ZX81

The ZX81 is a home computer that was produced by Sinclair Research and manufactured in Dundee, Scotland, by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and designed to be a low-cost introduction to home computing for the general public. It was hugely successful; more than 1.5 million units were sold. In the United States it was initially sold as the ZX-81 under licence by Timex. Timex later produced its own versions of the ZX81: the Timex Sinclair 1000 and Timex Sinclair 1500. Unauthorized ZX81 clones were produced in several countries.

The ZX81 was designed to be small, simple, and above all, inexpensive, with as few components as possible. Video output is to a television set rather than a dedicated monitor. Programs and data are loaded and saved onto compact audio cassettes. It uses only four silicon chips and a mere 1 KB of memory. It has no power switch or moving parts, with the exception of a VHF TV channel selector switch present in some models. It has a pressure-sensitive membrane keyboard. The ZX81's limitations prompted a market in third-party peripherals to improve its capabilities. Its distinctive case and keyboard brought designer Rick Dickinson a Design Council award.

The ZX81 could be bought by mail order preassembled or, for a lower price, in kit form. It was the first inexpensive mass-market home computer to be sold by high street stores, led by W. H. Smith and soon many other retailers. The ZX81 marked the point when computing in Britain became an activity for the general public rather than the preserve of businessmen and electronics hobbyists. It produced a huge community of enthusiasts, some of whom founded their own businesses producing software and hardware for the ZX81. Many went on to have roles in the British computer industry. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers and earned a fortune and an eventual knighthood for the company's founder Sir Clive Sinclair.

Features

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 wide by 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 uncommitted logic array (ULA) chip from Ferranti, and an 8 KB ROM providing a simple Sinclair BASIC interpreter. The entire machine weighs just . ''ZX81 Operating Supplement'' (1982) Early versions of the external RAM cartridge contain 15 KB of memory using an assortment of memory chips, while later versions contain 16 KB chips, but the lowest addressed kilobyte is disabled.

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. For example, the P key combines the letter P, the " character, and the BASIC commands PRINT and TAB. 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.

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. 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. This stores each data bit as a number of pulses followed by an inter-bit silence of 1300 µs. Each pulse is a 150 µs 'high' then a 150 µs 'low'. A '0' bit consists of four pulses while a '1' bit is nine pulses, so the baud rate varies between 400 bit/s for all '0's and 250 bit/s for all '1's. A file with equal amounts of '0's and '1's would be stored at 307 bit/s (38 bytes/sec). This provides a somewhat temperamental storage medium for the machine, which has no built-in storage capabilities. The ZX81 requires 420 mA of power at 7–11 V DC, delivered via a custom 9 V Sinclair DC power supply.

The ULA chip, described by the ZX81 manual as the "dogsbody" of the system, 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.

The ZX81's built-in RF modulator can output a video picture to either a UHF 625-line colour or monochrome television (used in the UK, Australia, and most western European countries). France required a slightly modified version of the machine to match the positive video modulation of SECAM sets, while the US and Canada required a different ULA chip and modulator to cope with their 525-line VHF (NTSC) television systems. 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 and the tape output, producing the stripes as an interference pattern of sorts. 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 interference produces the zigzag stripes.

The unexpanded ZX81's tiny memory presents a major challenge to programmers. Simply displaying a full screen takes up to 793 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 are able to achieve a surprising amount with just 1 KB. One example is the ''Super Micro Chess Engine'' by Stefano Maragò, which includes all of the rules of chess. The ZX81 conserves 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 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.

Comparisons with other computing devices

The following table provides a comparison between the capabilities of the ZX81 and various other competing microcomputers that were available in June 1981, about the time that the first ZX81 orders were delivered. The prices given are as of June 1981 from Your Computer UK.

N.B.: Commodore PET includes monitor and cassette recorder. The Video Genie includes a built in cassette recorder. The Compukit 101 is an Ohio Scientific SuperBoard made under licence in Britain. Compukit 101 and MicroTan are kit computers and are not supplied with a case.

The Atari 400 and TRS-80 model I were officially only expandable to 16 KB, but expansion to 48 KB was available from third party suppliers.

The ZX81 and the other systems referred to as having block graphics have a resolution of 256 × 192 pixels, there are 32 columns × 22 rows of text and graphics can be addressed in quarter character blocks.

History

Background

Clive Sinclair, a former radio kit salesman, established his first company, Sinclair Radionics, in 1962. The company made its name producing a wide range of cheap electronics aimed at the hobbyist market. Its products include amplifiers, radios, multimeters and other items which were generally sold in kit form to hi-fi enthusiasts and other electronics hobbyists. The company entered a new market in 1972 when it launched the first "slimline" pocket calculator, the Sinclair Executive. It was a major success that Radionics followed up by launching a wide range of pocket calculators. The company's subsequent expansion made it Europe's biggest calculator manufacturer by 1975.

