A CPU-Less Computer With A Single NOR-Gate ALU

November 23, 2020 by 13 Comments

We see a lot of discrete-logic computer builds these days, and we love them all. But after a while, they kind of all blend in with each other. So what’s the discrete logic aficionado to do if they want to stand out from the pack? Perhaps this CPU-less computer with a single NOR-gate instead of an arithmetic-logic unit is enough of a hacker flex? We certainly think so.

We must admit that when we first saw [Dennis Kuschel]’s “MyNor” we thought all the logic would be emulated by discrete NOR gates, which of course can be wired up in various combinations to produce every other logic gate. And while that would be really cool, [Dennis] chose another path. Sitting in the middle of the very nicely designed PCB is a small outcropping, a pair of discrete transistors and a single resistor. These form the NOR gate that is used, along with MyNor’s microcode, to perform all the operations normally done by the ALU.

While making the MyNor very slow, this has the advantage of not needing 74-series chips that are no longer manufactured, like the 74LS181 ALU. It may be slow, but as seen in the video below, with the help of a couple of add-on cards of similar architecture, it still manages to play Minesweeper and Tetris and acts as a decent calculator.

We really like the look of this build, and we congratulate [Dennis] on pulling it off. He has open-sourced everything, so feel free to build your own. Or, check out some of the other CPU-less computers we’ve featured: there’s the Gigatron, the Dis-Integrated 6502, or the jumper-wire jungle of this 8-bit CPU-less machine.

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The Descendants Of Ancient Computers

May 31, 2020 by 13 Comments

Building computers from discrete components is a fairly common hobby project, but it used to be the only way to build a computer until integrated circuits came on the scene. If you’re living in the modern times, however, you can get a computer like this running easily enough, but if you want to dive deep into high performance you’ll need to understand how those components work on a fundamental level.

[Tim] and [Yann] have been working on replicating circuitry found in the CDC6600, the first Cray supercomputer built in the 1960s. Part of what made this computer remarkable was its insane (for the time) clock speed of 10 MHz. This was achieved by using bipolar junction transistors (BJTs) that were capable of switching much more quickly than typical transistors, and by making sure that the support circuitry of resistors and capacitors were tuned to get everything working as efficiently as possible.

The duo found that not only are the BJTs used in the original Cray supercomputer long out of production, but the successors to those transistors are also out of production. Luckily they were able to find one that meets their needs, but it doesn’t seem like there is much demand for a BJT with these characteristics anymore.

[Tim] also posted an interesting discussion about some other methods of speeding up circuitry like this, namely by using reach-through capacitors and Baker clamps. It’s worth a read in its own right, but if you want to see some highlights be sure to check out this 16-bit computer built from individual transistors.

Who Could Possibly Need An FPGA With 9M Logic Cells And 35B Transistors?

September 24, 2019 by 101 Comments

Xilinx recently announced the Virtex UltraScale+ VU19P FPGA. Of course, FPGA companies announce new chips every day. The reason this one caught our attention is the size of it: nearly 9 million logic cells and 35 billion transistors on a chip! If that’s not enough there is also over 2,000 user I/Os including transceivers that can move around 4.5 Tb/s back and forth.

To put things in perspective, the previous record holder — the Virtex Ultrascale 440 — has 5.5 million logic cells and an old-fashioned Spartan 3 topped out at about 50,000 cells — the new chip has about 180 times that capacity. For the record, I’ve built entire 32-bit CPUs on smaller Spartans.

That led us to wonder? Who’s buying these things? When I first heard about it I guessed that the price would be astronomical, partly due to expense but also partly because the market for these has to be pretty small. The previous biggest Xilinx part is listed on DigKey who pegs the Ultrascale 440 (an XCVU440-2FLGA2892E) at a cost of $55,000 as a non-stocked item. Remember, that chip has just over half the logic cells of the VU19P.

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The Math That Makes Computers Go, Built On A Tiny PCB

October 18, 2018 by 20 Comments

A computer is, at its core, just a bunch of transistors wired together. Once you have enough transistors on a board, though, one of the first layers of abstraction that arises is the Arithmetic Logic Unit. The ALU takes in two sets of data, performs a chosen math function, and outputs one data set as the result. It really is the core of what makes computers compute.

An ALU is built into modern processors, but that wasn’t always how it was done. If you’re looking to build a recreation of an early computer you may need a standalone, and that’s why [roelh] designed an ALU that fits in a square inch piece of circuit board using five multiplexer chips and two XOR chips.

