Replicating the PDP-11/70

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The 1975 PDP-11/70 was the first computer with unix and C. But there is much more to make this an interesting minicomputer playground. The PiDP-11 uses a modified simh on a Pi, hiding behind a replica front panel. It runs everything: From Unix v5 to 2.11BSD to RSX-11, multi-user, vector graphics and all. CAD design and injection molding were new territory for me; I felt that a replica had to the exact same slanted, white panel frame to reproduce the original's spirit . Project completed. The kit is now at We're still adding to the machine's built-in "software museum", join the PiDP-11 Google Group for more!

In terms of electronics and software, the PiDP-11 is similar to my PiDP-8. The PiDP-11 turned out more costly to make, because of the custom case (injection molding!). I hit the upper limit of the design goal: to stay under $250.

The molds for switches and case will hopefully be useful for other PDP-11 projects as well. The idea is to have these parts available as 'open-source molds'. If such a concept exists. :)

(update - in fact, a plug-in FPGA board for the PiDP-11 is being worked on as part of the PDP2011 project, in case simh on a Pi does not meet your needs).

  • 1 × Raspberry Pi +/2/3/3+
  • 64 × 5mm LEDs, red
  • 37 × Diodes 4148
  • 1 × UDN2981A or equivalent Interface and IO ICs / Peripheral Drivers and Actuators
  • 30 × Custom made switches

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  • Making kits in the jungle - an update after 4 years

    Oscarv08/16/2022 at 15:30 0 comments

    The 4000th PiDP-11 kit was sent out last month, which amazes nobody more than me. It has taken over a significant chunk of my life. And I thought it would be fun to write a bit about what you can expect as a Maker when you go into kit making. From what I hear, my experience is not unusual at all.

    In the Internet age, no niche is too small. Not even making replicas of a 50 year old computer. Apparently. I thought that I'd make this kit for one or two years, and interest would wane quickly. Not so - four years on, interest in the kit is just the same. The problem is, logistics take over your life once you start making a kit like this. My home started to resemble a live-in factory and neighbours wondered about trucks offloading stuff into my garage...

    I had plenty of time over the past few years to do it all myself, as illness in the family made me a full-time carer. In some sense, the timing was perfect. But as (sadly) that carer role comes to an end, I had to think about how I could keep making the kits without it soaking up quite so much time. I'd rather focus on development than on logistics.

    So, starting from January, my kit making endeavours move to another phase. A weird one, but that is in line with this whole project. With three friends, production of PiDP-11 kits will become somewhat of a development project in the Panamanian jungle (because, why not, up in the Swiss mountains or down in the Panamanian jungle, same difference). We've set up a micro-factory that will start making a few of my kits and once that is all up and running, could serve as a production base for other Makers as well. Thanks to the Panama Canal and the help of the Panama Postal service, it turns out this is actually an amazingly efficient location logistics-wise. I know, weird idea but so is making PDP-11 clones. And on a small scale, it is pretty cool to think that my Maker activity will provide some useful activity in a small village. To be continued!

  • Finally, a DECtape unit for the PiDP-11

    Oscarv12/22/2019 at 23:51 0 comments

    Well, a pixelated one.

    Rene Richarz just released an animated TU-56 DECTape unit for the PiDP-11. In the form of a rack-mounted HDMI display. Much time was spent thinking of how to replicate a DECtape unit for the PiDP-11, but Rene did what makes most sense: have a high-fidelity DECtape animation instead, on a low-cost HDMI screen.

    DECtape was crucial in the early days of DEC, and having a timing-exact virtual unit gives a pretty realistic experience of life before floppy disks:

    Thank you, Rene! Apparently, a TU-77 magtape unit is forthcoming. It's a good thing that the new Raspberry Pi 4 has two HDMI outputs.

  • Ancient unix - 2.11BSD updates

    Oscarv09/17/2019 at 21:33 0 comments

    The past couple of weeks have seen some interesting developments on the ancient unix side of the PDP-11. We have the high-resolution Tektronix vector display with loads of software and C library of course, the http daemon (running on just simulated 1970s hardware, mind you!) has been boosted with CGI, and quite a few other patches and goodies have been worked on (even the Ingres database has been brought to life).

    Here is the thread with the latest disk image download: (link)

  • Supporting the Pi 4

    Oscarv08/19/2019 at 19:49 0 comments

    I just updated the firmware to support the new Pi 4, which has a slightly different way of enabling the internal pull-up resistors on the GPIO.

    So... where the Pi 3B+ was 6 times faster than the real 11/70 at 7241 dhrystones/second, versus 1250 for the 1975 original... you can now enjoy a 12x speed boost versus 1975. For those who like to be as authentic as possible, use a Pi 2! Or use the 'set throttle' configuration to slow down the Pi 4. Or run multiple 11/70s in parallel. At least the Pi 4 shows there's still progress in computing and 2.11BSD unix feels like lightning :)

    The other nice thing of the Pi 4 is that you can use one HDMI display for a faithful CRT display (using cool-retro-term) and a second one for the very cool Tektronixvector tube simulation, which really uses the 4K resolution of the Pi 4:

  • Building the PiDP-11: Youtube video

    Oscarv05/07/2019 at 15:50 0 comments

    Beige-O-Vision just made a very nice 4-part Youtube movie showing how to build the PiDP-11. Worth mentioning here, as it surely will save builders a lot of time. Since nobody reads written Building Instructions anymore these days...

