A Thoroughly Modern Sinclair ZX80

At the end of the 1970s, the 8-bit home computer market had been under way for several years. Companies like Apple and Commodore had produced machines that retain a cult following to this day, and there was plenty for the computer enthusiast to get to grips with. As always though with a new technology, the trouble was that an Apple II or a Commodore Pet wasn’t cheap. If you didn’t have much cash, or you were a young person with uncomprehending or impoverished parents, they were out of reach. You could build a computer from a kit if you were brave or technically competent enough, but otherwise you were out of luck.

As you might imagine, the manufacturers understood that there was an untapped market for cheaper hardware, so as we entered the new decade a range of budget machines that appeared to satisfy that demand. Gone were internal expansion slots, dedicated monitors and mass storage, for cheap keyboards, domestic TV monitors, and home cassette recorders. 1980s teenagers would have computers of their own, their parents safe in the knowledge they were educational while the kids themselves were more interested in the games.

The SInclair ZX80. By Daniel Ryde, Skövde [CC-BY-SA-3.0], via Wikimedia Commons.
The SInclair ZX80. Daniel Ryde, Skövde [CC-BY-SA-3.0], via Wikimedia Commons.
The veteran British budget electronics entrepreneur [Clive Sinclair]’s first mass-market home computer offering was the ZX80, a PCB stuffed with 74 logic chips and a Z80 processor, with a membrane keyboard occupying one end. Its flimsy white plastic console case and characteristic blanking of the display while it did any work failed to endear it to consumers, but it set the basic architecture for [Sinclair]’s ZX line of computers including the wildly succesful ZX Spectrum.

If you want the ZX80 experience the real thing is rather rare, but thanks to Tynemouth Software there is a chance you may soon be able to get your hands on a modern version. They’ve produced a prototype of a ZX80 in the form factor of a ZX81 main board, and their write-up makes for a very interesting read.

The UHF modulator of the original is gone, replaced by a phono socket for the video output. Also missing is the ZX expansion edge connector, and the 3.5mm jack used by Sinclair for power has been replaced by a more conventional power connector. This is a first spin of the board, and they admit to a couple of errors. A line between chips is missing, and a keyboard connector is reversed.

The original machine’s 1K has been upgraded to a healthy 16K so there is no need for a wobbling RAM pack, and both ZX80 and ZX81 ROMs are on board. The ZX81 circuitry for SLOW mode is missing though, so gaming will be off the cards on this machine.

Perhaps most interestingly the original 74LS chips give a far better performance than the newer 74HC variants. It seems the original’s economy of components was dependent on the analogue properties of the LS series. In an age when every possible custom chip is at our disposal we sometimes forget how far the designers of those days had to push things to reduce their BoM costs.

There is a promise of more to come in future versions of this board, so it’s one to keep an eye on. Meanwhile give it a read, for it contains a fascinating fun-down of the design of a venerable budget computer.

This appears to be the first ZX80 we’ve featured here, but we’ve brought you ZX81 projects aplenty. There was this ’81 networked with an ESP8266, or this vintage Maplin I/O board completed 30 years later.

Via Hacker News.

52 thoughts on “A Thoroughly Modern Sinclair ZX80

    1. Does anyone recall the 1st “ZX80”? It was a kit called the “MicroACE”. I found one in an electronics magazine, which I cannot recall the name of. I ordered one and built it. I was in high school at the time and had little $$$’s to spend. I think I paid US$49 for it but other history museums on “the net” claim it sold for US149 to US169.

      I found the 2KB of RAM limiting and added another 6KB using the expansion connector and some 1KBx4 RAMs all hand-wired on a vector board. It worked 1st time, which amazed me because there were so many solder points. Murphy granted me a pass to success on the 1st try?