One of the commonly used components for this purpose, the 74LS181 ALU, is not in production anymore. [roelh’s] ALU is intended to be a small footprint replacement of sorts, and can perform seven functions: ADD, SUB, XOR, XNOR, AND, OR, A, B, and NOT A. The small footprint for the design is a constraint of our recent contest: Return of the Square Inch Project. Of course, this meant extra design challenges, such as needing to move the carry in and carry out lines to a separate header because there wasn’t enough space on one edge.

Exploring the theory behind an ALU isn’t just for people building retrocomputers. It is integral to gaining an intuitive understanding of how all computers work. Everyone should consider looking under the hood by walking through the nand2tetris course which uses simulation to build from a NAND gate all the way up to a functioning computer based on The Elements of Computing Science textbook.

If you’re a homebrew computer builder, it might be worthwhile to use one of these ALUs rather than designing your own. Of course, if building components from scratch is your thing we definitely understand that motivation as well.

Won’t Somebody, Please, Think Of The Transistors!

April 6, 2018 by 146 Comments

At what age did you begin learning about electronics? What was the state of the art available to you at the time and what kinds of things were you building? For each reader these answers can be wildly different. Our technology advances so quickly that each successive generation has a profoundly different learning experience. This makes it really hard to figure out what basic knowledge today will be most useful tomorrow.

Go on, guess the diode!
Go on, guess the diode!

Do you know the forward voltage drop of a diode? Of course you do. Somewhere just below 0.7 volts, give or take a few millivolts, of course given that it is a silicon diode. If you send current through a 1N4148, you can be pretty certain that the cathode voltage will be that figure below the anode, every time. You probably also have a working knowledge that a germanium diode or a Schottky diode will have a lower forward voltage, and you’ll know in turn that a bipolar transistor will begin to turn on when the voltage between its base and emitter achieves that value. If you know Ohm’s Law, you can now set up a biasing network and without too many problems construct a transistor amplifier.

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Homemade Computer From 1970s Chips

June 16, 2017 by 39 Comments

Sometimes it starts with a 555 timer and an op-amp. Other times with a small microcontroller. But the timing’s not so great and needs a dedicated timing crystal circuit. And maybe some more memory, and maybe the ATtiny should be swapped out for some 74LS-series chips. And now of course it needs video output too. Before you know it, you’re staring at a 40-chip computer that hearkens back to a simpler, yet somehow more complex, time of computing. At least that’s where [Marcel] is with his breadboard computer based on 1970s-era chips.

For what it does, this homebrew computer is relatively simple and straightforward. It gets 8 bits of processing power from 34 TTL chips. Another 6 round out the other features needed for the computer to operate. It is capable of rendering 64 colors in software and has more than enough memory for a computer of this sort. So far the only recurring problem [Marcel] has had has been with breadboard fatigue, as some of the chips keep popping out of the sockets.

This is a great project for anyone interested in homebrew or 8-bit computing, partially because of some of the self-imposed limitations that [Marcel] imposed on himself, like “only chips from the 70s”. It’s an impressive build on its own and looks to get much better since future plans call for a dedicated PCB to solve the issue with the worn-out breadboards. If you’re already invested in a project like this, don’t forget that the rabbit hole can go a little deeper: you can build a computer out of discrete transistors as well.

Relay Computer Starts With An Adder That Makes A Racket

February 17, 2017 by 49 Comments

Computers built using discrete logic chips? Seen it. Computers from individual transistors? Impressive, but it’s been done. A computer built out of electromechanical relays? Bring on the ozone!

The aptly named [Clickity Clack]’s new YouTube channel promises to be very interesting if he can actually pull off a working computer using nothing but relays. But even if he doesn’t get beyond the three videos in the playlist already, the channel is definitely worth checking out. We’ve never seen a simpler, clearer explanation of binary logic, and [Clickity Clack]’s relay version of the basic logic gates is a great introduction to the concepts.

Using custom PCBs hosting banks of DPDT relays, he progresses from the basic AND and XOR gates to half adders and full adders, explaining how carry in and carry out works. Everything is modular, so four of his 4-bit adder cards eventually get together to form a 16-bit adder, which we assume will be used to build out a very noisy yet entertaining ALU. We’re looking forward to that and relay implementations of the flip-flops and other elements he’ll need for a full computer.

And pay no mind to our earlier dismissal of non-traditional computer projects. It’s worth checking out this discrete 7400 logic computer and this all-transistor build. They’re impressive too in their own way, if a bit quieter than [Clickety Clack]’s project.

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