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a computer w/o a microprocessor

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Homebrew Micros

Although the sun has well and truly set on the world of 8-bit, proprietary computer systems, there is still life in the trusty old Z80, 6502, and 6809 microprocessors. At night, hidden in attics, garages and electronics dens, hobbyists toil away on new designs using the old silicon. This page provides a brief overview of the Homebrewing universe. Although the text is based around my own collection of trinkets, links are provided for some of the things I discovered elsewhere on the Net.

Homebrewing with Old Silicon

 

The obvious way to build a vintage computer is to use vintage parts. In other words, you pick up a Z80, or a 6809 and glue it together with an EPROM and some RAM. The question is how authentic you want to be exactly. Use the real Z80, or maybe a souped-up Z180? And for RAM: do you really want to go with 64Kbit DRAMs? Probably, you replace eight of them with a single 512KB SRAM - which costs next to nothing and is trivial to hook up to a CPU. The other question is whether you want to work with floppy disks. Why not replace them with much simpler, much more durable CF or SD media?

Early days: the P112

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The P112 was maybe the first retro-computer - at least, the first I became aware of.

 

In 1996, Dave Brooks offered the board, which was designed around the Z180 CPU and a SMC Super-IO chip of the type normally used in PC's. The P112 was as high-spec as Z80 computers could get at that time, and some very interesting operating system development work was done on it. The B/P BIOS supported one of the finest Z-System implementations (progressing where CP/M had left off), and the latest version of UZI is aimed at the P112 platform as well. So you can either look at the P112 as the very last Z80 machine before they all went the way of the dinosaur and became 'retro', or as the first retro-computer.

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In 2004-5, Dave Griffiths decided the time had come for a new production run and offered new, sightly revised kits. I have one of these, proudly shown to the right in its custom micro-tower case. It's upgraded with Terry Gulczynski's version of the GIDE hard disk interface. A new production run took place in 2012-13. Who knows what the future will bring...

OZ/1

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I will shamelessly wedge my own Z80 creation from 2005 in here. Not because it is any good, but because I can get away with claiming that it represents what was happening around that time. Quite a few hobbyists popped up around the internet, describing how they finally got around to do something they had been thinking of for years: build their own Single-Board Computer (SBC). It must be a natural progression: you had an interest in computer hardware, you had built up some collection of nice old machines - so what do you do next? Build something simple yourself, because you realise that a small 8-bit micro only needs to contain a few parts and wiring them together is not actually all that hard.

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So here's my version: a Z80 is tied together with a static RAM and an EPROM. Two serial ports are provided by the Zilog SIO. The machine boots by copying a live 64K ROM image (with CP/M and DDTZ) into RAM, thus providing a comfortable debugging environment straight away. Then, an IDE interface is bolted on, made up from a few logic gates straight onto the CPU bus. Actually, the IDE never worked well. Er, it never worked. Doesn't matter, by then I had discovered the N8VEM project and left plans for a bigger, better, grander OZ/2 in the back of my mind.

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As said, the story is pretty representative for what was happening all over the internet. Soon after completing my OZ/1, I could download at least half a dozen other designs. Most of them were also based on the Z80, and - frankly - it got boring. As one of these builders said: by now, everyone and his mother has built a simple Z80 CP/M system. True enough, but it is still a journey worth travelling if you want to learn about the nuts and bolts of computer hardware. Do it once - then go off and join N8VEM.

Replicas of vintage machines

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Some of the most interesting computer designs of the 70s and 80s have been honoured by people offering modern replica kits. These range from functional replicas - not necessarily identical-looking - to perfect replicas that can actually cost a lot of money to populate with authentic parts of the past (do you know how expensive the exact-right capacitors for an Apple I are?)

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​Notable in this category are the KIM-1 and Apple 1 kits from Vince Briel, and the even more exacting replica kits created by Mike Willegal (Apple 1, Scelbi-08 and Grant Stockly (the Altair and Kenbak).

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Spare Time Gizmo offers a PDP-8 workalike,shown to the left, but not in my collection alas. It's the one picture on this site of stuff I just wish I had.

Homebrewing with New Silicon

 

How authentic is authentic? Is homebrewing really tied to using Old Silicon? In the above examples, it's already clear that a lot of Homebrew machines make compromises. They use a single modern SRAM instead of 8 tiny DRAMs, as would have to be done in the early 80s. And with the arrival of suitably powerful microcontrollers, these started to be built in as drivers for VGA and PS/2 keyboards. The next and last step is then to replace the CPU itself. With something like an atMega running a 6502 simulator inside. You lose something, no doubt. But you also gain something: you can now tinker with the insides of the CPU and not just glue it onto your homebrew board. There is something deeply satisfying about extending your 6502 simulator...

KIM Uno: my own replica of the KIM-1

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The KIM Uno is my own excursion into building replicas. Similar to the Altair clones described above, it does not use the original chip set of the replicated machine, but instead uses a microcontroller to emulate the original hardware. In this case, it was an Arduino Pro Mini (an atMega328 at heart). It allows you to recreate a KIM at absolutely minimal cost: using a $2.50 microcontroller results in a total cost of less than $10...

 

The atMega328 is just powerful enough to fit in a KIM plus some extra ROMs with software. Compared to the real KIM-1, you have Wozniak's disassembler, Jennings Microchess and a few other utilities immediately available. For the rest, it's a pure KIM-1. Click here to go to the KIM Uno project pages.

Interlude: New Hardware for Old Machines

 

It's a valid question: why make New Old Computers when Old Old Computers - the real ones - are available everywhere? Why not spend your energy in making new hardware for the old machines? Here are some good examples. But the list grows rapidly, it's by no means complete anymore...

