Circuits board from a PDP-11 minicomputer with inset terminal display

PDP-11/34 Restoration And The Virtue Of Persistence

The wildly successful PDP-11 minicomputer was a major influence on the evolution of computing throughout the 1970s. While fondly remembered in modern day emulation, there’s nothing like booting up the real thing, as [Jerry Walker] explores in his video series on restoring a PDP-11/34. Examples of PDP-11 hardware are becoming increasingly rare, which makes restoration and preservation of remaining equipment even more critical. [Jerry] has gone to exhaustive lengths to restore his PDP-11/34 to working condition, painstakingly troubleshooting wire-wrapped backplane and replacing suspect ICs across the entire system. With scant documentation on some of the cards, it was often a matter of sheer will and technical know-how that saw the system eventually come back to life.

If you’ve got a couple of hours, make sure to check out the entire series of videos documentation the restoration over on YouTube. If you’ve ever thought about restoring vintage computers, this series offers an insight into the satisfying yet oh-so-tedious process of chasing down broken traces and faulty logic. Exorcising the demons from decades-old computers is almost never straightforward, but [Jerry] demonstrates that persistence can yield exciting results. After the break is the latest installment of this series, which shows the system booting into the RT-11 operating system from floppy disk.

If you don’t have the time or real estate to restore a real PDP-11, you might want to check out modern hassle-free replicas. Or, if we’ve piqued your interest in restoring minicomputers, don’t miss what we had to say about previous PDP-11 resurrections, like this PDP-11/04.

Continue reading “PDP-11/34 Restoration And The Virtue Of Persistence”

A PDP 11 By Any Other Name: Heathkit H11 Teardown And Repair

[Lee Adamson] is no stranger to classic computers. He recently picked up a Heathkit H11A which, as you might remember, is actually a PDP-11 from DEC. Well, technically, it is an LSI-11 but still. Like a proper LSI-11, the computer uses the DEC QBus. Unlike a lot of computers of its day, the H11 didn’t have a lot of switches and lights, but it did have an amazing software library for its day.

[Lee] takes us through a tour of all the different cards inside the thing. It is amazing when you think of today’s laptop motherboards that pack way more into a much smaller space. He also had to fix the power supply.

Continue reading “A PDP 11 By Any Other Name: Heathkit H11 Teardown And Repair”

Spacewar! On PDP-11 Restoration

If you want to play the original Spacewar! but you don’t have a PDP-1 nearby, then you’re in luck — assuming you have a PDP-11, that is. [Mattis Lind] has successfully restored a PDP-11 port of the game from PDF scans of the source code, which was thought to have been lost to the trash bins of DECUS (Digital Equipment Computer Users’ Society). Fortunately, [Mattis] learned that [Bill Seiler], one of the original authors, had saved a printout of the assembly language. Using a combination of OCR and manual transcription to retrieve the code, [Mattis] took a deep dive into cleaning up the errors and solving a whole lot of system library and linking issues. Adding to the difficulty is that his PDP-11 is slightly different from the one used in 1974 when this port was written.

The project was not all software — [Mattis] also needed to make a pair of joysticks, which he made from a handful of items found on AliExpress. As you can see in the video below, he indeed got it all working. [Mattis] is no stranger to the PDP-11 world. We wrote about his PDP-11 restoration project back in 2015, a quest that took over 18 months.

Continue reading “Spacewar! On PDP-11 Restoration”

A Mini-ITX PDP-11

The PDP-11 is perhaps the most important computer in history. This was the king of all minicomputers, and once you get past the amazing front panels of the 11/20, 11/40, and 11/70, you’ll find PDP-11s everywhere. Heathkit sold one. It was the smallest computer that could run Unix. There were desktop versions sold as DEC Professionals. I have been told Ticketmaster — the entire backend of all event ticket sales in the United States — still runs on PDP-11s.

One of the interesting bits of the PDP-11 is the miniaturization that happened over the course of its development. Over time, the Unibus processor cards of the earlier models were shrunk down into a single chip. This PDP-on-a-chip was then cloned by the Soviets, and like most vintage Eastern European electronics, they’re all readily available on eBay.

For his Hackaday Prize entry, [SHAOS] is taking one of these chips and turning it into a modern machine. The PDPii is a project to bring the PDP-11 back to life in the form of an Open Source motherboard with a Mini-ITX motherboard. Is it game-changing? No, not really; you could buy a desktop PDP-11 thirty years ago. This project, though, is taking new old stock chips you can buy for ten dollars and turning it into something resembling a modern system. Finally, Ticketmaster can upgrade.

The design of this project doesn’t quite meet the spec for the Mini-ITX form factor; it’s based off the RC2014 backplane Z80 computer, but desktop computer cases are cheap, as are power supplies, and I’m sure someone out there knows how to fit an eight inch floppy in a five and a quarter inch hole.

