sinclair zx81
               Sinclair ZX81

The Sinclair ZX81 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 produced by Sinclair Research and manufactured in Dundee, Scotland, by Timex Corporation.

It was a huge success, and more than 1.5 million units were sold before it was discontinued. The ZX81 found commercial success in many other countries, notably the United States where it was initially sold as the ZX-81. Timex manufactured and distributed it under licence and enjoyed a substantial but brief boom in sales. Timex later produced its own versions of the ZX81 for the US market: the Timex Sinclair 1000 and Timex Sinclair 1500. Unauthorized clones of the ZX81 were produced in several countries.

The ZX81 was designed to be small, simple, and above all inexpensive, 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 compact audio cassettes. It had only four silicon chips and a mere 1 KB of memory. The machine had no power switch or any moving parts, with the exception of a VHF TV channel selector switch present on early “ZX81 USA” models and the Timex-Sinclair 1000, and it used a pressure-sensitive membrane keyboard for manual input. Its distinctive case and keyboard brought designer Rick Dickinson a Design Council award.

Early Price :  £69.95 (1981)
Today’s Price :  £188 (2019)

The Sinclair ZX81 was a game changer for both the world and the company. 

Sinclair ZX81: Overview

Developer :  Sinclair Research
Manufacturer :  Timex Corporation
Type :  Home computer
Release date :  5 March 1981 (38 years ago)
Discontinued :  1984
Units sold :  More than 1.5 million
Display  :  Monochrome display on UHF television
OS (Operating System) :  Sinclair BASIC
CPU :  Z80 at 3.25 MHz
Memory :  1 KB (64 KB max. 56 KB usable)
Storage :  External cassette tape recorder (average 300 bps)
Power Supply :  9V DC
Dimensions :  6.6 inch deep X 1.6 inch high
Weight :  350 gms (12 oz)

Sinclair ZX81: Machine build up

sinclair zx81 interior
              Revised Sinclair ZX81 PCB

The ZX81 has a base configuration of 1 KB of on-board memory that can officially be expanded externally to 16 KB. Its single circuit board is housed inside a wedge-shaped plastic case measuring 167 millimetres (6.6 in) deep by 40 millimetres (1.6 in) high. The memory is provided by either a single 4118 (1024 bit × 8) or two 2114 (1024 bit × 4) RAM chips. There are only three other onboard chips: a 3.5 MHz Z80A 8-bit microprocessor from NEC, an ULA (uncommitted logic array) chip from Ferranti, and an 8 KB ROM providing a simple BASIC interpreter.

The front part of the case is occupied by an integrated 40-key membrane keyboard displaying 20 graphic and 54 inverse video characters. Each key has up to five functions, accessed via the SHIFT and FUNCTION keys or depending on context. The ZX81 uses a standard QWERTY keyboard layout. The keyboard is mechanically very simple, consisting of 40 pressure-pad switches and 8 diodes under a plastic overlay, connected in a matrix of 8 rows and 5 columns.

sinclair zx81 keyboard
    Sinclair ZX81 PCB Revision 3 keyboard

The ULA chip has a number of key functions that competing computers share between multiple chips and integrated circuits. These comprise the following:

  • Synchronising the screen display.
  • Generating a 6.5 MHz clock, from which a 3.25 MHz clock is derived for the processor.
  • Outputting an audio signal to a cassette recorder in SAVE mode.
  • Processing the incoming cassette audio signal in LOAD mode.
  • Sensing keystrokes.
  • Using memory addresses provided by the CPU to decide when ROM and RAM should be active.
  • Controlling general system timing.

Sinclair ZX81: Ports and sockets

The ZX81’s primary input/output is delivered via four sockets on the left side of the case. The machine uses an ordinary UHF television set to deliver a monochrome picture via a built-in RF modulator. It can display 24 lines of 32 characters each, and by using the selection of 2×2 block character graphics from the machine’s character set offers an effective 64 × 44 pixel graphics mode, also directly addressable via BASIC using the PLOT and UNPLOT commands, leaving 2 lines free at the bottom.

sinclair zx81 ports
Left side and rear views of the Sinclair ZX81

Two 3.5 mm jacks connect the ZX81 to the EAR (output) and MIC (input) sockets of an audio cassette recorder, enabling data to be saved or loaded. The ZX81 requires 420 mA of power at 7–11 V DC, delivered via a custom 9 V Sinclair DC power supply.

