Happy birthday, ZX81

Thirty years ago today the Sinclair ZX81 was introduced to the world. The world’s first affordable personal computer, it wasn’t in fact the first home computer I had owned. That had been a Nascom 1, a Zilog Z80 based machine with just 1Kb of RAM, which I built from a kit – all 1,400 soldered joints of it, using the same Antex soldering iron I still use today.

The ZX81 was also offered as a kit as well as a ready built version, so naturally I ordered the kit. I seem to remember it cost £49 – much less than the Nascom. I don’t remember how many soldered joints there were, but there were only four main chips. It was a much easier project to build. The ZX81 also came with just 1Kb of RAM. But unlike the Nascom, it had a built-in BASIC interpreter so you could still do more with that 1Kb and you didn’t have to program it in assembly code.

Innovative design was used to cut the cost of the ZX81. For example, instead of a dedicated display processor the Z80 CPU generated the display. Whenever your program executed, the screen went blank. The screen was an ordinary black and white TV. Programs were loaded and saved using a cheap cassette recorder. That was read and written by the CPU too, which generated wild patterns on the screen while it neglected its display duties. The keyboard was a plastic membrane type. It was horrible to type on.

Later I upgraded the memory to 48Kb using a third party RAM pack (the standard RAM pack sold by Sinclair was only 16Kb.) This, too was built from a kit. Like the Sinclair one, it fixed to the rear of the machine using an edge connector, with no other fixing. Every ZX81 owner is familiar with the term “wobbly RAM pack”. One accidental jolt could interrupt the connection and crash the computer losing all your work. Ah, those were the days!

As a radio ham, I naturally was interested in writing ham radio software for the ZX81. I wrote several programs including a morse tutor, which used a machine code routine for sending the actual code. I think I have written morse tutors for every type of computer I have owned – it’s amazing that I am still so bad at reading the code! An article describing the morse tutor and a memory keyer for the ZX81 was published in Short Wave Magazine and was one of my first published articles.

I can still remember the excitement of home computing in those early days. Today’s PCs, vastly more powerful and capable though they are, just aren’t as interesting. Back then, home computing was very much a hobbyist’s game. We were pioneers. Now everyone and his granny has a computer, and programming has become more or less a job for professionals. I do miss those old days!

Thank you, Clive Sinclair, for bringing us these wonderful toys. Happy 30th birthday, ZX81!

Julian Moss, G4ILO, is a regular contributor to AmateurRadio.com and writes from Cumbria, England. Contact him at [email protected].

10 Responses to “Happy birthday, ZX81”

  • Matt W1MST:

    Reminds me of my first computer, the Texas Instruments TI-99/4A. What a machine for $500!

  • Demetre Valaris - SV1UY:

    I got mine in the beginning of 1982 for 16.00 drachmas. I still have it enclosed in a DKtronics keyboard-case with it’s 16K RAM and a 1K EPROM with RITTY RTTY program from G4IDE. This computer in conjunction with a homemade RTTY Terminal Unit and an 81 LED Matrix as a SCOPE was my main RTTY station for many years.

    73 de Demetre SV1UY

  • Ron N9XEQ:

    Brings back great memories. I had a TS 1000, then went to a Sinclair 1500, which I still have, a Sinclair 2068 and finally a Sinclair QL. What great machine that was. I wrote many programs and really started my computer journey there.
    I always looked down my nose at Apple, but today that is about all I use. I just wish that their was more Mac support for Ham Radio.

  • Eugene Kramer-WA9TZL:

    Great picture and article on the ZX81. I too built one and then purchased the Timex/Sinclair as a back up for the original. Then I purchased the Timex/Sinclair thermal printer for “hard copies” of my work. Then I purchased the MemoPak with a whomping 64K of RAM, but I used a strip of velcro to prevent the “wobble” and loose the work. I even purchased the Timex/Sinclair tape recorder that was made to provide a closer setting for its audio input levels. I built the RAND converter to save and load the audio saved data in 30 seconds instead of 15-20 minutes onto the cassettes.

    I wrote programs for “ham calls” for quick retrieval during QSO’s but there were several “cassette based” programs for inventory (which could have been modified) , so the Z80 chip was indeed the heart of this qreat under $100.00 work horse.

  • N7QAX:

    Does anyone still have any RTTY programs for the ZX81?

  • Ric - IK8MCX:

    > N7QAX wrote:
    > Does anyone still have any RTTY programs for the ZX81?

    …And relative hardware? 😉
    …And schematics? 😀

  • Wim - PAØWDW:

    Many years ago I made a special RTTY-interface for the ZX81. It works in combination with the RTTY-program from Scarab. I have loaded this Scarab program into my ZX81. The special RTTY-interface is connected between the external 16 k RAM and the ZX81.
    The description of that RTTY-interface is published in Practical Wireless, July 1983. *)
    My ZX81 as an RTTY-machine for receiving and transmitting is still working fine. I am still active with the ZX81 on the HF-bands.
    Apart from the above mentioned things you need an RTTY-converter, like ST-5, ST-6, or DJ6HP, etc.) between your transceiver and the ZX81 setup.

    If you are seriously interested I can mail you a scan of the Practical Wireless article. And also I can mail you the Scarab RTTY-program as mp3-file.

    *) Practical Wireless, July 1983, “RTTY with the ZX81”, Dick Ganderton G8VFH

    73, Wim PAØWDW

  • Bill N7QAX:

    Wim, I came across the same article in practical wireless, found the same program on EBay. I also built the interface. I got it working pretty well. Thanks for asking

  • jourgen LB2SA:

    Wim,is it posible to get a copy of the article from PW and a copy of the rtty program for the zx 81 and the interface.
    I have the zx 81 and the rtyy81-4 module from scarab, but no doc. or program.

    73 de LB2SA jourgen.

  • Wim - PAØWDW:

    Hello Jourgen, in my message of 28 June 2015 I announced that I can mail the Scarab RTTY-program as mp3-file. I am sorry to say that it is a mistake, because mp3 is causing compression which makes the file useless. Instead of mp3 I shall convert the Scarab RTTY-program into a wav-file, which is working errorless.
    I shall send you all documentation and a wav-file with the Scarab RTTY-program via WeTransfer.
    73, Wim PAØWDW

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