An ALU As A Desktop Calculator Has Stunning Style From Days Gone By

June 7, 2021 by Jenny List 9 Comments

Those of you with an interest in microcomputer history will know that there is a strong crossover between the path of electronic calculator evolution and the genesis of the integrated CPU. Intel’s 4000 was famously designed for a calculator, and for a while in the 1970s these mathematical helpers were seen as the wonder of the age. [Simon Boak]’s calculator is a curious throwback to that era, as it’s not a decimal calculator as we’d know it but a hexadecimal device that simply computes using the functions of the famous 74181 ALU chip.

An ALU, or to give it its full name an Arithmetic Logic Unit, is a component of a CPU with two inputs and one output that can perform any of a range of binary functions upon the two inputs and return the result on the output. This calculator has two of them for eight bits of raw adding power, with a hexadecimal keypad for setting the inputs and a set of 7-segment displays for showing the results. It’s housed in an achingly retro folded sheet metal console case with wooden end pieces that would have graced any engineer’s desk with pride back in about 1975. We may not need one, but we really want one!

If the 74181 is a mystery to you then fear not, because chip master [Ken Shirriff] has produced some handy explanation work on its operation.

Thanks [Ted Yapo] for the tip.

A Commodore 64 As You Have Never Seen One Before: Game Boy Form Factor!

June 5, 2021 by Jenny List 23 Comments

It’s now nearly four decades since the iconic Commodore 64 8-bit computer saw the light of day, and the vintage format shows no sign of dying. Enthusiasts have produced all kinds of new takes on the platform, but it’s fair to say that most of them have concentrated on the original style keyboard console form factors. A completely different take on a Commodore 64 comes from [UNI64] in the form of the Handheld 64, a complete Commodore 64 in a Game Boy style form factor that uses the original 64 chipset.

It achieves this improbable feat by sandwiching together several PCBs, with a tactile switch keyboard and LCD display at the top. It appears to bring the 64 ports out to headers, and the ROM cartridge port to an edge connector socket at the top of the device. A departure from the 1980s comes in using a Raspberry Pi Zero to emulate a 1541 floppy drive though.

Sadly unlike a Game Boy there’s no onboard battery, but don’t let that take away from the quality of this feat. A forum post from [3D-vice] has a set of decent-quality pictures, and even if you don’t have a handy 64 chipset lying about we’re sure you’ll still appreciate them. If home made 64s are your thing, take a look at how you can build one without Commodore parts.

Thanks [pbuyle] for the tip!

Build A New ZX81

May 28, 2021 by Al Williams 17 Comments

[Retro Shack’s] ZX81 died, and while he tried to figure out the fix, he decided to build a new one. Of course, building a circa-1980-something computer from new parts is a bit daunting. Unless you start with an existing design that has it all ready to put together.

The PCB looks great and we like that the silkscreen shows acknowledgments of projects that helped the designer, [Alejandro Sebastian]. The case is, of course, 3D printed. At first, the power LED didn’t work, but voltages looked correct and the board powered up.

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Reading Floppies With An Oscilloscope

May 20, 2021 by Al Williams 6 Comments

There’s a lot of data on magnetic media that will soon be lost forever, as floppies weren’t really made to sit in attics and basements for decades and still work. [Chris Evans] and [Phil Pemberton] needed to read some disks that reportedly contained source code for several BBC Micro games, including Repton 3. They turned to Greaseweazle, an interface board that can dump just about any kind of floppy disk if it is attached to the right drive. The problem is that Greaseweazle couldn’t read the disks due to CRC errors. Time to break out the oscilloscope and read the disk manually, which is what they did.

Greaseweazle provides a nice display of read sectors and shows timing coming from the floppy read head. The disk in question looked good with reasonably clean timing clocks except in the area of one sector. At that point, the clocks degenerated into noise. Looking on the disk, it was easy to see why. The actual media had a small dent in it.

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Inside The VIC-20

May 19, 2021 by Al Williams 52 Comments

Commodore machines are well-loved around here, but usually when you think Commodore, you think about the Commodore 64, or maybe the PET or Amiga. But the Commodore 64 had an older sister, the VIC 20. This was the first computer to sell a million units and has a lot in common with its better-known successor. The machine was only made for a few years, and [Dubious Engineering] has been restoring one over a few videos. In the video below, he opens it up for a look inside, among other things.

If you want to get straight to the opening, you’ll need to fast forward about 5 and a half minutes. The keyboard pulls off and a nice old-fashioned set of cables made from individual wires connect to the skinny main board with all the smarts on it. No ribbon cables or flex PCBs!

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Linus Åkesson’s ‘A Mind Is Born’ Commodore 64 Demo In Just 256 Bytes

May 19, 2021 by Maya Posch 36 Comments

It would be an understatement to say that the Commodore 64 demo scene is quite amazing. For those who are unaware, a ‘demo’ in this context is essentially a technological demonstration. Usually to show off particular effects or other (visual) properties that either push the limits of the platform on which it is being run, or use its hardware in a special fashion. In the case of [Linus Åkesson]’s A Mind Is Born demo, the challenge was to do as much as possible in 256 bytes, while providing an audiovisual experience.

Although at first glance 256 bytes may sound like a lot to work with, this code has to generate the entire melody that is output via the Commodore 64’s SID audio chip, while simultaneously generating an attractive visual pattern. This is quite an undertaking, as the video capture of the result (included after the break) makes clear. The secret sauce here is to make use of the C64’s SID audio & VIC-II video chips.

Driven by a 60 Hz timer interrupt, the three voices of the SID are used to play the kick drum and bass, melody and drone respectively, creating the 64 total bars of the music using a linear-feedback shift register (LFSR). This means that the melody is in a sense randomly generated, but deterministically enough to sound pleasing to the human ear.

For the visual side, the C64 runs in Extended Character Mode, using fonts along with a background color to create interesting patterns using what is essentially a cellular automaton algorithm. While there are some visual glitches due overwriting of video data, and a race condition, these end up adding to the charm. The resulting audio track is pretty catchy too, and absolutely worthy of a listen.

Thanks for the tip, Johannes!

(That banner image? That _is_ the whole code.)

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Repairing A Vintage HP 9825 The Hard Way

May 19, 2021 by Jonathan Bennett 16 Comments

[CuriousMarc] is at it again, this time trying to undo the damage from a poorly designed power circuit, that fried the internals of his HP 9825 computer. (Video, embedded below.)

The power supply on this particular model has a failure mode where a dying transistor can lead to 13 V on the 5 V line. This causes all the havoc one would expect on the internals of a 1970s era portable computer. This particular computer is rather rare, so instead of calling it a lost cause, our protagonist decides to replace the faulty transistor, install a proper overvoltage protection circuit, and then start the tedious hunt for which chips actually let their magic smoke out.
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