when you do they’re lined up side-to-side in a way to fit through the holes easily so you’re not surprised when it comes to final assembly, and another trick of installing all the LEDs into the holes, mounting the front panel, and flipping the whole thing over on its face with a satisfying “clack” while they all fall forward into their respective holes. A slight tap on one or two to get them to line up as well, and then you can solder every one in place – and all of them will be at the perfect height and angle to fit correctly on the front. Of course in the end when I was muscling things into place I ended up bending one of them the wrong way, but I was able to get it back pretty easily and everything looked fine in the end.

A few comments about the device itself. The brain of the whole operation is an Arduino Due which is preprogrammed with the software, which is easily reflashed if there’s a software update or you completely hose things and need to start over. There’s also an SD card with a bunch more software installed on it that can be loaded. The programming port is routed to the back panel with an extension panel-mount cable so you can always get to it easily for programming or powering it up, and the default serial connection from the Altair is routed to that connection as well. There are other options too, including one serial port that is also presented on the back panel, and another which connects to another card mounted on the back which contains a VT100 emulator running in a PIC microcontroller with a VGA port for display output and a USB port to connect a keyboard. I found I didn’t have the right cable to try connecting my VT320 to the serial port (I used to use it connected to my computer to receive syslogs, so the custom cable I have for it is a null modem), and I haven’t dug an old monitor out with a VGA port on it to try the VT100 emulator. Maybe I’ll do that in the next day or so, but then again the next project will be here on Tuesday so it might wait until after that.

What can be done with it? Well, it’s a clone of an Altair 8800, so anything you can do with that, you can do with this. I used the panel switches to enter in a couple programs and saw them work, and could demonstrate binary math and how assembly language works, how to debug a program by stepping through the code to look at the instructions entered, how to step through execution one step at a time to see how it works, etc. I even adjusted the internal settings so the emulated serial port would “act” more like a real serial port and spit out data to the terminal at a whopping 110 baud as it should be. Then I toggled in a bootloader, executed it, simulated loading a paper tape drive with 4k BASIC and hit play. Sure enough it took around 17 seconds for the first stage to load, then another 6.5 minutes while the second stage pulled in the data, finally outputting a satisfying “MEMORY SIZE?” to the terminal. I’m looking forward to exploring more about some of the operating systems that came and went before I got into computing, since my first foray into computing was an IBM PCjr with cartridge BASIC and PC DOS 2.10, right around when CP/M was making its exit.

Overall I’m extremely pleased with my choice. The kit is well designed and the manufactured parts done with precision, the instructions were easy to follow and the end result is a joy to play with and look at – so far everyone has commented that it’s one of the coolest looking things in the shack, and that’s while looking at an HF radio too. My only regret is that I didn’t buy the ESP-01S module that would provide for getting the device on my local network so I can just telnet into it; since I needed the nylon bolts that were missing from my kit, I asked if he could provide them with the module if I ordered it and he said yes, so that’s all on the way. I’ll have to decide if I want to commit to not using the standard serial port, or go through the process of soldering onto the TX/RX LEDs on the Arduino to basically squeeze one more serial port out of the device and have access to all three. I mean, I got it for hacking, why not hack right?

Part of me wants to make a video of using it or something, but I don’t feel like digging out a good camera to record it, or edit (because I’m a bit of a stickler for things being Just Right when it comes to that kind of production). Maybe the mood will strike me later.

Edit 2025/03/02: There’s three more photos at the end of the gallery from a follow-up addition which I cover in another post. Didn’t make sense to make a new album just for that.

Could it be? Yes, it could.
Somethin’s comin’, somethin’ good, if I can wait.

–Stephen Sondheim, “Something’s Coming”

[Personally I prefer the version by Yes, but that’s just me]

I’ve been wanting to get a project or two I can solder and tinker with, because I’m getting that itch. I’d keep looking at things like various radio kits, but the biggest problem with kit radios is that they tend to be directed at CW operators which I am not – even the most powerful kit radio I could easily grab ahold of would do max 10W SSB, and that’s not enough to make meaningful contacts for me. So while it would be fun to assemble and test, I probably wouldn’t use it much after that. And recently I realized that I’d been saving up some money but without a goal to save for. That can be a dangerous combination, so I went looking for something that would scratch that itch to build and tinker again.