      Eventually, the cheap “membrane keypad” started to wear out and luckily, I found a HEATHKIT full-size keyboard at a local HAM-fest. After cutting traces and rewiring the HEATHKIT’s switch array to match the MicroACE, I wired it into the MicroACE’s keyboard connections. It worked well and I had a nice full-size keyboard for my MicroACE,

      Here’s a link: https://www.flickr.com/photos/angeljim46/6229960260/

      The MicroACE was my 1st computer. Later, I designed and hand-wired one using the INS8073 with TinyBASIC. I spent alot of time designing various interfaces for it, including a floppy-disk interface, which I was never able to get running. Eventually I came across an inexpensive an APPLE ][+ clone called an ORANGE ][+, which had some 1/2-height floppy-disk drives that were REALLY quiet compared to APPLE’s drives. The ORANGE ][+ had FORTH in ROM and a built-in “Z80 softcard”.

      About a decade later, I purchased a TIMEX version along with the RAM expansion pack, thermal printer and a bunch of games off of eBay for about US$15. Sadly, I no longer have either of those units. :(

      I do not know if Clive Sinclair actually designed the ZX80 or if he purchased the design and resold it as a ZX80 because I do not recall the ZX80 being available when I purchased the MicroACE.

      Peace and blessings,
      Johnny

      1. Yep, a MicroACE was my first computer. Dad bought it and did most of the assembly work. I remember getting an upgrade kit for it that took it up to ZX-81 status. Wish I still had it, but we ‘turned it in’ as part of a promotion Commodore ran to get I think $100 or $200 off the price of a shiny new C-64, which years later begat an Amiga. Good times.

        1. Heh, I remember that, ZX81s were $50 assembled by that time and ppl were buying them and turning right around and turning them in for the $100 credit by ppl with no previous machine.

        2. I never followed the VIC-20 or C-64 till years later. I started working for a local computer store as a tech and eventually purchased an APPLE //e even though I already had an APPLE clone at the time called an ORANGE ][+. Thanks for sharing.

        1. Yes they all (zx80, Spectrum, and later variants) were very very popular here in Spain. I bought an Spectrum in 1982 and got the most I could get out of it in the following years. I programmed machine code, built relay in/out boards, etc…

      2. I did almost the exact same thing. You could buy the ZX80 as a kit. If I recall correctly, it was about £100 when I bought it in 1979.

        I still have it. I suspect it even still works. I’ll have to find a monitor with composite video in….Hmmm.

        I built the kit and it didn’t work. Nothing I did would make it fly. Sinclair would take it in and make it work for you for a quite reasonable sum. The problem? A resistor for the kbd was in a feed-through next to where it should have been.

        I did the same trick with a really nice reed switch based Honeywell keyboard. No key bounce. Awesome feel. Was incredible. Used wire wrap wire and a socket to get a ribbon cable out of the machine to the kbd. I peeled all the traces off of the kbd circuit board and connected the switches with wire wrap wire soldered point to point.

        The modulator worked on channel 3 as I recall. I yanked that off the board pretty quick, put a phono plug to the video output and connected it to the demodulated video on a 20″ TV I had gotten from the local repair shop and fixed. It’s a miracle it didn’t blow up.

        All those mods lead me to putting it in a nice metal box. In doing so, I seem to have lost the pretty white polystyrene top case. If these folks decide to vacuform some new ones, I’m all over it.

      3. I bought my Timex 1000 for around $30 (closeout price) at Montgomery Wards. Later I bought the 16K memory also for $30 at the same place. My first “real” computer! I subscribed to a fan magazine and copied/ran a lot of the programs from it.
        Then, I saw an article in the magazine about adding a full size keyboard. I ordered a surplus keyboard and wired it in, thereby breaking the computer… (sigh!)
        Later I bought a used Commodore VIC-20, and bought a new Atari 400.

  1. Beautiful work!

    A lot of effort has gone into aesthetics here and it looks better than the messy home computers of that era. Same orientation for all the components in a group: Resistors all the same way, Axial capacitors!

    I would add a poly-switch and shunt diode as the input polarity is selectable but only because a have burnt a couple of protos before they’re finished.