    The next three parts as Youtube links:

    Part 2 (soldering), Part 3 (mounting the switches), Part 4 (wrapping up)

    Thank you, Beige-O-Vision!

  • Vector Graphics on the PiDP-11

    Oscarv04/18/2019 at 12:30 0 comments

    [Rene] wrote a very pretty emulator for the Tektronix 4010 vector graphics display. And got everything set up inside 2.11BSD so you can actually use it. It's a great step forward: not only can the PiDP-11 now do vector graphics on BSD, but the C libraries are included as well! It turns out that making cool graphics takes only a tiny bit of C...

    We already had BSD unix networked on the PiDP-11, so this is becoming a pretty top-of-the line scientific minicomputing setup ;)

    One of the next challenges is to get uucp going on Unix v7, as a bridge between the Pi and the PDP-11 sides of the PiDP-11... nontrivial.

  • Big Update

    Oscarv01/15/2019 at 23:38 1 comment

    The new software (and software museum) distribution is currently in beta stage. Use your PiDP-11 with RSX-11 as a web server, surf the internet with 2.11BSD, play lunar lander on a simulated vector display, follow the Lions Commentary on a real Unix v6... things are developing rapidly at the moment.

    That is also why the manual is in .odt format. It's being added to constantly. Best opened in MS Word, but LibreOffice on the Pi is OK too. Have a look:

    This will be an exciting year as the collective effort to polish up the PDP-11 software museum gathers pace. With >800 builders, stuff gets done!

  • MagPi review, real core memory and on to historical Unices

    Oscarv12/22/2018 at 22:10 0 comments

    Much has been happening now the hardware is done.

    First comes a software update with polished-up boot disk images for unix v5, v6, v7, 2.11 BSD and Ultrix, hopefully next week. Got to do it before January 1st, Unix's 50th birthday!

    Then, Jörg has released the UniBone card. This thing lets Linux drive the old Unibus directly. It is possible to have a software CPU simulation run inside an old, potentially dead, PDP-11 and let it use its real core memory as well as any real PDP-11 disks... this needs hooking up to the PiDP :)

    Lastly, many thanks to PJ Evans for publishing this article in the latest MagPi edition! Hopefully that gets new people sucked into the dark world of PDP-11 computing.

    (note - MagPi is a truly Open Source publication, Creative Commons (BY-SA-NC 3.0))

  • PiDP-11 Updates

    Oscarv11/22/2018 at 22:48 0 comments

    Well, today was a great day - the day that the 600th PiDP-11 saw the light of day! I'll celebrate this weekend at the Vintage Computer Festival Zürich (, that's a shameless plug...)

    But there are hardware developments too. Soon, we'll have the PDP2011, Sytse's FPGA version of the PDP-11, as a plug-in module. So you can choose whether you like a Raspberry Pi simulating inside, or a FPGA hardware version to blink the front panel.

    And Jörg is cranking out the first production UniBones - a reasonably mad device that can plug in to a real DEC Unibus. Either to emulate any storage peripheral for a real PDP-11, or to hook up any real PDP-11 hardware onto the simh running inside the PiDP-11. Yes, you will be able to connect an RK-05 to the PiDP-11... who'd have thought that two years ago!

    Here, somewhere at the end of the last row, is PiDP-11 #600 waiting for the postman. One more grainy late-night picture:

  • Done! The PiDP-11 kit is live

    Oscarv05/13/2018 at 13:58 3 comments

    Ouch - the last update here was almost a year ago to the day.

    As it turns out, injection molding is not for noobs. Many thanks for all the help I got from here, including the massively helpful final check before the mold was made. It only took me 350 revisions of the CAD model, and 30 more from others... And it took custom-made brass inserts to hold the bolts in the case. So Iearned a lot, but it can be summarised: if you consider injection molding for a hobby, don't.

    But! The first 20 PiDP-11 kits went out to beta testers, and other than a slightly too-short hex spacer, no problems during the final shake-out! So here it is, the brand-new PiDP-11/70:

    Over the last few weeks, I've made a web site for the kit, and a Google Group is live as well. Now, the focus shifts to polishing up the (currently pre-alpha) software, and kits will go out from next month. Here are the kit contents:

    Stupidly I forgot to put the back panel of the case in the picture, but it's on my site anyway. That back panel taught me about laser cutting (press-out slots for connectors etc), and after injection molding, that is the way to go for any following project. Easy, low-cost and very precise results.

    I'll post some more updates in the coming weeks. As the PiDP-11 is a simh BlinkenBone device, I just wrapped up a virtual PiDP-11 with Jörg's virtual 11/70 front panel. That'll add to the fun as we go about building a right proper PDP-11 Software Museum into the software. From RT-11 to Unix v5/6/7 via Ultrix to 2.11BSD to RSX-11M Plus with networking, it should all be there of course!

