Remaking the PDP-8/I using the Pi, simh & a replica front panel

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Of all the PDP-8s, the 8/I is my favourite.
Alas, getting a real one is impractical: they are impossible to obtain, and just as hard to maintain. As I learned from my KIM-1 clone, making a replica is a very good way to get involved with the innards of a vintage machine. So here is my scale 2:3 replica.

Goal: a low cost, full replica of the PDP-8/I.

Design: a Raspberry Pi, custom front panel PCB to mount it behind, and 26 custom made toggle switches. A modified simh emulator drives the front panel.

Reason: PDP-8 replicas can get very expensive very quickly. The idea was to do the exact opposite: a faithful replica using minimum parts. Made possible by the fact that the computing engine is available for as little as $5 (if you use a Pi Zero) and in software, there's no better replica of PDP-8 functionality than the simh emulator.

Result: Pi drives the 92 leds and 26 switches from the simh emulator. Only a few ARM instructions are necessary to update the front panel during a PDP instruction cycle. An acrylic front panel faithfully reproduces that of the original.

Project details on my web site.

  • 1 × Raspberry Pi (Plus to 3B or Zero/Zero W) needs 40 pin GPIO connector
  • 26 × Toggle switches
  • 12 × 390 Ohm resistors
  • 27 × 4148 Diodes
  • 1 × UDN2981A Interface and IO ICs / Peripheral Drivers and Actuators

View all 8 components

  • DECtape on the PiDP-8

    Oscarv12/23/2019 at 01:53 1 comment

    A lot of PDP-8s were run on DECtape - the venerable TU56. Rene Richarz just released his tu56 simulation for the PiDP-8. It's hard to describe - with the TU56 whirring on an animated small HDMI display, you really get a feel for Life Before Disks:

    The simulation is timing-exact, and all controls work just as on the real one. Get one of these cheap small HDMI displays, and you can make a PDP-8 rack with tape drives!

  • A B compiler for the PDP-8...

    Oscarv10/14/2019 at 22:27 1 comment

    New software for a 50 year old computer doesn't appear every day. A full B compiler should be too much to ask. But despite the minimal hardware, Robert Clausecker managed this feat, and as opposed to C, the simpler B suits the 8 very well.

    Details are here (PDF) & here; a video presentation is here: (link).

    It's a great introduction to the PDP-8 instruction set, and how to build a compiler for it.

  • The 2019 PiDP-8

    Oscarv09/17/2019 at 15:43 0 comments

    It's already out since July, but just to log the improvements:

    • There's actually a lot to learn about kit-making: how to prevent beginners from messing up soldering a computer kit for the first time. So it was time for a revamp of the PiDP-8, putting in all those lessons. Easy to mount replica switches; a LED cover panel to quickly put in some 89 LEDs; but the PCB redesign actually took in most of the lessons-learned: when you don't know who is going to solder up your boards, better make wide traces, spaced far apart. Kits will be built with 450C soldering irons used by plumbers. And ensure you can diagnose problems over email. So. Bold statement... Anyone can build this kit now ;)
    • The software got an update to run on the new Pi 4, although that's gross overkill if all you want is a PDP-8. The Pi Zero makes more sense. But most PiDP-8s serve other purposes in the background - file/web/media server and whatnot. Then, the Pi 4 is very nice. And it finally has the graphics oomph to run a picture-perfect CRT simulator for that glorious Serial Terminal feeling.
    • The fancy vector graphics (spacewar, but also a 2019 fast-vector Game of Life) are now in and provide a nice fancy demo mode for all that 1960s 12-bit-power.

    Nothing has changed in the looks of the PiDP-8 once it's completed. No need to get the hardware update if you already have one! But thank you for all the lessons I learned.

    Now the race is on to kit #3000. Or, secret ambition, to one day reach kit #3841, because that's when there are finally more PiDP-8/I's than PDP-8/I's.

  • Major Software Update

    Oscarv05/12/2017 at 23:36 0 comments

    Well, it's been a year since the last PiDP-8 update. A new software version, in a proper repository, has been set up by Warren. Many thanks not only for that effort, but also for extending the software quite a bit, including Ian's fab lamp emulator. Lamp emulator? Yes. The PiDP has LEDs but the real PDP-8/I had lamps. Lamps glow, but LEDs flicker. If you want, now the PiDP has glowing LEDs.

