A Gentle Introduction To Fortran

August 4, 2025 by Maya Posch 75 Comments

Originally known as FORTRAN, but written in lower case since the 1990s with Fortran 90, this language was developed initially by John Backus as a way to make writing programs for the IBM 704 mainframe easier. The 704 was a 1954 mainframe with the honor of being the first mass-produced computer that supported hardware-based floating point calculations. This functionality opened it up to a whole new dimension of scientific computing, with use by Bell Labs, US national laboratories, NACA (later NASA), and many universities.

Much of this work involved turning equations for fluid dynamics and similar into programs that could be run on mainframes like the 704. This translating of formulas used to be done tediously in assembly languages before Backus’ Formula Translator (FORTRAN) was introduced to remove most of this tedium. With it, engineers and physicists could focus on doing their work and generating results rather than deal with the minutiae of assembly code. Decades later, this is still what Fortran is used for today, as a domain-specific language (DSL) for scientific computing and related fields.

In this introduction to Fortran 90 and its later updates we will be looking at what exactly it is that makes Fortran still such a good choice today, as well as how to get started with it.

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The Scourge Of Fake Retro Unijunction Transistors

August 4, 2025 by Maya Posch 19 Comments

We all know that it’s easy to get caught out by fake electronic components these days, with everything from microcontrollers to specialized ASICs being fair game. More recently, retro components that were considered obsolete decades ago are now becoming increasingly popular, with the unijunction transistor (UJT) a surprising example of this. The [En Clave de Retro] YouTube channel released a video (Spanish, with English dub) documenting fake UJTs bought off AliExpress.

These AliExpress UJTs were discovered after comments to an earlier video on real UJTs said that these obsolete transistors are still being manufactured and can be bought everywhere, meaning mostly on AliExpress and Amazon. Of course, this had to be investigated, as why would anyone still manufacture UJTs today, and did some Chinese semiconductor factory really spin up a new production line for them?

Perhaps unsurprisingly, some tests later and after a quick decapping of the metal can, the inside revealed a bipolar transistor (BJT) die (see top image on the left). Specifically, a PNP BJT transistor die, packaged up inside a vintage-style metal can with fake markings claiming it is a 2N2646 UJT.

The video suggests that scams like these might be because people want to get vintage parts for cheap, and that’s created a new market for people who would rather get scammed than deal with the sticker shock of paying for genuine new-old-stock or salvaged components. For example, while programmable unijunction transistors (PUTs) like the 2N6028 are still being manufactured, they cost a few dollars a pop in low quantities. UJTs used to be common in timer circuits, but now we have the 555.

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TDA7000 die shot, with labels. Credit: Ken Shirriff

Reverse-Engineering The TDA7000 FM Radio Receiver IC

August 3, 2025 by Maya Posch 5 Comments

A wristwatch featuring the TDA7000 FM radio receiver IC. (Credit: Philips Technical Review)

During the 1980s a lot of consumer devices suddenly got a lot smaller as large-scale integration using semiconductor technology took off. This included radios, with Philips’ TDA7000 FM radio receiver IC being the first to cram most of what you’d need for an FM radio receiver into a single chip. Recently, [Ken Shirriff] had a poke at analyzing a die shot of the TDA7000, reverse-engineering its functional blocks. How did the Philips engineers manage to miniaturize an FM radio? [Ken] will show you.

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Why Cheap Digital Microscopes Are Pretty Terrible

August 3, 2025 by Maya Posch 43 Comments

The depth of field you get with a cheap Tomlov DM9 digital microscope. Pictured is the tip of a ballpoint. (Credit: Outdoors55, YouTube)

We have all seen those cheap digital microscopes, whether in USB format or with its own screen, all of them promising super-clear images of everything from butterfly wings to electronics at amazing magnification levels. In response to this, we have to paraphrase The Simpsons: in this Universe, we obey the laws of physics. This applies doubly so for image sensors and optics, which is where fundamental physics can only be dodged so far by heavy post-processing. In a recent video, the [Outdoors55] YouTube channel goes over these exact details, comparing a Tomlov DM9 digital microscope from Amazon to a quality macro lens on an APS-C format Sony Alpha a6400.

