A Z80 CPU board built on a piece of prototype board with an edge connector

Designed From Scratch And Fully Handmade: The Modular Coleman Z80 Computer

While the phrase “I built my own computer” might sound impressive to the uninitiated, anyone with an interest in modern computer hardware knows that there’s really not much to it: buy a case, a motherboard with a CPU, some RAM and peripherals, and you’re pretty much there. What’s way more impressive is designing a complete computer system from the ground up, as [Joshua Coleman] just did when he built the Coleman Z80.

And when we say “from the ground up”, we mean it: everything down to the system bus was hand-drawn by [Joshua] himself. It does share something with modern PCs though: a strictly modular design. There’s a Z80 CPU board, a ROM and RAM board, and even two modules that you could describe as a video card and a sound card. All of these are built on prototyping boards with a 40-pin edge connector and hooked up to a single backplane carrying the main system bus.

Designed as an experimentation platform, the Coleman Z80 has many features that enable testing and debugging, such as an adjustable clock generator and a few beautiful vintage LED displays that show the status of the main bus. Input and output are mainly through a serial link and a 16×2 LCD, but [Joshua] is already planning a keyboard interface and composite video output to give it that proper 1980s home computer vibe. The software is currently limited to a ROM monitor that enables basic I/O commands, but with 256 KB of RAM there’s plenty of potential for writing useful software.

Just as impressive as the design itself is the fact that this was [Joshua]’s first electronic design project; we’ve certainly seen worse first projects! Over the years we’ve featured several cool homebrew Z80 computers, such as a super-minimalistic board, a modular system based on the powerful eZ80, and this cute little one that fits inside an Altoids tin.

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Talking To A Texas Instruments Calculator

Texas Instruments is a world-class semiconductors company, but unfortunately what they are best known for among the general public is dated consumer-grade calculators thanks to entrenched standardized testing. These testing standards are so entrenched, in fact, that TI has not had to update the hardware in these calculators since the early 90s. They still run their code on a Z80 microcontroller, but [Ben Heck] found himself in possession of one which has a modern ARM coprocessor in it and thus can run Python.

While he’s not sure exactly what implementation of Python the calculator is running, he did tear it apart to try and figure out as much as he could about what this machine is doing. The immediately noticeable difference is the ARM coprocessor that is not present in other graphing calculators. After some investigation of test points, [Ben] found that the Z80 and ARM chips are communicating with each other over twin serial lines using a very “janky” interface. Jankiness aside, eventually [Ben] was able to wire up a port to the side of the calculator which lets him use his computer to send Python commands to the device when it is in its Python programming mode.

While there are probably limited use cases for 1980s calculators to run Python programs, we can at least commend TI for attempting to modernize within its self-built standardized testing prison. Perhaps this is the starting point for someone else to figure out something more useful to put these machines to work with beyond the classroom too. We’ve already seen some TI-84s that have been modified to connect to the Internet, for example.

Thanks to [Nikša] for the tip!

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Retro Serial Terminal Uses Modern Chips To Get CP/M Machine Talking

The hobbyists of the early days of the home computer era worked wonders with the comparatively primitive chips of the day, and what couldn’t be accomplished with a Z80 or a 6502 was often relegated to complex designs based on logic chips and discrete components. One wonders what these hackers could have accomplished with the modern components we take for granted.

Perhaps it would be something like this minimal serial terminal for the current crop of homebrew retrocomputers. The board is by [Augusto Baffa] and is used in his Baffa-2 homebrew microcomputer, an RC2014-esque Z80 machine that runs CP/M. This terminal board is one of many peripheral boards that plug into the Baffa-2’s backplane, but it’s one of the few that seems to have taken the shortcut of using modern microcontrollers to get its job done. The board sports a pair of ATmega328s; one handles serial communication with the Baffa-2 backplane, while the other takes care of running the VGA interface. The card also has a PS/2 keyboard interface, and supports VT-100 ANSI escapes. The video below shows it in action with a 17″ LCD monitor in the old 4:3 aspect ratio.

We like the way this terminal card gets the job done simply and easily, and we really like the look of the Baffa-2 itself. We also spied an IMSAI 8080 and an Altair 8800 in the background of the video. We’d love to know more about those.

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Hacking An Obsolete Yet Modern Calculator

The gold standard for graphing calculators, at least in the US, are the Texas Instruments TI-84 series. Some black sheep may have other types, but largely due to standardized testing these calculators dominate the market. Also because of standardized testing, these calculators have remained essentially unchanged for decades. While this isn’t great for getting value for money, it does mean that generations of students have been able to hack on these calculators to do all kinds of interesting things as [George Hilliard] outlines.

Even before the creation of these graphing calculators, the z80 processor behind them was first produced over four decades ago and was ubiquitous in the computer scene at the time, which also lends to its hackability. There’s plenty to catch up on here, too, from custom TI games that trick the two-tone display into grayscale to Game Boy emulators that can play Zelda since the TI and Game Boy share the same processors. There are also several methods of running native code or otherwise “jailbreaking” these devices to run arbitrary code.

