Huge 74181 Is A Classic ALU You Can Actually Understand

November 18, 2017 by Jenny List 22 Comments

You can no longer buy a brand-new 74181, they’ve been out of production for years. All is not lost though, for [Dave’s Dev Lab] have created a facsimile of one on a printed circuit board, using modern single-gate 74-series chips.

Why on earth would you want an oversized replica of an outdated logic chip from nearly five decades ago, we hear you ask? The answer lies in education. If you were to embark on learning about the internals of a microprocessor by taking a modern example such as the one that powers the device on which you are reading this, you would find it to be a daunting task. Over six decades of progress in computer technology have delivered the performance enhancements that put a supercomputer in your smartphone, but at the expense of a contemporary microprocessor being an extremely complex machine which you can’t peer into for any level of understanding.

Simple enough to work your way through the logic

The starting point for the student of microprocessor internals often lies in the past. The technology of the early 1970s holds the fundamentals from which a modern processor can be understood, but remains simple enough to grasp in its entirety as a beginner. Registers, instruction decoders, counters, and an arithmetic/logic unit, or ALU. And for decades the 74181, as an all-in-one 4-bit ALU on a chip that you might have found in a minicomputer at the turn of the 1970s, represented the most convenient way to teach the operation of these devices. Electronic engineers and computer scientists of all ages will have encountered them as they gained their qualifications.

The PCB version of the 181 faithfully follows the original, but with modern 74LVC gates laid out as they would be in the circuit diagram of the chip, and LEDs to show logic state at the different parts of the circuit. Thus when it is used to teach ALU operation it can show every part of the device in detail in a way a real 74181 would never have done.

If the 74181 has caught your interest, we’ve previously brought you [Ken Shirriff]’s reverse engineering of the device in detail using breathtaking images of the silicon.

Blast From The Past With Space Station PROM Reader

November 9, 2017 by Lauren Faris 11 Comments

The Ursa Major Space Station SST282 is a dinosaur of a digital reverb. Okay, so maybe 1978 isn’t ancient yet, but it is getting to the point where one has to worry about the possibility of component failure. At least that’s what [Obsoletetechnology] thought when they created a backup of its memory contents.

As can be seen from some of Hackaday’s previous articles, a part does not have to be an older one to fail. However, there is no such thing as being too paranoid when it comes to older parts reaching their lifetime. Especially when there is valuable memory involved. Each bit of PROM memory is locked by a fuse on its location grid to store permanent data. To be able to read this and collect the respective data, a Raspberry Pi 3 PROM reader was created.

The SST282 uses 3 TTL-level 74xx series Schottky PROM memories on board that hold RAM lookup tables. In the case that these failed, all of the subsequent information would be lost since there are no surviving memory dumps online. Fortunately we are interested only in gathering their contents, so the PROM reader schematic is fairly rudimentary. The chip’s address and data buses connect to a Pi’s GPIO header, and the only other thing to note is a 74LS541 TTL level shifter that converts the Pi’s 3.3V output to the PROM’s 5V TTL level.

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Hackaday Prize Entry: A Mess Of VGA On A Breadboard

November 6, 2017 by Brian Benchoff 16 Comments

Before all our video games came over the Intertubes, before they were on CDs, and before they were on cartridges, video games were all discrete logic. Pong was the first and you can build that out of several dozen logic chips. The great [Woz] famously built Breakout out of 44 simple chips.

For [Marcel]’s entry to the Hackaday Prize, he’s taking the single board microprocessor-less computer to the next level. He’s building a multi-Megahertz 64-color computer on a breadboard. What’s the capacitance of a breadboard? Just ask [Marcel].

The design of this disintegrated computer has just about everything you could want in a discrete CPU. There is no microcontroller or complex chips like the 74181 ALU, there’s pipelining with sometimes two instructions per clock, decoding with diodes, and a 60 Hz, 64 color VGA output and four sound channels. There’s only about 40 TTL chips on this board.

The project logs for this Hackaday Prize entry are a treat in themsleves, ranging from topics to the implementation of NES controllers to getting rid of the breadboard and turning this computer into something like a vintage game system, but with a custom CPU and instruction set. It’s an amazing build, and an awesome project for the Hackaday Prize.

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Homemade Computer From 1970s Chips

June 16, 2017 by Bryan Cockfield 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.

