pdp-11

27 Articles

Booting A Desktop PDP-11

March 23, 2025 by 22 Comments

Ever heard of VENIX? There were lots of variants of Unix back in the day, and VENIX was one for the DEC Professional 380, which was — sort of — a PDP 11. The 1982 machine normally ran the unfortunately (but perhaps aptly) named P/OS, but you could get VENIX, too. [OldVCR] wanted to put one of these back online and decided the ST-506 hard drive was too risky. A solid-state drive upgrade and doubling the RAM to a whole megabyte was the plan.

It might seem funny to think of a desktop workstation that was essentially a PDP-11 minicomputer, but in the rush to corner the personal computer market, many vendors did the same thing: shrinking their legacy CPUs. DEC had a spotty history with small computers. [Ken Olsen] didn’t think anyone would ever want a personal computer, and the salespeople feared that cheap computers would eat into traditional sales. The Professional 350 was born out of DEC’s efforts to catch up, as [OldVCR] explains. He grabbed this one from a storage unit about to be emptied for scrap.

The post is very long, but you get a lot of history and a great look inside this vintage machine. Of course, the PDP-11 couldn’t actually handle more than 64K without tricks and you’ll learn more about that towards the end of the post, too.

Just as a preview, the story has a happy ending, including a surprising expression of gratitude from the aging computer. DEC didn’t enjoy much success in the small computer arena, eventually being bought by Compaq, which, in turn, was bought by Dell HP. During their heyday, this would have been unthinkable.

The PDP/11 did have some success because it was put on a chip that ended up in several lower-end machines, like the Heathkit H11. Ever wonder how people programmed the PDP computers with switches and lights?

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

January 22, 2024 by 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)
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” →

Check Out This PDP-11 Running Unix With A Teletype Terminal

January 16, 2024 by 19 Comments

If you’ve spent a few years around Hackaday, you’ve probably seen or heard of the DEC PDP-11 before. It was one of the great machines of the minicomputer era, back when machines like the Apple ][ and the Commodore 64 weren’t even a gleam in their creator’s eyes. You’ve also probably heard of Unix, given that so many of us use Linux on the regular. Well, now you can see them both in action, as [HappyComputerGuy] fires up real Unix on a real PDP-11/73… with a real Teletype Model 33 to boot!

It’s a fascinating dive into the tech of yesteryear, with a rich dose of history to boot. It’s mindboggling to think that video terminals were once prohibitively expensive and that teletype printers were the norm for interacting with computers. The idea of interacting with a live machine via a printed page is alien, but it’s how things were done! We’re also treated to a lesson on how to boot the PDP-11 with 2.11BSD which is a hilariously manual process. It also takes a very long time. [HappyComputerGuy] then shows off the Teletype Model 33 rocking the banner command to great effect.

It’s awesome to see this hardware as it would really have been used back in its heyday. Computing really was different before the microcomputer format became mainstream. It’s not the only PDP-11 we’ve seen lately, either! Video after the break.

Continue reading “Check Out This PDP-11 Running Unix With A Teletype Terminal” →

PDP-11 Trouble With A Ruthless Power Supply Issue

January 15, 2024 by 9 Comments

After [David Lovett] of [Usagi Electric] was donated a few cars full of DEC PDP-11 minicomputers of various flavors and vintages, he passed on most of them to loving homes, but kept a few of them himself. One goal of this being to put together a PDP-11 system that could be more easily taken to vintage computer shows than the ‘rollable’ PDP-11s he had access to prior. Of 1980s PDP-11s, the first-generation Large Scale Integration (LSI) PDP11/03 system (so-called Q-Bus models) is among the smallest, taking up about as much space as a 1980s desktop PC, while supporting the second generation LSI PDP-11/23 cards. It all seemed so easy until [David] tried testing the PDP-11/03’s PSU and everything went south.

Despite having access to the circuit diagrams of the PSU, figuring out what was going wrong was an absolute nightmare for [David], after some easy fixes involving replacing a blown fuse and bulging capacitors failed to deliver salvation. Reading through the comments to the video, it would seem that people are generally confused about whether this PSU is a linear, switching or some other configuration. What is clear is that with the absolutely massive transformer, it looks more like a linear power supply, but with a lot of protections against over current and other failure modes built-in, all of which rely on transistors and other components that could have gone bad.

