Flush from my success at repairing the Spectrum and with a fresh understanding of the Spectrum’s architecture, I started research on the Spectrum ULA. The ULA, or Uncommitted Logic Array, was a chip technology developed by Ferranti that permitted manufacturers to more easily make relatively large scale combinational logic chips. Whilst the principle is essentially the same as a CPLD, the design process was more complicated, requiring Ferranti to overlay an interconnect layer during the final stage of chip fabrication.
Ferranti ULAs can be found in a number of 80s computers including the BBC Micro and Acorn Electron. Sinclair were early adopters of this technology and used the ULA to incorporate the logic for the Spectrum I/O (keyboard, sound, cassette interface and video) along with the CPU clock signal and NMI interrupt generation.
Whilst researching this I discovered this great book “The Spectrum ULA: How to design a microcomputer” by Chris Smith, which was the inspiration for the Spectrum Harlequin clones. This book describes the ULA in much more detail, reverse engineering it and illustrating how its function can be replicated using off-the-shelf logic chips. This book was the inspiration for the Harlequin clone.
I bought my Harlequin kit from Ben Versteeg at Byte Delight in February 2020 for £74.99 plus £6 shipping. I was initially tempted to buy the motherboard PCB only and source the components myself, but this worked out slightly cheaper. The kit included all components with the exception of the ROM and case.
Incidentally, I discovered that I’d already spoken to Ben in the past during my Spectrum 48K repair – I’d purchased some RAM chips from him before he’d set up Byte Delight. Small world.
The Harlequin PCB is currently installed in a Spectrum+ case.