Sinclair ZX81

The Sinclair ZX81, developed by Sinclair Research and manufactured by Timex Corporation in Dundee, Scotland, was the first home computer sold in the United States for under $150.

Launched in England on March 5, 1981, the ZX81 was designed to be a low-cost introduction to home computing for the general public.

The 1KB on-board memory could officially be expanded externally to 16KB. The ZX81 reduced the 21 individual integrated circuits of the ZX80 to four: the 3.5 MHz Z80A 8-bit microprocessor from NEC, an uncommitted logic array (ULA) chip from Ferranti, an 8 KB ROM containing Sinclair’s BASIC interpreter and 1K of RAM. The ULA alone replaced 18 chips in the ZX80. And the ZX81 weighed even less than the ZX80, just 12 oz.

Like the ZX80, it had a flat, membrane keyboard and connected to the user’s television set. It produced a black-on-white video signal and comparatively low resolution graphics. The ZX81 added a “slow” mode that generated a constant screen display and but computation to the vertical blank time.

Clive Sinclair with the ZX81.


Sinclair’s 8K BASIC was a more complete but not-quite full BASIC. Features like READ, DATA and RESTORE were still missing but the math package was improved, incorporating trigonometric and floating point functions.

Where the ZX80 could only handle integer numbers (and within a limited range), the ZX81 could operate on very large numbers.

String handling and other aspects of the BASIC were vastly improved as well.

Video Display

Like the ZX80, the screen displayed 32 characters across by 24 lines. The first 22 lines displayed the output of the program and the last two were the “input area”, where users entered program lines or responded to prompts from the program.

Like the ZX80 before it, the CPU in the ZX81 generated the video signal. In FAST mode, the processor stopped generating the video signal when performing other computing tasks, resulting in the TV picture disappearing. Each key press resulted in a blank screen.

The ZX81 added a new, SLOW display mode. In SLOW mode, all computing takes place during the vertical blank period, a small window of time when the electron gun in a TV traces from the bottom of the screen back to the top.


The popularity of the ZX81 spawned a huge array of aftermarket accessories, including full-size keyboards, RAM, alternate language ROMs, printer interfaces and more. Many devices connected to the expansion port on the rear of the computer.


Clive Sinclair revealed the ZX81 to American readers in an article in New York Times, published in April 1981. The article revealed the company had contracted with Timex to produce both “minitelevisions” and the ZX81 at its Dundee, Scotland, plant.

Sinclair showed the ZX81 at the 1981 Summer Consumer Electronics Show, which garnered some positive press for the computer.

Reviews of the ZX81 noted many of the same limitations observed in the ZX80: its small size, limited memory and flat keyboard.

Popular Electronics said “we were surprised at the amount of computing power that Sinclair packed into such a small computer (you can carry this little wonder in a jacket pocket without making a bulge). The BASIC is as good as anything around in small computers.”


Despite its limitations, the ZX81 sold like gangbusters; more than 1.5 million units were sold worldwide. Even with its limitations, the ZX81 at one point sold faster than Timex could manufacture them for Sinclair.

Sinclair officially launched the ZX81 in the US on October 7, 1981 with 20,000 units available for sale, at a price of $149.95 assembled and $99.95 in kit form. As with the ZX80, they sold directly to the American market by mail order, via ads in computer publications and popular consumer magazines.

Sales reached 15,000 a month by January 1982.

A deal with American Express to including marketing material and order forms in 2 million of their customer’s bills resulted in massive interest in the computer. 2,000 orders were received by noon the day following the offer and American Express sold more than 25,000 units in total.

By August 1982 Sinclair had lowered the American mail-order price of the assembled ZX81 to $99.95 and kit to $79.95, and its advertisements stated that “more than 10,000 are sold every week”.

Timex’s option to sell the computer directly, as the Timex Sinclair 1000, kicked in when Sinclair’s American sales reached 500,000 units.

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