Nearly half a century ago, Clive Sinclair’s Sinclair Research made history. It released the ZX81, one of the key home computers of the 1980s, as the first low-cost micro available to High Street shoppers. And you can express your love of early 80s tech with my latest retro-wear: the ZX81 keyboard shirt.

Work began on the ZX81 in September 1979 before its predecessor, the ZX80, had even gone on sale. Designer Jim Westwood’s goal was to cost-engineer the ZX80’s dozens of chips down into as few as possible. He succeeded, and that’s why Sinclair was able to release the ZX81 at just £69.95 for a pre-built unit or as little as £49.95 if you bought it as a kit.

According to the Bank of England, that’s equivalent to £275 in today’s money for assembled unit.

Westwood’s trick was to combine the ZX80’s many separated integrated circuits into a single chip: an Uncommitted Logic Array (ULA), a custom part that was pricey but still less that the cost of the separate parts. Eighteen of the ZX80’s 22 ICs were packed into one in the ZX81.

Westwood applied a second canny trick: he took advantage of the host Z80A CPU’s interrupt facility to trigger the display processing and keyboard scan routines, eliminating the irritating screen flicker seen on the ZX80.

“There was a big failing in the ZX80,” said Brian Flint, one of the hardware engineers working under Westwood. “When you hit a key the microprocessor stopped generating the signal to the TV set and went off and got the message from the key and did some calculations, so suddenly you would have no signal going out and you’d get a flicker on the screen. That meant that there was a flicker every time you touched something, but you sort of got used to it and almost felt it was natural after a while.”

Developing the ZX81

The lower chip cost allowed Sinclair to increase the on-board ROM capacity to 8MB, allowing the developer of the ZX80’s Basic interpreter, Nine Tiles, to add floating point maths and trigonometry routines to the new machine.

“As far as Clive was concerned, it wasn’t a question of what the machine ought to be able to do, but more what could be crammed into the machine given the component budget he’d set his mind on. The only firm brief for the 81 was that the 80’s maths package must be improved,” said Nine Tiles coder Steve Vickers — who would go on to co-design the ZX Spectrum and the Jupiter Ace — in 1985.

Vickers worked to expand the Basic keyword list, while Nine Tiles owner and chief programmer John Grant handled the links to hardware. Grant wrote ZX81 Basic’s immediate syntax checking. Vickers also penned the ZX81 manual.

During 1980, Nine Tiles was required to make a major change: the addition of printing code to support the new thermal printer that Sinclair was by now planning to release alongside the ZX81. Nine Tiles didn’t get its hands on a printer to test the code until October 1980, close to the point it was expected to deliver final code. Printing support required major ROM revisions and, in the process, the square root code was botched. The error would be found early in the new year, but ZX81s with updated ROMs wouldn’t start shipping until August. Not that too many early buyers noticed: Sinclair claimed that by September on 300 users had written in to request fresh ROM chips.

During the Winter of 1980/81, Nine Tiles completed the Basic and the manual. Meanwhile, Sinclair’s in-house industrial designer Rick Dickinson was working on the new computer’s styling. In contrast to the ZX80’s vacuum-formed plastic casing, Dickinson, with Sinclair’s support, pursued a metal casing for the ZX81 — decades before Apple’s metal MacBook Pros. But with inflation raging, it proved too expensive, and so it was the familiar injection-moulded casing, along with its low-cost membrane keyboard — a feature of Sinclair’s previous micros, the MK14 and ZX80 — that made the cut.

Launch

When the ZX81 was launched, in March 1981, Sinclair was churning out around 10,000 ZX80 kits a month, by now a year old. Production of the ZX81 commenced in April 1981, and quickly jumped to that total on the back of early, post-launch publicity. The expectation was that this number would quickly double.

By June, Sinclair was claiming to be punching out and shipping 18,000-20,000 ZX81s a month to the US alone, exceeding the combined sales of Tandy, Apple and Commodore.

In September, newsagent WHSmith took delivery of the first ZX81s that its Market Development Manager, John Rowlands, had agreed to stock, earlier in the year. With buyers able to walk into a familiar store and buy a home computer for what was then considered disposable — not to mention demo machines for a generation of kids to begin punching programs into — the micro’s success was assured.

Whether you owned a ZX81, were one of the kids who dreamed of getting one for Christmas, or weren’t even born then but have discovered the ZX81 more recently, then you can wear a ZX81 Keyboard T-shirt with pride. Order yours here.

You can view all my retro tech T-shirts here.