Fixing A PDP-11/03 Power Supply Is Easy When You Understand It

January 22, 2024 by Maya Posch 6 Comments

After we last saw [David Lovett] of [Usagi Electric], he was knee-deep in trying to fix a DEC PDP-11/03 power supply, which fortunately led to a fixed PSU and a very happy PDP-11/23 system installed in the enclosure, as he covers in today’s video. Previously, we had covered his debugging attempt of this very much dead power supply, which had led [David] down many fruitless rabbitholes. By the time he was taking various components off the board to try and induce certain results, he threw in the towel and went back to the drawing board, assisted with many community comments.

The 5V rail on a DEC PDP-11/03 power supply. (Credit: David Lovett)

Much of the confusion came down to not really understanding how this PDP-11/03 PSU design works, which isn’t that crazy in hindsight, considering how quaint it is. Although [David] originally focused on the +5V rail, a small detail that was in the schematics is that the 5V rail is based around a 7805 that has its ground referenced to the -15V rail.

It is this 7805 that provides a linearly regulated 5V rail up till its current limit, at which point the control transistor gets biased sufficiently to start conducting, which eventually triggers the driver transistor that is responsible for driving the pass switch transistor. This then charges L2 from the unregulated supply, which is used effectively as a switching mode power supply until the current across the 7805 drops sufficiently that it becomes the primary 5V rail source again. This repeats at a kHz rate, making it more or less an SMPS as we know it today, but heavily reliant on the -15V rail as can be observed in the schematic. Continue reading “Fixing A PDP-11/03 Power Supply Is Easy When You Understand It” →

Preserving Floppy Disks

September 15, 2023 by Bryan Cockfield 36 Comments

Time is almost up for magnetic storage from the 80s and 90s. Various physical limitations in storage methods from this era are conspiring to slowly degrade the data stored on things like tape, floppy disks, and hard disk drives, and after several decades data may not be recoverable anymore. It’s always worth trying to back it up, though, especially if you have something on your hands like critical evidence or court records on a nearly 50-year-old floppy disk last written to in 1993 using a DEC PDP-11.

This project all started when an investigation unit in Maryland approached the Bloop Museum with a request to use their antique computer resources to decode the information on a 5.25″ floppy disk. Even finding a floppy disk drive of this size is a difficult task, but this was further compounded not just by the age of the disk but that the data wasn’t encoded in the expected format. Using a GreaseWeazle controlled by a Raspberry Pi, they generated an audio file from the data on the disk to capture all available data, and then used that to work backwards to get to the usable information.

After some more trials with converting the analog information to digital and a clue that the data on the disk was not fragmented, they realized they were looking at data from a digital stenography machine and were finally able to decode it into something useful. Of course, stenography machines are dark magic in their own right so just getting this record still requires a stenographer to make much sense out of it.

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

PDP-11/34 Restoration And The Virtue Of Persistence

March 28, 2022 by Chris Wilkinson 12 Comments

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

November 22, 2021 by Al Williams 30 Comments

[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

May 14, 2021 by Chris Lott 10 Comments

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

April 26, 2018 by Brian Benchoff 34 Comments

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

April 26, 2017 by Jenny List 26 Comments

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.