The Sinclair ZX81 was released by Sinclair Research Ltd in 1981.
It was a development of the successful ZX80 released the previous year. The design was refined, retaining the membrane keyboard, but with a sleeker case designed by Rick Dickinson in what would become the Sinclair trademark black. Many of the discrete chips were rationalised onto a single Uncommitted Logic Array (ULA). These changes and a general drop in component prices allowed the ZX81 to be released for just £49.95 in kit form and £69.95 fully assembled - a reduction in cost of almost a third in just one year. Unlike earlier computers which had primarily been sold through mail-order, it was also distributed through high-street retailers such as the UK stationer WH Smith. This made it visible to a much wider market.
The computer was based around a Zilog Z80 CPU running at 3.5 MHz (though most ZX81s actually shipped with an NEC clone of the Z80). 1Kb of RAM was included as standard, though this could be expanded with external RAM packs up to a maximum of 16Kb.
An example of cost-cutting carried over from the ZX80 was that the expansion port to which RAM packs and other peripherals was connected was not a moulded connector, but merely an edge connector on an exposed section of motherboard. This could cause problems if the computer was knocked, resulting in the RAM pack being dislodged and work being lost. This was such a problem on the ZX81 that official Sinclair customer support suggested that it be held in place with Blu-Tack or electrical tape.
The ZX81 was designed to be connected to a standard television, rather than an expensive dedicated monitor and included an RF out. Graphics were black and white only, with a screen resolution of 32 characters by 22 characters. Each character was an 8x8 pixel block. Graphics characters such as ▌or ▄ were available which allowed primitive graphics to be drawn. Developers later discovered how to display graphics that addressed each pixel individually, allowing graphics with a resolution of 256x192 to be displayed. The flicker problem of the ZX80 was solved through the use of hardware which allowed the video update and CPU processing to happen simultaneously, this was known as SLOW mode. The original ZX80 screen update method was still available in the ZX81 and was known as FAST mode, as it allowed the processor to be dedicated entirely to processing tasks without updating the screen.
The ZX81 had no sound output, though later peripherals were released by third parties which added a sound capability.
The operating system and BASIC software was included on an 8Kb ROM. BASIC commands were not typed in full, rather pressing a key when a keyword was expected caused an entire keyword to appear. For example, pressing "I" would cause "INPUT" to appear. The keywords in a BASIC program were stored in memory as a single character, resulting in a significant reduction in memory storage required.
Data storage was by connecting a standard domestic cassette tape recorder to connectors on the side of the ZX81. This also allowed for relatively cheap distribution of early commercial software, including games.
The ZX81 also marked Sinclair's first major foray into the American market. Initially sold as the Sinclair ZX81 by mail order, Timex later licensed the design and sold it through retail outlets, as had been successfully done in the UK. Timex already assembled ZX81s in their Dundee factory and their slightly upgraded version of the computer (with 2Kb RAM as standard) was released in the US market as the Timex Sinclair 1000. Unfortunately the cheap build quality did not survive the trip across the Atlantic well, and apparently only about a third of computers arrived in operational condition. Clones, both official and unofficial, were released in many countries around the world.
The ZX81 was superseded by the Sinclair ZX Spectrum. This was a much more substantial jump in capability than from the ZX80, including colour graphics, sound and a more substantial keyboard.