The edge connector or external interface at the rear of the ZX81 is an extension of the main printed circuit board. This provides a set of address, control, and data lines that can be used to communicate with external devices. Enthusiasts and a variety of third-party companies make use of this facility to create a wide range of add-ons for the ZX81.

Sinclair ZX81: Marketing

The marketing of the ZX81 was handled by Sinclair’s long-standing marketing agency Primary Contact (now part of Ogilvy & Mather), which had provided marketing services for Sinclair since 1971 and was to continue doing so until 1985. Sinclair’s entry into the nascent home computing market gave Primary Contact a major challenge – how to market a product simultaneously at hobbyists and at the “man on the street“, who probably had little or no computer literacy. The answer was to pursue what the journalist David O’Reilly of MicroScope magazine described as a single-minded “user-friendly strategy.” Chris Fawkes, one of Primary Contact’s directors, explained: “We brought personal computers to the mass market by showing that you didn’t have to be a whizzkid to use one”. As Clive Sinclair put it in a 1982 interview with Your Computer.

Sinclair Research’s launch advertising for the ZX81.

High-profile advertising was central to the marketing campaign. Although Sinclair Research was a relatively small company, it had a long-standing policy of using large-scale advertisements that stood out in stark contrast to the more muted advertisements of other manufacturers. Superlatives, exhortations, appeals to patriotism, testimonials, eye-catching drawings and photographs on double-page spreads, varying from month to month, were used to drum up mail order business for Sinclair.

The launch advertising for the ZX81 illustrates this approach. A photograph of the ZX81 alongside the official Sinclair peripherals dominated the centre of a double-page spread. The value for money of Sinclair’s products was emphasised by the prices being printed in larger type than any other text on the spread. The ZX81’s benefits were promoted with the aspirational slogan “Sinclair ZX81 Personal Computer – the heart of a system that grows with you“. The advertisement highlighted ZX81 BASIC Programming, the manual written by Steve Vickers, as “a complete course in BASIC programming, from first principles to complex programs.”


The ZX81 had an immediate impact on the fortunes of Sinclair Research and Clive Sinclair himself. The company’s profitability rose enormously, from a pretax profit of £818,000 on a turnover of £4.6 million in 1980–81 to £8.55 million on a turnover of £27.17 million in 1981–82. Clive Sinclair became one of the UK’s highest-profile businessmen and a millionaire, receiving a £1 million bonus on top of a salary of £13,000. He received a knighthood in the Queen’s Birthday Honours and the Young Businessman of the Year award in 1983.

Even 30 years after launch, the ZX81 has a German user forum, and one in English. New hardware and software continue to be developed for the ZX81.


Clive Sinclair’s relationship with the NEB was fraught due to conflicting notions about which direction the company should go. Radionics had begun a project to develop a home computer but the NEB wanted to concentrate on the instrument side of the business, which was virtually the only area where Radionics was profitable. Sinclair disagreed vehemently with what he characterised as the view “that there was no future in consumer electronics”. This and other disputes led to Sinclair resigning from Radionics in July 1979.

While he was struggling with the NEB, Clive Sinclair turned to a “corporate lifeboat” in the shape of an existing corporate shell under his exclusive control – a company called Ablesdeal Ltd., which he had established in 1973 and later renamed Science of Cambridge. It became a vehicle through which he could pursue his own projects, free of the interference of the NEB. Despite his later success in the field, Sinclair saw computers as merely a means to an end. As he told the Sunday Times in April 1985, “We only got involved in computers in order to fund the rest of the business”. In an interview with Practical Computing, Sinclair explained:

“I make computers because they are a good market, and they are interesting to design. I don’t feel bad about making them or selling them for money or anything, there is a demand for them and they do no harm; but I don’t think they are going to save the world”.



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