Some commercially available replicas that fall into this category are Vince Briel's miniature Altair 8800 clone, as well as Mike Douglas' perfect-looking Altair 8800 Clone. They use an atMega and a PIC microcontroller, respectively, to simulate the original hardware. And if you crave to experience the Kenbak-1, look no further than the Kenbak-uino.

Links​

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A small sidestep: new peripherals for vintage computers

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Homebrewing is nice, but making a vintage computer be all it can by adding things like fast mass storage or internet access is an equally honourable enterprise.

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  • Equipping a Commodore 64 with the SD2IEC + Jiffy ROM makes your tired old C64 a joy to use again - especially because you now have a file transfer mechanism between C64 and your PC. The Commodore Flyer is an IEC peripheral for all Commodores, providing fast Jiffy-compatible storage plus a smart Internet access mechanism.

  • The Apple II gets a stunning boost from the CFFA 3000, which provides virtual floppy and hard disks.

  • For the BBC Model B, you can buy MMC mass storage and flexible NVRAM/Flash ROM boards from Ctorwy31's eBay shop.

  • MSX computers get a perfect floppy disk replacement + hard disk capability through Padial's SD/MMC Reader. Just mail him on the address on his front page.

  • The Atari ST can use two SD cards instead of a hard disk through the slightly unfortunately named Ultrasatan. I bought one from Lotharek, highly recommended.

  • S-100 IDE boards can provide mass storage to antique S-100 systems too. Although you'll have to adjust the BIOS to incorporate it into CP/M, it is not all that hard to do.

​Not the same as homebrewing, but this had to be be mentioned somewhere in here...

Homebrewing with FPGAs

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With the arrival of cheap FPGA prototyping boards, it is tempting to virtualise vintage hardware. Not simulate it in a program running on a microcontroller - clone it in hardware.  It does away with the physical aspect of soldering stuff together (which may be good or bad, depending on how you define Homebrewing) but it is still very much Vintage Hardware - in fact, with FPGA homebrewing you get to dig inside the chips, not just wire them together.

Jeri Ellsworth's C-One was an early attempt, followed up with the more successful C64DTV in 2004 - actually, it's a C64 clone built into a joystick. It was sold in toy stores for $25, but it turned out that the development team had left some solder points inside to hack it into a proper C64 clone...

But these days, mass-produced FPGA development boards cost around $150 and have everything on board you need to virtualise any 8 bit machine. See Alex Freed's site for an Apple II, John Kent's site for various 680X systems, or Symbos.de for a turbocharged CPC with multitasking Z80 operating system.

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CPC-TREX

 

Shown here is my Terasic T-REX C1 board (around $100 in 2009) running CPC-TREX. It is the ultimate CPC computer, running a 24MHz Z80, virtual disks and hard disks on a CF card and connects to a VGA and PS/2 keyboard. That is quite something already, but its operating system is probably the high point in Z80 operating system development.

Grant Searle's Multicomp

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Shown to the left is my build of Grant Searle's Multicomp (click for high-res details). Based on the cheapest FPGA board (as of 2014), Grant's project is a perfect introduction to FPGA programming for beginners that happen to have an interest in vintage computers.

 

The Multicomp project is like a set of Lego bricks: you pick your virtual CPU (6502, Z80 or 6809), your choice of display (VGA+PS/2, serial terminal, or 80 column video display). And you can 'click in' the CHDL code for an SD card as mass storage. Grant went all the way up to a 25Mhz Z80 CP/M system (see here for my hard disk image for it), AG5AT from the N8VEM group then brought up the CUBIX operating system for the 6809.

 

In short, the Multicomp is the best way to enter the FPGA retrocomputing game. For about $25 for the Altera FPGA board, plus maybe $15 in connectors and wires, you're set with one of the fastest CP/M computers around. See my blog page on the Multicomp for more details.

Haute Cuisine Homebrewing

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Most people's ambitions go about as far as mine: you build a simple Single-Board computer, bring it up with some operating system, and conclude that you, too, could do this. But a select few go further, much further. The links below showcase some directions into which the art of Homebrewing can become truly Haute Cuisine:

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  1. Going all the way: a full-fledged portable system with a custom top-quality Operating System
    This does not fit in the clean time-line that this page tries to present. But a true masterpiece of homebrewing is Dave Dunfield's 6809 portable computer, on which he developed Cubix - an excellent portable OS with complete tool chain including C compiler. Built in the early 80s, but with a trace straight into the present: the N8VEM 6809 board runs Cubix. So does the FPGA Multicomp. And Dunfield's site even offers a full emulator to try Cubix on. You have to seriously wonder how Microsoft's MS-DOS came to rule the world if this can be done by a single man.

  2. Build a CPU: Why not?
    The Magic-1 is a full minicomputer with a CPU built from TTL chips. I am so in awe I can't even describe it here. The home-built computers webring, maintained by Dave Brooks of P112 fame is a good place to start on this topic.
     

The N8VEM project: Homebrewing as a team effort

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Still, one challenge remains: if you want homebrewing to be an enduring hobby instead of a one-off project, there should be some perspective beyond putting together a computer and switching it on. Working all on your own, taking the next steps can get progressively more difficult: building graphics subsystems or adding state-of-the-art microcontrollers to create ‘Frankenstein’ systems: blends of old and new technology that can do something useful, like automate your home. This is where the N8VEM group comes in: a collaborative effort where various projects come together - and where both experienced hacks and less fanatical newcomers can chip in. See the N8VEM pages for more information.​

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