    I first thought those rows of 74LSxx chips were (D)RAM. Then I realised they’re the glue logic so I bet the bottom layer track layout is really busy, especially under the glue logic/video generation.

    1. Thank you, I like a neat looking board. The trace routing is not as neat as I would like, but I’ll spend more time on that on the next revision. I did toy with adding a bridge rectifier on the input, rather than the jumpers, may go with that, then it doesn’t matter which way around it goes.

  2. The ZX80/81 when it hit the 90s was hacked a lot because it was among the few computers, that they released the entire ROM source code, so that you could customize it and burn your own custom rom, to an EPROM. I modded my TS1000 to double the clock speed replacing the Z80A with a Z80B processor. The same was done with the TRS80 model 1 system of same era. The other mod was to Y split the composite video to an RCA jack, and connect it to a Composite monitor or hack up a small B&W tv set which I did. I mounted my TS1000 to to top of a cassette case, and put an L bracket and 2 clips to secure the dreaded 16K ram expansion pack so that it would not constantly crash. Apart from countless ways to crash it on purpose, it was ahead of its time in the amount of software you could purchase. I always looked forward to hours of typing in programs from magazines.

    1. “The ZX80/81 when it hit the 90s was hacked a lot because it was among the few computers, that they released the entire ROM source code…”
      I had a ZX Spectrum as my first computer. I never learned machine code – still haven’t – but the complete ROM was published and disassembled into assembly language during the machine’s heyday. A copy of the book is now available online:
      https://www.microbe.cz/docs/CompleteSpectrumROMDisassemblyThe.pdf

      1. After your comment above, I have spent the last 3 days or so looking into the hardware and how it could be made. I am not quite up to speed with the ULA chip but it has been specified in Verilog. Unfortunately I write VHDL.

        I am tempted to make a Specie with standard parts but using a CPLD (if that is big enough) for the ULA and glue logic.

        I am now wondering of an Altera EPM570 (240-570 macros and is 5v tolerant) is enough to cover for the ULA and glue logic. I think someone did the ULA in an Xilinx XC96144XL (144 macros and 5 volt tolerant) so the the Altera chip should be enough.

  3. I had to mod the RF modulator, hundreds of them, to get better video quality!

    I desoldered the modulator, open it up, changed a resistor, soldered it back on the board.

    I got 4 of them for my own use and got paid $15/hour!!

  4. Also I had one zx81, being the only guy on scool with a computer.
    As a radio amateur I tried to fit is the qth locator grid distance calculator program. It would not fit in the small 1k memory . I did solderd a 2nd set memory chips on top of the one’s allready mounted, and got 2k of memory. result of having 1.5k or so of netto program space. You never forget your first computer!

  5. Why did the ZX80 look so much more awesomer than the ZX81?
    Still have a ZX81, extended keyboard, 16k ram, printer – the works.
    Funniest thing I read years ago was along the lines of “….gaffer tape a small length of a broom handle to the ZX81, and you can make your own ping pong bat”

  6. “The explorer’s guide to the ZX-81 and Timex Sinclair 1000” is what was mostly responsible for my corruption. I built a keyboard out of a full size industrial keyboard for it as the cunama and other units were out my budget. Soldered wires into each individual keyswitch to make a matrix. It worked, but was fragile as was the official ram pack until everything was glued onto a thick bit of mdf to stop wobble at the edge connector. I did the shunting the rom into ram by piggybacking a ram chip onto it which pushed the rom ascii charset into ram and let you do your own graphics sets.
    Who can forget masses of asci chars in a rem statement to be compiled into z80. Looking up each mnemonic and its op code and finding the ascii out for it. What a relief to discover compilers afterwards :-)
    And software distributed on the front of computer magazines on a thin floppy vinyl 45 record, to be recorded onto a tape, and mostly loaded 3rd or 4th time. And mazogs. I have quilted cushions in my gamesroom with the spiders and character etc from mazogs made for me as a present, old 8 bit ascii graphics lending itself perfectly to that craft!
    Still got a zx81, but dont use it much now, fire up a emulator on a wii or something. I introduced my son to sinclair basic like that…
    Tynemouth offer a usb keyboard option for using a zx81 keyboard/case as a keyboard, I expect they’ll do a run to cover both repro zx81 and zx81 keyboards for modern computers for hipsters that once again want to experience the true delight of a keyboard undersized with no tactile feedback that hurts your fingers after a short period.