Although in round 1 the PDP-11/03 PSU won the battle, we hope that once round 2 commences [David] will have had the proverbial training montage behind him (set to ‘Eye of the Usagi’, probably) and will manage to get this PSU working once more.

Continue reading “PDP-11 Trouble With A Ruthless Power Supply Issue” →

Long-Distance Gaming Over Packet Radio

March 27, 2023 by 25 Comments

The amateur radio community often gets stereotyped as a hobby with a minimum age requirement around 70, gatekeeping airwaves from those with less experience or simply ignoring unfamiliar beginners. While there is a small amount of truth to this on some local repeaters or specific frequencies, the spectrum is big enough to easily ignore those types and explore the hobby without worry (provided you are properly licensed). One of the best examples of this we’ve seen recently of esoteric radio use is this method of using packet radio to play a game of Colossal Cave Adventure.

Packet radio is a method by which digital information can be sent out over the air to nodes, which are programmed to receive these transmissions and act on them. Typically this involves something like email or SMS messaging, so playing a text-based game over the air is not too much different than its intended use. For this build, [GlassTTY] aka [G6AML] is using a Kenwood TH-D72 which receives the packets from a Mac computer. It broadcasts these packets to his node, which receives these packets and sends them to a PDP-11 running the game. Information is then sent back to the Kenwood and attached Mac in much the same way as a standard Internet connection.

The unique features of packet radio make it both an interesting and useful niche within the ham radio community, allowing for all kinds of uses where data transmission might otherwise be infeasible or impossible. A common use case is APRS, which is often used on VHF bands to send weather and position information out, but there are plenty of other uses for it as well.

Continue reading “Long-Distance Gaming Over Packet Radio” →

Reliving Second-Hand Nostalgia With The PDP-11

March 19, 2022 by 31 Comments

Levy’s Hackers: Heroes of the Computer Revolution is something like required reading for the hacker subculture, and Hackaday by extension. The first section of that book is all about early hackers and their adventures with the PDP-1 at MIT. The PDP-11 has earned a special place in hacker history for being the minicomputer used to write the first Unix. We’re always amazed to find how many of our readers have stories about programming PDP microcomputers, usually the PDP-11. Those of us young enough to have missed out on the PDP experience often have something of a second-hand nostalgia for the old machines. An exceptionally detailed article over at Ars Technica promises to get us started reliving the glory days, even if it is for the first time.

It turns out that there’s an emulator for the old minicomputers, the History Simulator, abbreviated SimH. The article gives step-by-step instructions to get the emulator running, booting Unix 2.11 on a virtual PDP-11. The fun doesn’t stop there. The write-up includes an intro the the PDP-11 hardware, and a crash-course to assembly programming for the beast. It’s a great look at how the stack, branching, and subroutines work under the hood. Most of it still applies to computing today, so it really is worth the read.

Looking for more PDP-11 lore? Check out our coverage of DEC’s history.

The image above is “PDP-11” by ToastyKen, and is licensed under CC BY 2.0

History Of Digital Equipment Corp And Bonus PDP-11 Replica Build

March 8, 2022 by 58 Comments

[RetroBytes] takes us on a whirlwind tour of the history of the Digital Equipment Corporation (DEC), its founder Ken Olsen, and during intermission builds up a working replica of the PDP-11 from a kit. DEC was a major player in the early computer industry, cranking out a number of models that were both industrial workhorses and used in computer laboratories to develop many of the operating systems and tools whose descendants we still use today. On top of that, DEC’s innovative, employee-friendly, and lightweight company structure was generally well-liked by its employees and a welcomed departure from the typical behemoths of the day.

This video takes us from the beginnings of DEC and its roots in MIT up to the PIP-11 era, highlighting major architectures and events along the way such as the PDP-1, PDP-8, and PDP-11. [RetroBytes] says he has a DEC Alpha sitting on the sidelines, so there may be a few follow-up videos in the future — perhaps one on the VAX as well.

We’ve covered this particular PDP-11 replica last year, and if these replica kits are your cup of tea, check out our coverage of kit designer [Oscar Vermeulen]’s presentation. Have you ever used real PDP or VAX computers? Let us know your war stories in the comments below.

Continue reading “History Of Digital Equipment Corp And Bonus PDP-11 Replica Build” →