[Joshua Coleman] likes to design his own computers. Sometimes, that means drawing up bus architectures, memory maps and I/O port pinouts. Other times, he can focus his efforts more on the general aesthetics, as well as on building a great set of peripherals, as he shows in his latest ColemanZ80 project. Thanks to the RC2014 architecture defining most of the essential features of a classic Z80 computing platform, [Joshua] was able to design a modern retrocomputer that’s not only genuinely useful, but also looks as if it came off a production line yesterday.
The external design is a sight to behold: bright red laser-cut acrylic pieces form a neat, semi-transparent case with ventilation slots on the sides and lots of blinkenlights on the front. Inspired by 1970s classics like the Altair 8800, the front panel gives the user a direct view of the machine’s internal state and allows simple command inputs through a series of tumbler switches. The CPU, RAM and other basic devices are housed in one case, with all the expansion modules in a second one, linked to the mainboard through a 40-wire flatcable.
Although the mainboard closely follows the RC2014 design, [Joshua] went through a lot of effort to tune the system to his specific needs. The expansion boards he built include an NS16550 UART to replace the default 68B50, a battery-backed real-time clock, a YM2149-based sound card and even a speech synthesizer module built around the classic SP0256 chip, of Speak & Spell fame. An even more unusual feature is the presence of an AM9511, one of the earliest math coprocessors ever made, to speed up floating-point calculations. All of these modules were built entirely by hand on prototype boards: we can barely imagine how much time this must have taken.
Output devices include a VGA adapter courtesy of a Raspberry Pi Pico as well as a regular 4-digit 7-segment LED display and a set of classic HP “bubble” LEDs. [Joshua] runs several demos in his video (embedded below), ranging from computing the Mandelbrot set to playing chiptunes on the YM2149. There’s plenty of scope for further expansion, too: [Joshua] plans to build more peripherals including a floppy drive interface and a module to operate a robotic car.
If you have a soft spot for a Z80 computer but want a new operating system experience, try Zeal. You can watch a demo of the open-source OS in the video below.
As you might expect, the whole system is written in Z80 assembly language. The features you expect are there: files, directories, device drivers, a clock, and even memory banking to support up to 16M of memory. The work isn’t totally done, nor is the initial target computer — Zeal — but it looks like a great piece of work so far and will be of interest to anyone who has a Z80.
The microprocessor feature which probably most enables the computing experience we take for granted today is protected mode. A chip with the required hardware can run individual software processes in their own environments, enabling multitasking and isolation between processes. Older CPUs lacked this feature, meaning that all the resources were available to all software. [Andy Hu] has done the seemingly impossible with a Zilog Z80, enabling a protected mode on the chip for the first time in over four decades. Has he found an elusive undocumented piece of silicon missed by every other researcher? Not quite, but it is a clever hack.
The Z80 has two address spaces, one for memory and the other for I/O. He’s taken the I/O request line and fed it through a flip-flop and some logic to call a hardware interrupt the first time an I/O call is made or when a RST instruction is executed. Coupled with a small piece of memory for register contents, and he’s made a Z80 with a fully-functional protected mode, for the cost of a few logic chips. It’s explained in the video below the break, and we hope you agree that it’s rather elegant given the resources in hand. It’s too late for the commercial 8-bit machines of the past, but it would be interesting to see what today’s retrocomputer designers make of it.
In case you weren’t around in the 80s, or you happened to blink, you may have missed the Mattel Aquarius computer. [Nick Bild] has a soft spot in his heart for the machine though and built the Aqua cartridge to make the Aquarius into a more usable machine.
Originally equipped with a mere 4 KB of RAM and a small, rubbery keyboard, it’s not too surprising that the Aquarius only lasted five months on the market. [Nick] decided on the cartridge slot to beef up the specs of this little machine given the small number of expansion ports on the device. Adding 32 KB of RAM certainly gives it a boost, and he also designed an SD card interface called Aqua Write that connects to the Aqua cartridge for easily transferring files from a more modern machine.