    The repository is here: PiDP-8 repository

    Download the latest ready-to-run SD card image here: SD boot image

    Any feedback is welcomed at the PiDP forum: PiDP-8 forum

    Thanks Warren, and all others who have contributed!

  • Finally - boot time down to 10 seconds

    Oscarv03/04/2016 at 23:46 0 comments

    The Raspberry Pi is a great microcontroller - except for the fact that it boots up like a PC, taking 40 seconds. Which is not how a microcontroller should feel. Two months ago, I discovered pipaOS - a pretty standard Raspbian, but it boots up in less than 10 seconds including wifi connectivity. So the latest version of the PiDP 'firmware' is based on this brilliant little distribution.

    On another note, in February I made the 3rd batch of PiDP kits, bringing the total in existence to about 700 now. Yay! Confirms again: in the age of the internet, there's no niche obscure enough not to bother with...

    The new design with custom switches has worked very well:

    Thanks to Adrian for providing the picture

    I will do one more batch of kits in April (hopefully), and then the backlog of kit orders is done. Finally! On to the PiDP-11.

  • New switches, new PiDP-8

    Oscarv01/19/2016 at 23:26 3 comments

    Well, it took about 4 months but I finally got myself custom switches for the PiDP... It turns out that making custom switches is not all that hard - all I did was make a Sketchup model (adapt an existing model, really) and the switch manufacturer did the rest reasonably painlessly.

    Here is the end result: the 2016 PiDP-8...

    As usual, I took the pictures in bad lighting so the color looks a bit off from what they really are. So now I know custom switches are not hard to arrange. If anyone is thinking of doing the same, I'm happy to share the details.

    Another improvement is in the software. The Raspberry Pi is a nice board but the standard Linux distribution is bloated - taking up to 40 seconds to boot. Which is annoying if you want to use it as a microcontroller. I found pipaOS - which boots into wifi in less than 10 seconds, so that's what the new firmware for the PiDP is build upon. Details are here.

  • New Replica 8/I Switches

    Oscarv10/30/2015 at 11:01 1 comment

    It seems my first 3D CAD adventure turns out OK. Here's a snapshot of a sample PDP-8/I replica switch straight out of the factory. The production versions will of course not be in black, but the shape is what I had hoped for. It'll still be a month before I have them ready for the kits, but - it's progress!

  • Next steps…

    Oscarv09/29/2015 at 14:03 0 comments

    Now I’ve gained some experience with CAD for the PiDP’s new switches, I’d like to explore the manufacturing aspects of ‘Kit Making’ a bit more. For a more ambitious PDP-11 replica - the other machine I always wanted but never had. The PDP-11/70 in particular had a very special form factor:

    You really want those cosmetics for a replica as well: the sculpted white frame, the peculiar switches. Over the last two weeks, two people offered to share the financial risk of making an injection-molded case. One being a right proper Venture Capitalist (although this is a hobby sponsorship for him too – thanks!), who also set up a meeting with the team of one of his companies, people who have proper expertise in the field of injection molding.

    Funny how one thing leads to another. If I look back over the past few months, it's amazing how much knowledge-sharing and help was available. People who offered to send someone to China to help source parts, people who beta-tested, supplied original parts to replicate… And how that really can lead to getting you into things you had no clue about just a few months ago!

    Although the PiDP-8 project will only be completed when the new switches are in, it’s drawing to a close. On to the next & bigger thing :)

  • Actual Production - Lessons learned the hard way

    Oscarv09/09/2015 at 13:47 0 comments

    The next batch of 250 kits is coming up. Just while I was enjoying VCF MidWest, I got bad news: the manufacturer mailed to say that actually, my switches had not been/would never be shipped. Their production molds were broken. OK... but I had just ordered 250 PCBs and front panels, which depend on the exact measurements of this particular switch. Ouch.

    Lesson 1: use standard parts. This one was non-standard, used by the Chinese military 10 years ago. Or something. You only find out afterwards...

    Lesson 2: if you buy cheap, get the parts in your hand before planning further.

    After three weeks of solution-seeking, things are looking up with the surprisingly helpful switch manufacturer! I will have to ditch the PCBs I made, but a new standard switch with custom-made switch caps is in sight. Custom-made... I learned some about that as well, because I had a go at a custom 3D design for the replacement switches:

    Lesson 3: Making a 3D design of something simple like a switch cap, then have it implemented on the other side of the world is not trivial. Not cheap either. And if you're a hobbyist, you quickly learn of the pitfalls of precision design. 'Nuf said... I have to wait and see how the new switches turn out. Manufacturing time is a month.