Enter some retro computing items. Some years ago when looking to populate my wish list with more items, I stumbled across some things that I didn’t know existed. Of course there’s old computing stuff out there, and I love playing with it, but some of it is too old to be reliably dealt with – too hard to source the parts, or too expensive, too big, all kinds of reasons it’s not a Good Idea. Well, some enterprising folks out there have come up with good ways to handle that via emulation and replicas, and I am here for it. The first one on the list is a replica of the IMSAI 8080, which many will know as the computer that David Lightman uses in the movie WarGames.

This replica is done by The High Nibble (love that name) and is available on his website. One of the main reasons I want one of these is of course the movie factor, but also because I love tinkering with old hardware, and why not? It has blinkenlights. That makes it better. One downside to this particular kit though is that it’s not readily available from what I can tell – the order form is more of an interest form, and after you submit your address you’ll get an email when a kit is available to purchase. It’s also a little more expensive than other items on my list, which is not a mark against this kit at all – I think it’s worth every penny, I just didn’t want to spend that many of them on one item at this time. So this one will stay on my list for another day, hopefully soon.

What’s next? Well, the IMSAI was one of the first “computer clones” (those who remember the early 90s to 2000s may recall when things went from “IBM or Apple” to “Send In The Clones” as everyone started making mostly compatible hardware). The original that it copied was the Altair 8800, and I remember seeing them as well when I was first getting into computing. You may have too, as it’s a pretty iconic look.

Adwater & Stir (another awesome name) offers the Altair-Duino which comes in two flavors. One is pretty close to the original in terms of space and expandability, and the other is slightly smaller and much more shelf friendly (and cheaper). Since I’m more interested in tinkering and building, and don’t plan to add expansion cards at the moment (nor do I care to have it take up a lot of space on a desk or cart somewhere), the smaller “experimenter” fits the bill quite nicely. It keeps the cool factor between the blinkenlights and the panel switches, where you can actually “toggle in” an entire program if desired but not necessary since a bunch of stuff is preloaded on the Arduino inside for you to cheat and load very quickly. But if you want to bit bang on the panel, go for it – and that’s exactly the kind of thing I like to read about. Reading those pages, I also saw a comment at the bottom of one of them that gave me a head scratch (or maybe a record scratch):

I also have to say “thank you” to Oscar Vermeulen of Obsolescence Guaranteed for his recreation of the PDP-8, which got me on this quest to recreate historic computers.

PDP-8 I thought? I don’t know much about the DEC machines, but they had blinkenlights too.. and I remember seeing them in the background of various podcasts where Steve Gibson was a guest (such as “Security Now”). So I moused over to there, and found something I could really get behind: the PiDP-11/70

The PDP-11/70 was a computer from Digital Equipment Corporation and was instrumental in the design and creation of Unix. It’s also one of the last machines to still have a proper front panel with lights showing address and data information (later PDP-11s had their panels replaced with blank panels since after the boot sequence the switches and registers were typically not needed). As you may guess from the name, the brain of this one is a Raspberry Pi running the ‘simh’ emulation package, so while there’s the faithful recreation of the panel and operation there’s a lot more power internally to do other things, including running a full Unix if you so desire (nevermind the fact that the Pi itself runs Linux anyway).

So armed with all of this, what to do? Well, I counted up the savings I had sitting here and realized a bit of good news – I had enough for the PiDP-11 and the Altair-Duino. So I did what any self-respecting geek would do and ordered them both. Plus a new Raspberry Pi 4B since the only one I had sitting here is a 2B which, while it’s plenty powerful enough to run the emulator, doesn’t have built-in wi-fi (and the cost of a dongle is a portion of the cost of a whole Pi at that point). The Altair and the Pi will be here Saturday, and I hope early enough that I can get a good start working on it, while the PDP will arrive next Tuesday and probably have to wait until evenings and/or the weekend for its time to shine. I’ll be taking photos during the process to document it and share here, because.. well, this is just too freaking cool not to share. Oh, and if you’re curious about the term “blinkenlights”, it comes from a long line of posters and such that would be placed around machinery as a WWII-era joke. See an example below, or read the entry from The New Hacker’s Dictionary that explains it well.