The key feature for this Mini-ITX backplane PDP-11 is a redesign of the Q-bus found in later PDPs to something that’s a bit smaller, a bit cheaper to manufacture, and still has all the relevant pins accessible. With some reconfiguring of the baroque DEC standards, [SHAOS] came up with the Bread-Board Friendly Q-bus Extended, or BBQ-Bus+. The next step for this project is gathering up a few PDP-11 compatible Russian КР1801ВМ2 CPUs and going to town on the architecture of what is probably the most replicated computer design ever.

A Modern Day PDP-11 Front End

Hands up if you feel your spiritual home is in front of a terminal with a “DIGITAL” logo on it.  It’s a name that has long ago been subsumed into first Compaq and then by extension HP, but it’s one with a lot of history when it comes to computing.

From the start of the electronic computing age, there were the computers we’d probably now describe as mainframes. Big computers that cost the GDP of a small country, filled an entire floor of a building, and could only be found in government departments, universities, and large companies. By the 1960s, the technologies existed to build computers that broke this mould, could be bought within the budget of a smaller organisation, and for which you didn’t need a huge air-conditioned basement to house. These so-called minicomputers were the great revolution of that era because they bought the fruits of computing into everyday business, and probably the most successful of the companies that produced them was the Maynard, Massachusetts-based Digital Equipment Corporation, or DEC.

DEC produced a succession of minicomputers in their PDP line, of which the most successful was their PDP-11 series. These were 16-bit minicomputers that remained in their product line from their launch in 1970 through to the early 1990s, and were available in a succession of configurations and physical form factors. The famous view of a PDP-11 is of a set of floor-to-ceiling racks, but there were also standalone terminal models, and desktop models. One of these, a PDP-11/03 from 1975, has come into the hands of [Joerg], and he’s used it to craft his LSIbox, the PDP11/03 card frame packaged with a BeagleBone for access via a modern-day interface. It’s a build in the vein of modern tube audio amplifiers that feature the retro hardware on the top of their cases, the card frame is exposed as a feature on top of a white case that is featureless except for a genuine PDP-11/03 front panel.

You might ask why anyone would do this in order to run PDP-11 software when the BeagleBone could almost certainly emulate the vintage hardware much faster than the real thing. But to take that view is to miss the point; the PDP-11 series are a seminal part of computing history, and to have genuine PDP-11 hardware on your desk is quite an achievement.

We’ve shown you a few PDP-11 projects in the past. There was this minimalist PDP-11 implementation using one of the later integrated PDP-11 processors, and we’ve seen a faithful reproduction of an earlier PDP-11 front panel powered by a Raspberry Pi.

A PDP-11 On A Chip

If you entered the world of professional computing sometime in the 1960s or 1970s there is a high probability that you would have found yourself working on a minicomputer. These were a class of computer smaller than the colossal mainframes of the day, with a price tag that put them within the range of medium-sized companies and institutions rather than large corporations or government-funded entities. Physically they were not small machines, but compared to the mainframes they did not require a special building to house them, or a high-power electrical supply.

A PDP-11 at The National Museum Of Computing, Bletchley, UK.
A PDP-11 at The National Museum Of Computing, Bletchley, UK.

One of the most prominent among the suppliers of minicomputers was Digital Equipment Corporation, otherwise known as DEC. Their PDP line of machines dominated the market, and can be found in the ancestry of many of the things we take for granted today. The first UNIX development in 1969 for instance was performed on a DEC PDP-7.

DEC’s flagship product line of the 1970s was the 16-bit PDP-11 series, launched in 1970 and continuing in production until sometime in the late 1990s. Huge numbers of these machines were sold, and it is likely that nearly all adults reading this have at some time or other encountered one at work even if we are unaware that the supermarket till receipt, invoice, or doctor’s appointment slip in our hand was processed on it.

During that over-20-year lifespan of course DEC did not retain the 74 logic based architecture of the earliest model. Successive PDP-11 generations featured ever greater integration of their processor, culminating by the 1980s in the J-11, a CMOS microprocessor implementation of a PDP-11/70. This took the form of two integrated circuits mounted on a large 60-pin DIP ceramic wafer. It was one of these devices that came the way of [bhilpert], and instead of retaining it as a curio he decided to see if he could make it work.

The PDP-11 processors had a useful feature: a debugging console built into their hardware. This means that it should be a relatively simple task to bring up a PDP-11 processor like the J-11 without providing the rest of the PDP-11 to support it, and it was this task that he set about performing. Providing a 6402 UART at the address expected of the console with a bit of 74 glue logic, a bit more 74 for an address latch, and a couple of  6264 8K by 8 RAM chips gave him a very simple but functional PDP-11 on a breadboard. He found it would run with a clock speed as high as 11MHz, but baulked at a 14MHz crystal. He suggests that the breadboard layout may be responsible for this. Hand-keying a couple of test programs, he was able to demonstrate it working.

We’ve seen a lot of the PDP-11 on these pages over the years. Of note are a restoration of a PDP-11/04, this faithful reproduction of a PDP-11 panel emulated with the help of a Raspberry Pi, and an entire PDP-11 emulated on an AVR microcontroller. We have indeed come a long way.

Thanks [BigEd] for the tip.