An 8-Bit Transport Triggered Architecture CPU In TTL

April 21, 2017 by Jenny List 37 Comments

When we are introduced to the internals of a microprocessor, it is most likely that we will be shown something like one of the first generation of 8-bit CPUs from the 1970s. There will be the familiar group of registers and counters, an arithmetic and logic unit (ALU), and an instruction decoder with associated control logic. A complex instruction set causes the decoder to marshal registers and ALU to perform all the various functions in the right order. CPUs may have moved on in many ways since the 1970s, but the block diagram of an 8080 or similar still provides a basic grounding for the beginner.

So when we tell you about another home-made CPU using TTL logic chips, you might expect it to follow this well-worn path. Fortunately though the hardware hacking community is always capable of springing surprises upon us, and [Szoftveres] has done just that with his design. It’s a one-instruction-set machine following a transport triggered architecture, and that means it deviates sharply from the conventional architecture described above. Each instruction is a move between the different physical functions of the processor, and computation is achieved by the physical functions working on the data as it is moved into them and presenting the result on their outputs ready to be moved elsewhere. The result is a computer that is in its own way beautifully simple, though at the expense of some inflexibility and lack of some hardware functions we take for granted in more conventional processors.

This machine has been built on a piece of stripboard, and has an accompanying board with display, keypad, and a modem. There is a small board based upon an ATmega8 microcontroller which performs the function of fast program loading, and can be removed once the code is loaded. Software can be written in a C-like language anc compiled using the compiler in his GitHub repository, and he has produced a YouTube video of the machine in operation. This project is well worth reading through in-depth, for its introduction to this slightly unusual architecture.

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Another 74XX Series CPU

March 28, 2017 by Adil Malik 13 Comments

[Jack Eisenmann] is no stranger to building impressive DIY CPU’s on vast stretches of breadboard. This time [Jack] has done away with the seventeen breadboards he used in his last 8-bit computer and instead has gone a step further and designed a set of generously utilised PCB’s for the CPU. The result is the DUO Enterprise.

The CPU design is based around an 8-bit data bus and a 24-bit address bus. As usual, a minimal yet carefully chosen instruction set allows [Jack] to do all the heavy lifting in software as part of the compiler and operating system he is working on. There is no sign of a display yet, instead the computer communicates via a dumb terminal. We love the aluminum foil for shielding! Check out the video, below, to see what we mean.

Over the years, we have seen many of [Jack]’s other CPU builds featured on Hackaday. One of his first designs was a 4-bit CPU that could play many games on a LED matrix.Later he did a much more impressive 8-bit CPU along with analog video output and an OS ofcourse. It could even play pong. He even built a Single Instruction Set Computer (SISC).

His final goal with DUO Enterprise is to allow anyone to utilise its computing power by submitting programs and calculations. Heads up [Jack], our neural net needs training soon.

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Explaining The Operation Of The 74181 ALU

March 27, 2017 by Jenny List 18 Comments

You will all no doubt be familiar with the 74 series logic integrated circuits, they provide the glue logic for countless projects. If you look back through old listings of the series you’ll find alongside the familiar simple gates a host of now obsolete chips that reveal their roots in the pre-microprocessor computer industry of the late 1960s, implementing entire functions that would now be integrated.

One of the more famous of these devices is the 74181, a cascadable 4-bit arithmetic logic unit, or ALU. An ALU is the heart of a microprocessor, performing its operations. The 74181 appeared in many late-60s and early-70s minicomputers, will be familiar to generations of EE and CS students as the device they were taught about ALUs on, and can now be found in some home-built retrocomputers.

[Ken Shirriff], doyen of the integrated circuit teardown, has published a piece taking a look at the 74181, in particular at its logic functions and the reason for some of them that are rather surprising. As well as the normal logic functions, for example the chip can do “(A + B) PLUS AB“. Why on earth you might think would an ALU need to do that?

The answer lies in the way it performs carrying while adding, a significant speed-up can be achieved over ripple carrying along a chain of adders if it can be ascertained whether a bit addition might generate a carry bit. He explains the function required to perform this operation, and suddenly the unusual extra function makes sense. Addition is transformed from a serial process to a parallel one, with a consequent speed increase.

It’s one of those moments in which you have to salute those logic designers from an era when on-chip real-estate was costly and every ounce of speed had to be teased from their designs. Give it a read, and have a go at the interactive 74181 simulator further down [Ken]’s page. We learned something from the article, and so may you.

We brought you the first part of [Ken]’s 74181 investigations earlier in the year. If you would like to see a 74181 in action, take a look at this 4-bit 74 logic single board computer.