Board design: Jim Westwood
ULA: Brian Flint
Software: Steve Vickers
Case design: Rick Dickinson

Dimensions: 167 × 174 × 40 mm (feet adds 2 mm)
Weight: 340 g

This successor to the ZX80 reduced the chip count from 21 to only 4, by incorporating the Ferranti ULA, a ‘glue chip’ and precursor of the modern day Gate Array IC.

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In January 1980, John Grant hired mathematician Steve Vickers to rewrite the ZX80 ROM for the ZX81, based on Grants earlier work. The commands DATA, READ and RESTORE were scrapped in favour of the ZX Printer commands LPRINT, LLIST and COPY.

“ZX80 8K BASIC” prototype keyboard overlay with READ, RESTORE and DATA

Users could save themselves £20 / $50 by building it from a kit.

ULA types 2C158E and 2C184E didn’t have a ‘back porch’ giving a dark, blurry and noisy image, whereas ULA type 2C210E did have a ‘back porch’, which improved compatibility with most TV’s. It is explained in this video.

There are 3 ROM types:

550 — Initial ROM has the ‘square-root bug’: SQR 0.25 would return 1.3591409 instead of 0.5

622 — Fixed the math bug, added an extra CALL to clear the workspace in the INPUT routine and improved the syntax checking routine by testing bit 6 and 7 of system variable FLAGS.
It also introduced a new bug, explained in chapter 19 of the ZX81 programming manual:

(memory address 16437 is system variable FRAMES_hi)

649 — Final ROM which fixed the PAUSE statement bug