    J-shifted P shifted P.

  7. “Also missing is the ZX expansion edge connector” – why? That means I/O is not possible? If that is the case, it is a big fail in that respect, hardware control being one of the foremost uses for old hardware. At least for me.

  8. ZX80 was the first computer that I ever coded on, I was 12. I owe it everything. I’m dyslexic and was failing at school, this little machine saved me for a life as a road sweeper.

    I recall having to put it in the fridge when they got a bit too hot so that I could get back to coding quicker. LoL :)

  9. I don’t think that was unusual, remember looking at specs back in the day and finding some particular LS parts were faster than HC on paper, like 11ns instead of 15 or something.

    1. Ahh, prodding the grey matter a bit more, yes, throughout most of the latter half of 20thC CMOS was a synonym for “slow as crap”, HC means “high speed considering it’s CMOS”, the LS was like a a poor mans F series, i.e. the fastest you would normally use unless for something special. LS was usually fast enough for most things.

      CMOS was used grudgingly when you really really had power problems.

      1. HC also has CMOS input levels, which are broadly compatible with TTL, but not really. HCT does better. LS is used here are there are a few RC circuits hanging off LS TTL outputs in the ZX80 design which don’t worth the same with CMOS totem pole outputs. Timing is sometimes tight as well, so best stick with gates with similar propagation delays to those in the original design. I’m planning to revamp the design where possible to make it more tolerant of different ICs, but for the moment its SN74xxN all round.

    2. Yes, there was inconsistency. The 74196 was a decade counter, good to at least 50MHz, but no particular power increase. It saw lots of use in the first digit of frequency counters.

      There was a need for low current, and a need for faster speeds, not at the same time initially, so things went both ways. So there was lots of opportunity to improve on things. That 74196 came later, so presumably design improved in the interim. Likewise moving to new families probably allowed smaller increments. I remember when S came along, a whole lot faster but more current drain, and few remember the L (low power) or H (faster but higher current) which were kind of stopgaps which didn’t last long. But I remember variants in the S series, so if you looked carefully you could find a faster flip-flop than the rest.

      Then the hybrids came, more chance to fiddle. And faster speeds with lower current, the LS series wasn’t as fast as S but was lower current and beat what had come before, even the H series. Then the goal was faster but lower current, kind of an x-y grid.

      CMOS went the same way, the 74C00 series just like the 4000 CMOS series except pinout to match ttl (but not electrically compatible), very slow, but in the end way more iterations than the ttl line.

      At some point, I found it very hard to follow.

      Michael

    3. Apples and oranges –
      HC (High speed CMOS) has CMOS voltage levels and is only high speed compared to normal CMOS
      HCT (High Speed CMOS but with TTL voltages. The rest is the same as above)

      LS (Low Power Schottky) has hysteresis and is used when you want to avoid any linear mode operation

  10. Beautiful board layout!

    Indeed the original ZX80 schematic relies on the analogue properties of the chips and some RC timing networks to reduce the number of logic gates needed. It works but it’s not a “clean” design. I guess every penny counted back then, especially when working for Sir Clive. Wilf Rigter has designed a clean discrete CMOS logic clone of the ZX80/ZX81 called the ZX97, which doesn’t need those analogue timing tricks. Later on, Rodney Knaap simplified that original ZX97 design into an easier to build ZX97 Lite variant. Both are 100% ZX81 compatible and feature some nice enhancements such as swappable memory ranges and ROM files.

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