Patching Together Logic Gates

August 25, 2023 by Bryan Cockfield 14 Comments

The digital world offers many advantages over its analog relatives, the use of boolean logic among them. Some of the functions, like NOT, OR, and AND are fairly straightforward and line up nicely with their linguistic counterparts. Others are more elusive, like XOR and NAND. For those just getting their start in digital logic, this teaching tool allows different logic gates to be wired together with patch cables.

While [David] first thought to use 74-series logic circuits directly, a much more versatile solution was to use configurable custom logic — a feature found in AVR DA-series microcontrollers that allows for the creation of custom logic circuits without the need for external hardware or complex programming. He went with an ATmega4809 which is capable of supporting twelve gates which are depicted graphically on the board, where the patch cables can be connected between inputs and outputs from a set of switches on the left to another set of LEDs on the right. The microcontroller continually polls for connections, applies the correct logic via a lookup table, and lights the appropriate LED.

Even with only twelve gates, the amount of real-world analogs that can be created with this teaching tool are numerous and varied, from simple things like displaying traffic light patterns in the correct order to implementing a binary adder. It’s an excellent way to get started in digital logic or understanding gates, and much simpler than dealing with 74-series chips on a breadboard like many of us might have done, but those logic chips can be powerful tools to have on hand even in the modern world of microcontrollers.

GLASNOST Is A Computer That Makes Transparency A Priority

July 17, 2023 by Bryan Cockfield 7 Comments

We live in a world where most of us take the transistor for granted. Within arm’s length of most people reading this, there are likely over ten billion of them sending electrons in every direction. But the transistor was not the first technology to come around to make the computer a possibility, but if you go to the lengths of building something with an alternative, like this vacuum tube computer, you may appreciate them just a tiny bit more.

This vacuum tube computer is called GLASNOST, which according to its creator [Paul] means “glass, no semiconductors” with the idea that the working parts of the computer (besides the passive components) are transparent glass tubes, unlike their opaque silicon-based alternatives. It boasts a graphical display on an oscilloscope, 4096 words of memory, and a custom four-bit architecture based only on NOT, NOR, and OR gates which are simpler to create with the bulky tubes.

The project is still a work in progress but already [Paul] has the core memory figured out and the computer modeled in a logic simulator. The next steps are currently being worked through which includes getting the logic gates to function in the real world. We eagerly await the next steps of this novel computer and, if you want to see one that was built recently and not in the distant past of the 1950s, take a look at the Electron Tube New Automatic Computer that was completed just a few years ago.

FPGA Plays Tic-Tac-Toe

June 24, 2023 by Bryan Cockfield 18 Comments

As computers get more and more powerful and artificial intelligence algorithms improve, few games remain where the best humans can reliably beat their electronic counterparts. In chess this barrier was passed in 2005 with the last human win against a computer, and recently humans lost to computers at go. Simpler games like tic-tac-toe have been solved for all possible positions for a while now, so even a simple computer will always win or tie the game. But that doesn’t mean that there’s nothing left to learn about these games as [Hayden] demonstrates with this tic-tac-toe game built entirely on an FPGA.

[Hayden] is making this as part of a college course on digital design, so it really starts at first principles for working with FPGAs. It’s programmed in Verilog on a Basys 3 board, which also hosts the switches used as the game’s input and handles the VGA video output as well. The build uses state machines to keep track of the moves played on each of the squares, and another state machine to keep track of whether or not the current game has been won. If so, it highlights the winning moves in red, and stops taking further inputs until it is reset. Some more logic ties everything together along with a customized VGA driver to produce the entire gaming experience.

A game like tic-tac-toe is a great way to master the fundamentals of a system like this before moving on to more complex programs, especially on an FPGA platform that might handle a lot of the things we take for granted on more traditional computing systems, such as the video output. If you’re interested in taking more of a deep dive into the world of FPGAs, we published a primer about them a few years ago that will get you started.

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Op-Amp Challenge: A Logic-Free BCD

June 8, 2023 by Dan Maloney 10 Comments

Of digital electronics, a wise man once said that “Every idiot can count to one.” Truer words have rarely been spoken, because at the end of the day, every digital circuit is really just an analog circuit with the interesting bits abstracted away. And to celebrate that way of looking at things, we’re pleased to present this BCD to seven-segment converter that uses no logic chips.

With cheap and easily available chips that perform this exact job, it might seem a little loopy to throw 20 LM324 op-amps at the job. But as [gschmidt958] explains, this is strictly for the challenge, plus it made a nice entry in the recently concluded Op-Amp Challenge contest. His work began in simulation, exploring op-amp versions of the basic logic gates — NAND, AND, OR, and NOT — all of which rely on using the LM324s as comparators. There were real-world curveballs, of course, not least of which was running out of the 10k resistors used for input averaging. Another plot twist was running out of time to order a PCB, which required designing one using MS Paint and etching it at home.

