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Happy Anniversary to the Sinclair ZX81, my first computer. I still have fond memories of building it from the kit, connecting it to a black and white TV for a monitor, and recording programs on cassette tape for use later.
The Sinclair ZX81 was a wonder in its time.
The ZX81 was a home computer produced by Sinclair Research and manufactured in by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair’s ZX80 and was designed to be a low-cost introduction to home computing for the general public. It was hugely successful and more than 1.5 million units were sold before it was eventually discontinued. The ZX81 found commercial success in many other countries, notably the United States, where Timex manufactured and distributed it under licence. Timex later produced its own versions of the ZX81 for the US market – the Timex Sinclair 1000 and Timex Sinclair 1500. Unauthorised clones of the ZX81 were produced in a number of countries.
The ZX81 was designed to be small, simple, and above all cheap, using as few components as possible to keep the cost down. Video output was to a television set rather than a dedicated monitor. Programs and data were loaded and saved onto audio tape cassettes. It had only four silicon chips on board and a mere 1 kB of memory. The machine had no moving parts – not even a power switch – and used a touch-sensitive membrane keyboard for manual input. The ZX81′s limitations prompted the emergence of a flourishing market in third-party peripherals to improve its capabilities. Such limitations, however, achieved Sinclair’s objective of keeping the cost of the machine as low as possible. Its distinctive design won awards in the UK and abroad.
The ZX81′s built-in RF modulator can output a video picture to either a UHF (PAL) television (used in the UK, Australia, and most western European countries except France) or a VHF (NTSC) television (used in the US and Canada). This could be pre-set either at the factory or during the assembly of the kit. Both the ZX81 and its predecessor, the ZX80, have a significant drawback in the way that they handle visual output. Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display. On the ZX80, this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place.
The ZX81′s designers adopted an improved approach, involving the use of two modes called SLOW and FAST respectively. In SLOW mode, also called “compute and display” mode, the ZX81 concentrates on driving the display. It runs the current program for only about a quarter of the time – in effect slowing the machine down fourfold, although in practice the speed difference between FAST and SLOW modes depends on what computation is being done. In FAST mode processing occurs continuously but the display is abandoned to its own devices – equivalent to the ZX80′s standard operating mode.
Another hardware quirk produced one of the most distinctive aspects of the ZX81′s screen display – during loading or saving, moving zigzag stripes appear across the screen. The same pin on the ULA is used to handle the video signal as well as the tape output, producing the stripes as an interference pattern of sorts. As well as this, the ULA cannot maintain the display during SAVE and LOAD operations, as it has to operate continuously to maintain the correct baud rate for data transfers.
The unexpanded ZX81′s tiny memory presented a major challenge to programmers. Simply displaying a full screen takes up 768 bytes, the system variables take up another 125 bytes and the program, input buffer and stacks need more memory on top of that. Nonetheless, ingenious programmers were able to achieve a surprising amount with just 1 kB. One notable example was 1K ZX Chess by David Home, which is somewhat misnamed as it managed to squeeze most of the rules of chess into just 672 bytes. The ZX81 conserved its memory to a certain extent by representing entire BASIC commands as one-byte tokens, stored as individual “characters” in the upper reaches of the machine’s unique (non-ASCII) character set.
The success of the ZX81 led almost immediately to enthusiasts producing a huge variety of peripherals and software. Clive Sinclair was “amused and gratified” by the attention the machine received but made little effort to exploit the demand, effectively ceding a very lucrative market to third party suppliers – a decision that undoubtedly forfeited a lot of potential earnings. W.H. Smith, for instance, was able to exploit a peculiarity of the ZX81; owners found that technically obsolete low-fidelity mono tape cassette recorders actually worked better as storage devices than higher-quality music systems. Smith’s cashed in by buying up stocks of cheap “shoebox” cassette recorders in the Far East, sticking the W.H. Smith logo on them and selling them with a generous mark-up as “data recorders”. Over 100,000 were sold in eighteen months.
Sinclair released only two official peripherals for the ZX81 – a 16 kB RAM pack (actually the same one previously released for the ZX80, but rebadged) and the ZX Printer, both of which plugged into the edge connector at the rear of the ZX81. They retailed at a launch price of £49.95 each but both had notable flaws. The RAM pack was top-heavy and was supported only by the edge connector. It had a habit falling out of its socket at crucial points and crashing the ZX81, losing anything that the user had typed in. Users turned to using sticky lumps of chewing gum, double-sided tape or Blu-Tack to cure what became known as the “RAM pack wobble” problem. The ZX Printer was a tiny spark printer that used two electrically charged styli to burn away the surface of aluminium-coated paper to reveal the black underlay. It worked reasonably well at first but its output deteriorated rapidly after a time.
Related articles
- Sinclair ZX81: 30 years old tomorrow (go.theregister.com)
- Thirty Years Ago the Future Arrived: The Sinclair ZX81 (designmind.frogdesign.com)
- What Computer History Should Be Saved? (spectrum.ieee.org)
I hope you’re going to write something as good for the Spectrum’s anniversary, this month!