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SharplyUnclear wrote 07/04/2021 at 15:42 point

I've just seen this project. I'd love to buy one (when they are back in stock) for my father who was one of the team of just six that designed the first PDP-11 prototypes at the Mill in the summer of 1968 (as a child, I didn't see him for six months). And, unsurprisingly, he was the first Field Service engineer in Europe when it shipped in 1970.

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peter jansen wrote 08/10/2019 at 08:07 point

Hi Oscar, my dad put his together (in about an afternoon, which tells you how excited he was to get it!), and he absolutely loves it.  Thanks for making this kit!  best wishes -- Peter

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Ken Yap wrote 05/19/2019 at 23:48 point

Nice work. Takes me back. The first Unix I used was on a PDP11/45. Strictly speaking one did not buy Unix as one would software these days. Software as a packaged product was in the future then. Rather institutions, academic mostly, purchased a license from Bell Labs to use it. Tapes came with source code and institutions made modifications so conferences started where programmers talked about what they did with Unix. Mostly to support teaching, but there were research uses. The beginnings of software code sharing.

It was only later that AT&T developed commercial versions.

I recount the origin of the term NUXI problem here:

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roelh wrote 04/18/2019 at 07:49 point

Hi Oscar, I am working on a TTL CPU with PDP11 instruction set. It would be great to connect it to your nice front panel ! My project can be found here: #Kobold retro TTL computer

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Oscarv wrote 04/18/2019 at 12:17 point

Roel, yes - I follow your project of course! If you want to use the PiDP as its front panel, you have one problem to overcome: the LED display is multiplexed. The simplest solution would be to sample your address & data lines with a microcontroller, which then sends the status of the lines through to the front panel.

Alternatively, just make a new PCB for inside the PiDP (keep the same PCB layout, just change the wiring, not too hard).
Let me know if I can help. Regards, Oscar.

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jacobnesau wrote 01/01/2019 at 19:42 point

2nd Greatest purchase in my life!

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Modzer0 wrote 05/13/2018 at 17:17 point

Oscar! I was actually wondering about the PiDP-11 a few days ago. I have a pair of J-11 processors and a few Russian clones from planning on participating in the PDPII project. What's the usable dimensions inside the case if a board was mounted on the back with about 1cm standoff? I can barely find time for personal projects, but I want to do a PDP-11 build with expansion so will add a port to the back, or have a new casing made that will fit the front panel and board.

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Oscarv wrote 05/13/2018 at 17:28 point

The back panel (which fits exactly in the case) is 313.5mm by 171.5mm. I can mail you the drawings if you want. You could mount a PCB behind it easily, flush to the panel. The guy behind the PDP2011 (FPGA version) is doing that; there is enough space for a PCB plus parts on the front; and the connectors mounted on the back of the PCB so they come out of the back panel. It would be amazing to fit a real J-11 board...

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Kevin Szabo wrote 01/25/2018 at 18:44 point

Will the final PiDP-11 console be the exact dimensions of the real PDP-11/70, or slightly smaller?  One reason i ask is that I have a physical PDP-11/70 front panel but it is lacking the white bezel that you are getting injection-molded.  If this bezel is close to the original dimensions then I can hopefully purchase one from you and upgrade my 11/70 panel :-)  And I would probably buy a full PiDP-11/70 kit anyway because it is so cool.

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Carl wrote 07/16/2017 at 13:18 point

Saw the PDP--8 project and, while reading, thought, "Man, I really would love to have a PDP-11 sitting on the bookshelf!" and then...  Thank you for all of the work you put into the details. Signed up with hackaday to follow and looking forward to the release!

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hackerday wrote 05/19/2016 at 20:12 point

Saw this at VFCe in Munich. Love it! Tell me when it's available, please.

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andy wrote 04/04/2016 at 01:46 point

Oscar- I would love to sign up for your PiDP-11 Early Adopter Program :-)...very excited about this project ... oh and thanks for the 2x PiDP8s by the way, one is assembled and working, the other will be a gift that I will surely assemble more quickly than the first one!  best wishes, Andy

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S.Edwards wrote 03/05/2016 at 01:11 point

Let me know as soon as you have a kit or an assembled product for sale.  I will buy at least one of them.  It was with a pdp11/70 that I learned Unix back in the 1970s.

pdp11/70 trivia: the unlabeled toggle switch (to the right of bit zero) is the secret "lamp test".  It is unlabeled because it was thought that LEDs, unlike the incandescent bulbs in earlier pdp models, would never fail.  When activated, all of the LEDs are lit.

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Andrew Wasson wrote 12/06/2015 at 01:40 point

Looking forward to this one. My PiPDP-8 went together extremely well and is happily blinking away. The PDP-11/70 front panel and switches sure is a thing of beauty. 

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Oscarv wrote 10/14/2015 at 19:53 point

Thanks! This one will hopefully teach me much more about manufacturing, CAD, and whatnot. Whole new world... Electronically, it's the same trick as the PiDP-8. 

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Benchoff wrote 10/14/2015 at 19:47 point

It begins! Can't wait to see it finished

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