By the late 1970s, however, Sinclair Radionics was experiencing serious difficulties. It lost its ability to compete effectively in the calculator market following the launch of a new generation of Japanese-produced calculators with liquid-crystal displays, which were much more capable and power-efficient than Sinclair's LED calculators. Projects to develop a pocket television and digital watch turned out to be expensive failures. The company made losses of more than £350,000 in 1975–76, bringing it to the edge of bankruptcy. In July 1977 Radionics was rescued by a state agency, the National Enterprise Board (NEB), which recapitalised it, provided a loan facility and took effective control of the company by acquiring a 73% stake.

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", specifically the development of the ultimately unsuccessful TV80 pocket television and C5 electric vehicle. In an interview with ''Practical Computing'', Sinclair explained:

Precursors: the MK14 and ZX80

By the late 1970s, American companies were producing simple home computer kits such as the MITS Altair and IMSAI 8080. This aroused interest among electronics hobbyists in the UK but relatively high prices and lower disposable income reduced the appeal of the American products. ''New Scientist'' stated in 1977 that "the price of an American kit in dollars rapidly translates into the same figure in pounds sterling by the time it has reached the shores of Britain". Off-the-shelf personal computers were also available for the high end of the market but were extremely expensive; Olivetti's offering cost £2,000, and the Commodore PET, launched in 1979, sold for £700. There was nothing for the hobbyist at the low end of the market. Sinclair realised that this provided a useful commercial opportunity.

Sinclair's first home computer was the MK14, which was launched in kit form in June 1978. It was a long way from being a mass-market product. Its very name – MK standing for "Microcomputer Kit" – was indicative of its origins as a product developed by, and for, hobbyists. It had no screen but instead used an LED segment display (though Science of Cambridge did produce an add-on module allowing it to be hooked up to a UHF TV); it had no case, consisting of an exposed circuit board; it had no built-in storage capabilities and only 256 bytes of memory; and input was via a 20-key hexadecimal keyboard. Despite the limitations of the machine it sold a respectable 10–15,000 units; by comparison, the much more expensive Apple II had only sold 9,000 units in the United States, a much bigger market, in 1978. This success convinced Clive Sinclair that there was an untapped market for low-cost computers that could profitably be exploited.

Sinclair followed up the MK14 by producing the ZX80, at the time the world's smallest and cheapest computer, which was launched in January 1980 costing £99.95 (equivalent to £390 at 2021 prices.) The company conducted no market research whatsoever prior to the launch of the ZX80; according to Clive Sinclair, he "simply had a hunch" that the general public was sufficiently interested to make such a project feasible and went ahead with ordering 100,000 sets of parts so that he could launch at high volume.

The ZX80's design introduced many key features that were carried over to the ZX81; as Sinclair himself later said, "the ZX80 was very much a stepping stone to the ZX81". Scot (Aug/Sept 1981) The design was driven entirely by the desired price – the machine had to cost less than £100 but still make a healthy profit. Its distinctive wedge-shaped white case concealing the circuitry and the touch-sensitive membrane keyboard were the brainchild of Rick Dickinson, a young British industrial designer who had recently been hired by Sinclair. As he later recalled of Sinclair's approach, "Everything was cost driven. The design was the face of the machine." BBC News (23 April 2007) The unconventional keyboard was the outcome of Sinclair's cost-cutting. It made use of a sheet of plastic, on which the keys were printed, overlaying a metallic circuit that registered when a key was pressed. This avoided the expense of providing a typewriter-style keyboard, though the design had many drawbacks when it came to usability and "feel".

Inside the case, there were many more similarities with the ZX81. Like its successor, it used the Z80A microprocessor and had only 1 KB of on-board RAM. It came with a specially written BASIC interpreter on a dedicated ROM chip and could use a television as a display. It relied on an ordinary cassette tape recorder for data storage. The main difference between the two machines lay in the internal software; when the ZX81 was released, ZX80 owners were able to upgrade by the relatively simple expedient of plugging a new ROM onto the circuit board.

The ZX80 was an immediate success, selling 20,000 units over the following nine months. Science of Cambridge was producing ZX80s at the rate of 9,000 a month by the end of 1980 and within 18 months of its launch the company had sold 100,000 units. Crisp (20 March 1982) The commercial success of the ZX80 made a follow-up product inevitable. The company was renamed Sinclair Computers in November 1980, reflecting its new focus, and became Sinclair Research in March 1981.

BBC Micro bid

The launch of the ZX81 was catalysed in part by the BBC
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