    Oh well. I can still do a run of kits in October, and then I'll have to take six weeks to respin and test a new PCB. At least I now have switches with standard footprints.

  • spacewar - the first video game - at VCF Midwest

    Oscarv08/21/2015 at 00:32 0 comments

    Got to show something new at VCF Midwest on August 29-30, so here is the PiDP-8 running spacewar, the world's first ever video game.

    See halfway on my page here for spacewar history. Note the homebrew controllers - clunky looks intended (see the originals here). The poor photography not intentional though. It's just very late at night... preparation for a VCF always happens at ungodly hours.

    The screen is a $20 HMDI TFT put in a matching case. Makes for a rack-mounted look. I still want to add some instrument features (A/D, D/A converters etc) so it becomes a proper lab peripheral, just like DEC offered back in the day.

    Intellectual credits go to Kyle Owen, who ported spacewar to SimH and also made a nice X/Y (oscilloscope) display emulator using Processing. The phosphorous effect deserves a video, really! I ported it to C on the Pi, as Processing has 'issues' with the little machine.

    Hope to see you at VCF Midwest next week! Once again, my suitcase will need space on the way back home from VCF MW. So if you're there and want to adopt one of the demo machines, please let me know :)

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  • 1
    Step 1

    ---------- more ----------Build Instructions are here (link to PiDP web site)

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Dr. Cockroach wrote 02/08/2017 at 17:42 point

Boy, I am doing it the hard way :-). This project is really nice, job well done :-)

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Jesse wrote 12/15/2016 at 02:32 point

I might be able to get you a bunch of ACTUAL PDP 8 parts if you would like. 

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Jerel Crosland wrote 12/01/2016 at 18:05 point

I hope I get a chance to buy one of these kits! I can't wait to build one. It is so authentic looking! The only things missing are the keyed power switch and panel lock! Oh, and the panel lights were rounded squares. I'm sure it was just the shape of the transparent spots in the acrylic overlay that allowed the lights to shine through. (The lights shown are simulated but are how the "nulljob" looked when it wasn't doing anything else, but look at the other lights to see how they looked.)

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Oscarv wrote 12/01/2016 at 20:11 point

Yes, you're right - I made a transparent slot for a row of 12 leds rather than 'windows' for each of them. It saved a lot in production cost and my main goal was to make this a low-cost gadget. Nice demo, BTW! Have you seen this:

It's a photorealistic PDP-8/I on your (touch)screen.

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Tobias wrote 03/31/2016 at 23:04 point

Great project! When I went to school, one of our assignments was to program an elevator control on a PDP (I think it was  PDP-11 but not sure). 

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cmduarte wrote 12/28/2015 at 16:03 point

Intersil made a cmos chipset version of the PDP8/i.  I have that kit (processor, 4kX12 bits of cmos ram, serial io chip, and maybe another io chip.  I never got around to using the chip kit. Send me a pm, and it can be yours for the shipping cost.  Note, I will have to dig it out of my loft storage, so it may take a little time to find it, heh.

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Oscarv wrote 12/28/2015 at 17:10 point

I'm probably (as in, certainly) too incompetent to put the chipset to good use. But there are quite a few people out there very eager for it - I think think is what the Spare Time Gizmo's PDP-8 replica needs. But the chip sets are as scarce as hen's teeth!

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Martian wrote 10/21/2015 at 02:09 point

Wonderful project, this is the kind of thing that I really enjoy and gives me inspiration for my own madcap ideas, it is a work of engineering art.  Also preserving our computing heritage is so important as well as fun.   Having a PDP at home is something that was once so far beyond our wildest dreams, you are making it a reality and in the price range many people can aim for, for that I cannot thank you enough.

I've just visited your web site, absolutely great stuff there.  When I can commit I will be begging for you to allow me to buy one.   Again, great project, really well done.

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Joel Fletcher wrote 09/11/2015 at 03:43 point

I was another of the fortunate ones who got to purchase one of the first batch of PiDP-8 kits. It was an extremely well done kit. The panel itself is a gem, and the system board is awesome. My first programming experience was learning FOCAL on an 8/L. Having a great time playing Adventure, blinking the lights, and writing PAL code.

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bluemontie wrote 09/11/2015 at 01:41 point

this will be awesome.  I love these old 'blinken light' computers.  Cant wait to get this.  it will be a lot of fun...  :)

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crockett.martin wrote 09/10/2015 at 10:54 point

An awesome, cost effective project, I can't wait to receive and start building this amazing project. 

