Microcomputing on the Cheap: The Sinclair ZX81/Timex Sinclair 1000 Turns 45 Today

Today marks a special day in the calendar of the cost-conscious home computing enthusiast: the Sinclair ZX81, sold in the US as the Timex Sinclair 1000, celebrates its 45th birthday.

"Reach advanced computer comprehension in a few absorbing hours," the launch advertisements for Sinclair Research's ZX81 microcomputer promised. A successor to the ZX80, with design work having started even before its predecessor's launch, the Sinclair ZX81 — relaunched in the US as the Timex Sinclair 1000, and later the upgraded Timex Sinclair 1500 — promised a low-cost entry into the burgeoning field of home computing, addressing criticisms leveled at the ZX81 while somehow bringing the price down even lower.

"All in all, I can only recommend the ZX81 to any intending purchaser — it is very good value for money," Peter Freebrey wrote in his review for Electronics Today International on the launch of the machine. "Anyone wanting to learn personally what a computer can do, without initially spending a fair amount of money, should seriously consider one."

"Uncle Clive [Sinclair] has come up with a lovely product which will have enormous appeal to people wanting to find out more about computers, but without it costing them an arm an and a leg," agreed David Tebbutt in his review for Personal Computer World. "The idea of producing a superior machine to the ZX80 and selling it for a lower price is absolutely wonderful. I'm full of admiration for the man."

"The ZX80 reduced the chips in a working computer from 40 or so, to 21," Sinclair explained of how the company had slashed the already-low price of the ZX80 in the design of its more capable successor. "The secret lies in a totally new master chip. Designed by Sinclair and custom-built in Britain, this unique chip replaces 18 chips from the ZX80!"

That "unique chip" was a precursor to the field-programmable gate array (FPGA), an off-the-shelf part from Ferranti called an uncommitted logic array (ULA). Like an FPGA, it allowed the user to "program" how its internal gates connect in order to create a semi-custom chip without the expense of actual semiconductor manufacturing; unlike an FPGA, it was a once-and-done operation that permanently fused the configuration into the chip.

Using the Ferranti ULA, Sinclair was able to dramatically reduce the number of components in the ZX81 compared to the ZX80 — a working ZX81 required only the ULA, a ROM, a Zilog Z80 or second-source NEC compatible, and a 1kB RAM chip — but the process was arguably taken a step too far, with the ULA replacing all components bar the CPU, RAM, and ROM. The ULA was designed to run at somewhere around 50% gate utilization, but in the ZX81 it was pushed much further. The result: a chip that ran hot, and the leading cause of dead ZX81s today.

"[Sinclair's chief engineer Jim] Westwood took what was basically a modification of the ZX80 and turned this into a Ferranti ULA 2000 series logic design, which he wire-wrapped into a prototype but had little success in getting it to work," Chris Smith writes in his book The ZX Spectrum ULA, an exhaustive deep-dive into the ZX81's successor. "In frustration, Westwood was forced to leave it in the hands of the new recruit, Richard Altwasser, while he took a week away from the office on business. Altwasser says he doesn't know who was more surprised when Westwood returned and unexpectedly found him in possession of a working prototype."

You can't fault Sinclair for trying to hit a price point, despite urging his engineers for an upgraded machine that no longer blanked the screen to read keyboard input: after launching the ZX80 as the first fully-functional microcomputer for under £100 (around $133 at current exchange rates, around $525 corrected for inflation), the ZX81 hit a new low at just £69.95 pre-assembled or £49.95 as a do-it-yourself through-hole soldering kit (around $93 and $66 respectively, or $333/$236 corrected for inflation.)

Those who bought it, however, would soon find themselves spending more. While the machine came with a power supply, you needed a cassette deck to load and save your programs — and the 1kB of memory, supplied as either a single 1kB chip or two 512-byte chips depending on what Sinclair could get the best price on, was cramped indeed. Some, like David Horne, would take it as a challenge: Horne's 1K ZX Chess requires only 672 bytes of RAM, and was the smallest game of computer chess around until Olivier Poudade crammed a chess engine into a 512-byte boot sector in 2015. Others took advantage of first- and third-party RAM expansion cartridges, which typically needed careful handling not to reset the machine partway through programming thanks to a fickle edge connector.

While anyone who's typed anything of length into the ZX81's flat, feedback-free keyboard will likely remember the process as more painful than pleasant, the ZX81 retains its fan base today. A number of projects have arisen to restore classic machines by recreating end-of-life parts like the Ferranti ULA, while others expand its capabilities with composite video output, joystick support, solid-state storage, and even high-resolution graphics capabilities. Even today, indie developers write and release games for the machine — though most, sensibly, require at least a 16kB RAM expansion.

With 45 years under its belt yet software development still ongoing, it's clear that Sinclair's ZX81 struck a chord with many a budget-conscious home computing enthusiast — to say nothing of its voluminous clones, released worldwide following reverse-engineering of the ULA.

Here's to another 45, ZX81.