Four Chips To Retro Perfection

Over the years, we’ve seen many people build a computer from the ground up. It’s always great, but this one takes the cake. I’m not just saying that because there’s a cute little ‘Z80 Inside’ logo on the silk screen, either. It’s a four IC Z80 computer, a tiny board, and [Just4Fun]’s entry into this year’s Hackaday Prize.

This single board computer is only four chips, the most important being the CMOS Z80 CPU. This is the same CPU as was found in the TRS-80 and the ZX Spectrum, both classics from the early days of computing. In addition to the PCU, there’s a Toshiba SRAM with 128 whole kilobytes of random access memories. A 74HC00 is thrown into the mix for glue logic, and everything else happens through a specially-programmed ATMega32A. This last chip provides a universal I/O subsystem, the EEPROM, and the 4/8MHz clock for the CPU.

Those four chips are really all you need for a fully functional computer, but you can do so much more with this little board. There’s a uCom board, or basically a ‘transparent’ USB-to-serial emulator that will allow you to upload a hex file to the board. Of course this means you can also connect it to a terminal, and with FuzixOS, there’s Unix for the Z80. It’s a wonderment of retrocomputing, and one of the best ways to build an old computer today.

24 thoughts on “Four Chips To Retro Perfection

        1. Yup. I planned to use cheapest CPLDs to recreate the sections containing rare ICs or complex circuits, and fill all the gaps with standard 74xx chips. Odra 1305 was basically british ICL9000 series computer recreated behind Iron Curtain by people who only had some hardware specs and Programmer’s Manuals. It even had version of COBOL where entire syntax was translated to polish. I have manual for that somewhere…

        1. The “glue” logic on a chip used in some of those 8-bit machines was precursor to CPLDs. And these are precursors to FPGAs. Some MCUs have simple CPLD components inside, for example CLCs in PICs or advanced FPGAs paired with ARM processors in single package. Anyway, using an CPLD or FPGA to recreate that “glue logic” done by GALs of the era or logic chips from 74xx or 4xxx families is not a big deal. Using an FPGA to recreate one of those old machines is just taking an easy way. Especially when there are soft cores available for free…

          1. The Sinclair ULA are a precursor of CPLD, the difference is like an EPROM and a Mask Rom, but I think that if they used a CPLD to emulate an ULA of the Speccy they had made a nice machine with a lot of games!

    1. I really agree. I, like, audibly sigh whenever I see one of these with a gratuitous modern IC thrown in. There’s a place for that too, but if you’re going to use a modern IC why not just make a faithful logical recreation on an FPGA?

    2. To Me, it’s a beautiful union of past and present. The “host” controller can be used as a “virtual I/O” device and emulate any peripheral you want, which is exactly what the MBC and MBC2 host controler does.

      I have the original Z80-MBC and I just purchased the bare PCB of the Z80-MBC2. The build went fine and everything functions without any issues upon the 1st power-up.

      In looking at the Z80 interface differences between the MBC and MBC2, the Z80 bus access is different. The MBC directly accessed the 1st 256 bytes of memory and “stuffed” programs into RAM then RESET the Z80 to execute. On the MBC2, a sequence of Z80 OPCODES is “inserted” during the M1 cycle, so in effect, the Z80 is “being fed” it’s program by the “host”. It’s an interesting software project for sure! :)

      Many thanks to JUST4FUN.

      Now where’s that FUSIX project for the MBC2 gone to? :)

      Peace and blessings.

  1. That is awesome! It makes me want to dig out my boxes of old 5.25″ disks to find my Borland Turbo Pascal and Wordperfect for CP/M. My first computer was a salvaged Xerox 820 with the display hacked from a salvaged 12V ATM tube using coax to run the signals to the monitor. Learned to code on that puppy and even picked up the 8086 addon board with CP/M 86 running so I could start a compile on one processor and then switch to the other to do something else. Crazy days.

  2. It should be noted that Zilog made a chip for minimal systems like this – the Z80 GLU. They were not popular, unfortunately, and are very difficult to find now.

      1. There was a z80/64180 variant which had built in basic via a serial port – essentially a one chip solution. Can’t remember the part number but I do remember lusting over it. Way over my teenager budget.

        1. Steve Ciarcia did a board based on the 64180. SB180, I think it was called.
          It was in BYTE magazine, back in the day.

          An update of that design would be an interesting read.

  3. I’d like to point out that Just4Fun entered an earlier version of this project in a HaD contest in 2017, and won a relatively small amount as a semi-finalist. They seem to have taken the prize money and invested it into making the project even better. Which I think is pretty cool…

    (Also, this isn’t just a “look, I made retro hardware” project. J4F has gotten an impressive amount of software working as well. The boards isn’t just a curiosity; it’s a USEFUL SYSTEM (as much as any Z80 ever was?))

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