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Norman wrote 09/10/2015 at 00:50 point

I built this kit and it wasn't challenging -- and I seldom did any soldering before.  Beyond the blinking lights, I've been using the project to teach kids now to program at the machine level because the PDP-8 only had 8 basic instructions.  I even connected it to a VT-420 Terminal for an authentic feel.

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unclebob wrote 09/09/2015 at 21:43 point

This project was a load of fun to build.  Two blissful nights of simple soldering, clipping, and painting.  And the result is _stellar_.  Now it sits on my shelf, running Adventure, happily bouncing the accumulator lights back and forth, waiting for me to walk into the well house.

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Andrew Wasson wrote 09/09/2015 at 21:40 point

It took all of a couple of days to assemble (I took my time) and it's one of the best thought out and executed kits I've ever built. It's sitting beside my workstation now, criticizing my Adventure playing "skillz". Well done! 

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RandyKC wrote 06/18/2015 at 00:44 point

I would love a bare bones kit. The September run would be fine. My thought is to make a laptop using this panel and a Raspi2.

Please let me know the cost of your kit.

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jens.andree wrote 05/20/2015 at 01:53 point

Wow! I never was able to lay my hands on a PDP-8 because when I grew out of my PET, VIC 20 and later C64 emerged on the market and the luster for already by then old computers was way less than the want for a new one, and that's what it's like for most of us I guess...

I'd be very happy to buy a kit from you and go about building my own PiDP-8/I for sure! Ok, most of us can spin our own boards but getting the acrylic done will probably be cumbersome to say the least, and thus I'm very excited to order a kit from you, as and when it's ready! Will you be able to supply the switches as well? I've had a quick look but found no suitable switches from my usual sources...

If I can have a PiDP-8/I and also at some later stage an (emulated) Altair 8800 up and running it would be awesome, next to my Commodore collection!!! :D

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counter.culture wrote 04/11/2015 at 02:27 point

i just watched your video.  i've got wood - to put it mildly.  if the price is right, i definitely will buy a kit.  holy smokes that thing is nice - time to break out the 35 year old reference card and start thinking in octal again.

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Joe McMorrow wrote 04/01/2015 at 12:56 point

My first experience of a Computer back in 1977 was a Teletype (with paper punch / reader) connected to a PDP at what was then the North Middlesex Polytechnic. I actually used to get to school early so I could have an hour feeding my BASIC Addiction before school started. We then progressed onto a Commodore 8k PET (Personal Electronic Transactor) - one PET for the whole school of around 800 kids!

I am delighted that there are those with abilities beyond my own still keeping this old Computer world alive and kicking!

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Jerel Crosland wrote 03/30/2015 at 22:17 point

The first computer I ever used was a PDP-8i. After teaching myself BASIC I discovered Orange Coast College allowed "community" accounts on their IBM 360 so I taught myself Fortran IV and APL. Then I went back to the PDP-8 and taught myself PAL-D, which is the assembler language used by TSS-8, a timesharing OS used on the PDP-8. One 4k (core) field (partition) would execute user code, while the other 4k field swapped out the previous user and swapped in the next user. Rebooting required toggling a bootstrap (RIM loader) in via the front panel which loaded the OS loader (BIN loader) from paper tape. Amazing to think this 12-bit machine is still even being thought of anymore. It was extremely significant in so many ways, yet largely unknown today. I can't wait to get my hands on one of these!

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DL101 wrote 03/29/2015 at 00:59 point

You should do a atx x86 pc based edition next that can be used as a regular pc!

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counter.culture wrote 03/28/2015 at 02:19 point

the pdp 11/04 was my first home computer in 1977 - my dad had one.  didn't have a fancy front panel to load the boot instruction sequence - you just typed it into the teletype, hit return and watch the paper tape reader hustle.  BASIC, bio-rhythm, lunar lander, adventure (colossal cave)...oh what the spoiled little brats of today are missing.

i also used the pdp 8.  i can't believe i turned people away who tried to give me all manner of these machines about 12 years ago (they're untouchable these days) - but the power to run them would cost a small fortune - better to emulate them.

very nice work with this project - your next assignment is to emulate one in fpga.

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Benchoff wrote 02/24/2015 at 23:59 point

Oh, man. This one is good. Be sure to click that 'submit to tips line' button when you get a few more project logs. This one is going up on the HaD blog for sure.

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