It looks like the world of TI hacking is alive and well now, and with several decades of projects to browse there’s always something new to find. As it stands, there may be more decades of these types of projects to come, since neither TI nor the various testing standardization companies and government agencies show any signs of changing any time soon.

Thanks to [Adrian] for the tip!

CRISS CP/M Provides Modern Hardware For A Classic OS

Today you might choose run Windows, Linux, MacOS or some other OS on your computer. Back in the 1980s however, you generally had little choice: a certain home computer came with a certain OS, and that was it. If yours was based on a Z80 processor, chances are it ran CP/M. While differences in hardware often made direct data exchange difficult, CP/M provided at least a basic level of software compatibility between various Z80-based computers. Although eventually supplanted by MS-DOS (which initially aimed to be compatible with CP/M), enthusiasts kept the classic OS running on old hardware throughout the 90s and even beyond.

[Igor] decided to make a 21st-century CP/M machine by designing the CRISS, a single-board computer based mainly on AVR microcontrollers. The CPU is a 20 MHz ATMEGA1284P, which imitates a 4 MHz Z80 through machine-code emulation. A pair of ATMEGA328s run the peripheral controller and a VGA output, so the CRISS can be used with modern monitors. True to its heritage however, the image is monochrome green-on-black, looking instantly familiar to users of Kaypros, Osbornes and other contemporary CP/M machines.

Software is loaded through an SD card that holds floppy images. The CRISS can directly run programs written for the Kaypro II and Robotron 1715 computers, although other platforms can be supported as well with a software upgrade. [Igor] shows it running programs ranging from the Turbo Pascal compiler to games like Xonix and Tetris.

Housed in a neat little case, the CRISS can communicate with standard PS/2 keyboards and serial printers. Even an Ethernet port is provided for those willing to experiment with network connectivity (a rare feature in the 1980s).

We love seeing modern retro builds like this; similar projects we’ve covered before include the compact ZZ80MB and the huge Z20X. Others have used different ways of running CP/M on modern hardware, such as booting it directly on a Raspberry Pi or emulating an Altair on an ESP32.

Teardown: VTech Smart Start

Regular readers may be aware that I have a certain affinity for vintage VTech educational toys, especially ones that attempted to visually or even functionally tie in with contemporary computer design. In the late 1980s, when it became obvious the personal computer was here to stay, these devices were seen as an affordable way to give kids and even young teens hands-on time with something that at least somewhat resembled the far more expensive machines their parents were using.

Much Smarter: VTech PreComputer 1000

A perfect example is the PreComputer 1000, released in 1988. Featuring a full QWERTY keyboard and the ability to run BASIC programs, it truly blurred the line between toy and computer. In fact from a technical standpoint it wasn’t far removed from early desktop computers, as it was powered by the same Zilog Z80 CPU found in the TRS-80 Model I.

By comparison, the Smart Start has more in common with a desktop electronic calculator. Even though it was released just two years prior to the PreComputer 1000, you can tell at a glance that it’s a far more simplistic device. That’s due at least in part to the fact that it was aimed at a younger audience, but surely the rapid advancement of computer technology at the time also played a part. Somewhat ironically, VTech did still at least attempt to make the Smart Start look like a desktop computer, complete with the faux disk drive on the front panel.

Of course, looks can be deceiving. While the Smart Start looks decidedly juvenile on the outside, that doesn’t mean there aren’t a few surprising technical discoveries lurking under its beige plastic exterior. There’s only one way to find out.

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Chip Transplant Brings Timex 2048 Back From Grave

The 1984 Timex Computer 2048 that [Drygol] recently got his hands on was in pretty poor shape. Not only did it have the mangled exterior that comes from several decades of hard use and furious typing, but the internals appeared to be shot as well, with the machine showing nothing but vertical lines when powered up. Thankfully, this retro computer virtuoso was more than up to the challenge of bringing it back from the brink.

After a good cleaning and the installation of a reproduction front panel, the Timex was already looking much better. Unfortunately [Drygol] says he doesn’t currently have the equipment necessary to touch up the graphics and lettering on the key caps, but the fact that he had to qualify that statement with “currently” has us all sorts of excited to see what he’s planning down the line.

A bevy of fresh chips.

Of course beauty is only skin deep, and this particular TC-2048 was still bad to the bone. [Drygol] had a hunch its Z80 processor was dead, but after swapping it and its socket out, the machine still wouldn’t start. Though he did note that the garbled graphics shown on the screen had changed, which made him think he was on the right track. He then replaced all the RAM on the board, but that didn’t seem to change anything.

There isn’t a whole lot else to go wrong on these old machines, so the final step was to try and replace the ROM. Sure enough, after installing a new Winbond W27C512 chip with the appropriate software burned onto it, the nearly 40 year old computer sprang back to life.

Another classic computer saved from the trash heap, but it’s all in a day’s work for [Drygol]. Over the years we’ve seen him perform meticulous repairs on computer hardware that any reasonable person would have given up on. Even if you’re not into retro hardware, his restorations are always full of fascinating tips and tricks that can be applied when repairing gadgetry from whatever era happens to tickle your fancy.