The demo video below shows the circuit at work, taking the BCD output of a 74HC393 counter — clocked by a 555, naturally — and driving a seven-segment LED. It’s honestly a lot of work for such a simple task, but there’s something satisfying about the whole project. We think [Widlar] would be proud.

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Hackaday Prize 2023: Building A Relay ALU

June 1, 2023 by Dave Rowntree 9 Comments

There’s much truth in the advice that, to truly understand something, you need to build it yourself from the ground up. That’s the idea behind [Christian]’s entry for the Re-engineering Education category of the 2023 Hackaday Prize. Built as an educational demonstrator, this is a complete arithmetic-logic unit (ALU) using discrete relays — and not high-density types either — these are the big honking clear-cased kind.

The design is neatly, intentionally, partitioned along functional lines, with four custom PCB designs, each board operating on 4-bits. To handle a byte-length word, boards are simply cascaded, making a total of eight. The register, adder, logic function, and multiplex boards are the heart of the build with an additional two custom boards for visualization (using an Arduino for convenience) and IO forming the interface. After all, a basic CPU is just an ALU and some control around it, the magic is really in the ALU.

The fundamental logical operations operating upon two operands, {A, B} are A, ~A, B, ~B, A or B, A and B, A xor B, can be computed from just four relays per bit. The logic outputs do need to be fed into a 7-to-1 bit selector before being fed to the output register, but that’s the job of a separate board. The adder function is the most basic, simply a pair of half-adders and an OR-gate to handle the chaining of the carry inputs and generate the carry chain output.

3D printed cable runs are a nice touch and make for a slick wiring job to tie it all together.

For a more complete relay-based CPU, you could check out the MERCIA relay computer project, not to mention this wonderfully polished build.

ChatGPT Powers A Different Kind Of Logic Analyzer

April 6, 2023 by Dan Maloney 17 Comments

If you’re hoping that this AI-powered logic analyzer will help you quickly debug that wonky digital circuit on your bench with the magic of AI, we’re sorry to disappoint you. But if you’re in luck if you’re in the market for something to help you detect logical fallacies someone spouts in conversation. With the magic of AI, of course.

First, a quick review: logic fallacies are errors in reasoning that lead to the wrong conclusions from a set of observations. Enumerating the kinds of fallacies has become a bit of a cottage industry in this age of fake news and misinformation, to the extent that many of the common fallacies have catchy names like “Texas Sharpshooter” or “No True Scotsman”. Each fallacy has its own set of characteristics, and while it can be easy to pick some of them out, analyzing speech and finding them all is a tough job.

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Retrotechtacular: Critical Code Reading, 70s Style

January 5, 2023 by Dan Maloney 30 Comments

Anyone who has ever made a living writing code has probably had some version of the following drilled into their head: “Always write your code so the next person can understand it.” Every single coder has then gone on to do exactly the opposite, using cryptic variables and bizarre structures that nobody else could possibly follow. And every single coder has also forgotten the next part of that saying — “Because the next person could be you” — and gone on to curse out an often anonymous predecessor when equally inscrutable code is thrust upon them to maintain. Cognitive dissonance be damned!

It’s a tale as old as time, or at least as old as programming has existed as a profession. And by 1975, poorly written code was enough of a problem that an outfit called Edutronics put together the animated gem Critical Program Reading: Structuring an Unstructured Program. It’s apparently Part 1 of a larger series on structured programming techniques, and comes to us by way of [Alec Watson], host of Technology Connections on YouTube, by way of his second channel, the delightfully named Technology Connextras.

The film’s three minimally animated characters, each of whom could have been the villain in an episode of Scooby Doo, are tasked by a stern-sounding narrator to analyze a fragment of pseudocode that’s written in a concoction of COBOL, PL/1, and a bunch of other languages. The code is a hot mess, but our heroes muddle through it line by awful line, making it more readable by guessing at more descriptive variable names, adding structured elements, and making logical changes to improve the program’s flow. The example code is highly contrived, to be sure, but the business logic becomes much clearer as our team refactors the code and makes it far more approachable.

For as much as languages have changed since the 1970s, and with all the progress we’ve made in software engineering, the lessons presented in this film are still surprisingly relevant. We loved a lot of the little nuggets dropped along the way, like “Consistency aids understanding,” and “Use symbols in a natural way.” But we will take exception with the statement “Wrong means poor structure” — we’ve ~~written~~ seen plenty of properly structured code that didn’t work worth a damn. We also enjoyed the attempt at socially engineering a less toxic work environment: “Use tact in personal criticisms.” If only they could learn that lesson over at Stack Overflow.

It’s not clear where [Alec] found this 16-mm film — we’d sure like to hear that story — but it’s a beauty and we’re glad he took the time to digitize it. We’re consistently amazed at his ability to make even the most mundane aspects of technology endlessly fascinating, and while this film may be a bit off from his normal fare, it’s still a great find. Continue reading “Retrotechtacular: Critical Code Reading, 70s Style” →