The Aqua Write uses an Arduino Mega 2560 to handle moving data between the SD card and the system’s memory. This is complicated somewhat because a “PLA sits between the Z80 and data bus that XORs data with a software lock code (initialized to a random value on startup).” [Nick] gets around this by running a small program to overwrite the lock code to zero after startup.
A lot of people had a Radio Shack TRS-80 Model I. This was a “home computer” built into a keyboard that needed an external monitor or TV set. Later, Radio Shack would update the computer to a model III which was a popular “all in one” option with a monitor and even space for — gasp — floppy disks. But the Model II was not nearly as common. The reason? It was aimed at businesses and priced accordingly. [Adrian] got a Model II that was in terrible shape and has been bringing it back to life. You can see the video of how he’s done with it, below.
The Model II was similar to the older “Trash 80” which had been used — to Radio Shack’s surprise — quite often by businesses. But it had more sophisticated features including a 4MHz CPU — blistering speed for those days. It also had an 80×25 text display and a 500K 8-inch floppy drive. There were also serial and printer ports standard.
There were a few interesting features. The floppy drive’s spindle ran on AC power and if the computer was on, the disk was spinning. In addition, there was bank switching so you could go beyond 64K and also you didn’t have to share your running memory with the video display. In theory, the machine could go beyond 64K since half the memory was bank switchable. In practice, the early models didn’t have enough expansion space to handle more than 64K physically.
Most retrocomputer builds seem to focus on either restoring old machines or rebuilding them from scratch. Either way, the goal is to get as close as possible to the original machine, and while we certainly respect those builds, there are other ways to celebrate the computers of yesterday, as this Z80 single-board computer nicely demonstrates.
[Ivan Farafontov]’s SBC is sort of a “Z80 that never was” build, one that would almost have been possible back in the heyday of 8-bit computing, and would have made quite a splash if it had. Most of the peripheral chips are from Zilog and would have been found in many of the Z80 machines of the day, like the TRS-80 and ZX Spectrum. Where it goes off the old-school path is with the video section, which uses an Atmel CPLD chip and a dual-port RAM to drive a VGA monitor. It still looks the part, though, with a 256×192 pixel, 16-color display. The compact video section helps keep the overall footprint of this machine pretty small, at least by the standards of the old machines. The machine is barely larger than its custom keyboard, which is populated with mechanical switches and really nice-looking custom keycaps, and everything fits into a 3D-printed case.
The demo that starts at the 4:30 mark of the video below will be a nostalgia storm for a lot of readers, starting as it does with a version of Boulder Dash that [Ivan] wrote from scratch, along with the tile editor he used to create the sprites for the game. All the design files and code are available if you want to build your own, of course. We recently featured another Z80 that never was, but [Ivan]’s machine really makes a statement with its compact size and its capabilities.
Very few retrocomputing projects are anything other than a labor of love. There’s really no practical reason to build a computer that is woefully inadequate for just about any task compared to even an entry-level PC today. But the lack of a practical reason to do something rarely stops a hacker, as with this nifty modular Z80-based rack computer.
Actually, there’s at least one area where retrocomputers excel compared to their modern multi-core gigahertz counterparts — and that’s nostalgia. That’s what [Ricardo Kaltchuk] was going for with his build, which started by finding a Z80 and an Intel 8251 USART in his parts bin. Those formed the core of what would become the “Proton” computer, a modular beauty built around 7 cm by 10 cm PCBs that plug into a backplane inside a rack made from aluminum angle. Aside from the power supply and the Z80 CPU, other modules include a RAM card with a zero insertion force socket for an EPROM, a mass-storage module sporting a 128 MB Compact Flash card, plus modules for standard serial and I2C comms.
The fit and finish are excellent, and the performance is impressive. The Proton runs CP/M and boasts a ton of old applications that will bring back some memories, like SuperCalc and dBase. We’d venture a bet that WordStar is in there someplace, or easily could be. The video below is a little rough, but shows everything off really well.
In some ways, the Proton reminds us of the RC2014, but its fit and finish are what bring this build home. That’s not to take away from the work [Ricardo] obviously put into documentation, though. The 62-page manual has every detail of every module, plus instructions for building one of your own.