First of all, the magnification levels listed are effectively meaningless, as you are comparing a very tiny image sensor to something like an APS-C sensor, which itself is smaller than a full-frame sensor (i.e., 35 mm). As demonstrated in the video, the much larger sensor already gives you the ability to see many more details even before cranking the optical zoom levels up to something like 5 times, never mind the 1,500x claimed for the DM9.

On the optics side, the lack of significant depth of field is problematic. Although the workarounds suggested in the video work, such as focus stacking and diffusing the light projected onto the subject, it is essential to be aware of the limitations of these microscopes. That said, since we’re comparing a $150 digital microscope with a $1,500 Sony digital camera with macro lens, there’s some leeway here to say that the former will be ‘good enough’ for many tasks, but so might a simple jeweler’s loupe for even less.

There are some reasonable hobby-grade USB microscopes. There are also some hard-to-use toys.

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Exploring VersaLOGIC Pre-LSI Logic Cards With The Data/620

August 2, 2025 by Maya Posch 3 Comments

Before the era of large-scale integration (LSI) semiconductor circuits, discrete logic circuits using the common diode-transistor logic (DTL) were still necessary and available in a format that was modular and reusable. [David Lovett] over at the Usagi Electric farm has two great examples that date back to the 1950s and 1960s, showing the jump in technology over the course of a mere decade.

The newer Varian Data Machines 620 from 1966 uses germanium diodes and transistors, while the 1956 Bendix G-15 uses germanium diodes with vacuum tubes, the latter effectively fulfilling the same purpose as the transistors. The main difference between the modules is the density, with a decade of technological improvements allowing for more than double the logic on similarly sized cards and a similarly impressive reduction in power usage.

Currently, [David] is working on reverse-engineering these so-called VersaLogic modules to be able to troubleshoot the Data/620 machine in his possession. The results of these efforts are being published on GitHub. Although you can think of these modules as more or less big versions of the 7400-logic ICs — which began to replace them in the Data/620I from 1967 — some of the circuits on the cards get pretty complex.

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Raspberry Pi RP2350 A4 Stepping Addresses E9 Current Leakage Bug

July 31, 2025 by Maya Posch 31 Comments

When Raspberry Pi’s new RP2350 MCU was released in 2024, it had a slight issue in that its GPIO pins would leak a significant amount of current when a pin is configured as input with the input buffer enabled. Known as erratum 9 (E9), it has now been addressed per the July 29 Product Change Note from Raspberry Pi for the A4 stepping along with a host of other hardware and software issues.

Although the PCN is for stepping A4, it covers both steppings A3 and A4, with the hardware fixes in A3 and only software (bootrom) fixes present in A4, as confirmed by the updated RP2350 datasheet. It tells us that A3 was an internal development stepping, ergo we should only be seeing the A4 stepping in the wild alongside the original defective A2 stepping.

When we first reported on the E9 bug it was still quite unclear what this issue was about, but nearly a month later it was officially defined as an input mode current leakage issue due to an internal pull-up that was too weak. This silicon-level issue has now finally been addressed in the A3 and thus new public A4 stepping.

Although we still have to see whether this is the end of the E9 saga, this should at least offer a way forward to those who wish to use the RP2350 MCU, but who were balking at the workarounds required for E9 such as external pull-downs.

Railway Time: Why France’s Railways Ran Five Minutes Behind

July 31, 2025 by Maya Posch 10 Comments

With us chafing at time zones and daylight saving time (DST) these days, it can be easy to forget how much more confusing things were in the late 19th century. Back then few areas had synchronized their clocks to something like Greenwich Mean Time (GMT) or other standards like London time or Paris time, with everyone instead running on local time determined by as solar time. This created a massive headache for the railways, as they somehow had to make their time schedules work across what were effectively hundreds of tiny time zones while ensuring that passengers got on their train on time.

In a recent video [The Tim Traveller] explains how the creation of so-called Railway time sort-of solved this in France. As railroads massively expanded across the world by the 1850s and travel times dropped rapidly, this concept of Railway time was introduced from the US to Europe to India, creating effectively a railway-specific time zone synchronized to e.g. London time in the UK and Paris time in France. In addition to this, French railways also set the clocks inside the stations to run five minutes behind, to give travelers even more of a chance to get to their train on time when stuck in a long goodbye.

By 1911, across Europe GMT was adopted as the central time base, and the French five minute delay was eliminated as French travelers and trains were now running perfectly on time.

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