The Ferranti files: 40 years of Sinclair secrets unearthed

TL;DR: The internal logic diagrams for Ferranti ULA chips, missing for more than 40 years, have now been made available to the community after an eBay buyer acquired them for £110 and passed them to the video’s creator, who negotiated IP permissions with Sky’s lawyers and painstakingly digitized them by hand. ## A Question Left Unanswered for 40 Years Sinclair’s ZX81, ZX Spectrum 48K, and 128K Spectrum Plus all used Ferranti ULA chips as their central peripheral devices. Through reverse engineering, the community has learned how these chips function and even produced replacements. But what exactly was happening inside them? The circuit diagrams describing their operating logic had never been found—at least as far as the creator knew. That remained the case for about 40 years. This story begins roughly 45 years ago, so we need to start at the beginning. ## From the ZX80 to the ULA: The Birth of a Custom Chip In 1980, Science of Cambridge released the Sinclair ZX80, the first product in a legendary computer series, hailed as unquestionably excellent value for money. All those cost savings came with some shortcomings, particularly the extremely annoying screen flicker every time a key was pressed. Inside this white machine were 21 chips, or 22 including the voltage regulator, all off-the-shelf commercial components. There were no custom chips—apart from the ROM containing Sinclair’s operating system. Next came the ZX81, with a completely different appearance: a black case and red lettering. The real point of interest was the circuitry inside—the 21 or 22 chips had been reduced to four or five, with all the magic concentrated in a single chip: the Ferranti ULA. All the complex logic in the ZX80 had been improved and incorporated into this one integrated circuit package. ## What Is a Ferranti ULA? Ferranti was a company dating back to the 19th century, with products spanning everything from defense to home computing. The product examined here is the Ferranti ULA, or “Uncommitted Logic Array.” There was an entire range of them, or even several ranges. Take a photograph of a 5C052 as an example. It does not come from any Sinclair computer, but it illustrates the operating principle well: a small piece of silicon is packed with multiple “cells.” Around the edges, near the bonding pads, are peripheral cells, while toward the center of the integrated circuit is a repeating grid of cells. Each cell can contain a certain amount of logic, subject to particular constraints. Ferranti’s customers were free to use them however they wished, and some cells might not be fully utilized—for example, the green cells in the upper-right corner of the photograph. The creator suspects customers did not necessarily have to specify everything at this level themselves, but more likely worked with Ferranti to complete the design. Ferranti supplied design manuals. The creator has only a 5R manual and has never seen manuals for the 5C, 6C, or 2C—if anyone has one, please get in touch. These manuals explained all the resources available to customers and how to design devices using ULA technology. One easily overlooked detail: even the ZX Printer, introduced during the ZX81’s lifetime, contains a Ferranti ULA, the 1026. Opening the cover reveals that it is the device’s only chip. ## The Evolution of the ULA Family Across Sinclair’s Product Line ### The ZX Spectrum and the “Dead Cockroach Modification” The Spectrum was a huge improvement over the ZX81, offering color, sound, and up to 48 kilobytes of memory. The ZX81 had just 1 kilobyte unless you owned a 16K RAM expansion pack. Inside the machine, eight memory chips sit in the lower-left corner, alongside the CPU, ROM, ULA, and some addressing logic, with a green daughterboard at the top providing an additional 32 kilobytes of memory. The earliest 5C102 ULA had a problem that required a factory modification, the famous “dead cockroach modification”—the modification board can be seen upside down beside the CPU. Even when the Issue 2 version went on sale, the manufacturer was still using up its stock of 5C102 ULAs, so dead cockroaches can still be found in some Issue 2 machines. In brief, the 48K Spectrum eventually used a 6C-series ULA. The Mark 7 was the last version produced and, in the creator’s opinion, the best. His collection includes a Mark 7 ULA with its top removed, and under a microscope you can almost make out the array structure inside. The NTSC Spectrum also deserves a mention. One of the components that had to be replaced when switching from PAL to NTSC was the ULA, because video generation had to conform to the NTSC standard. The creator has a machine fitted with a 6C011E ULA, distinguished by its metal-lined case, which relates to FCC standards, as well as a yellow NTSC Spectrum containing a 6C011E3 ULA. There were probably further revisions of the NTSC ULA. Ferranti also made ULAs for Interface 1 and the Micro Drive. The Sinclair flat-screen television—the miniature portable TV demonstrated by Sir Clive—also contained a Ferranti ULA. ### The 128K Era and the End of the Story Following the main product line brings us to the 128K Spectrum Plus “toastrack,” a technological leap: it moved from the 6C-series ULA to the 7K series, with more pins and greater capabilities, handling the machine’s 128 kilobytes of paged memory and providing RGB output. At this point, the story of Ferranti’s collaboration with Sinclair was drawing to a close—around 1986, the year the 128K “toastrack” was released. Amstrad acquired Sinclair that year, and shortly afterward Ferranti was also acquired and became part of Pi. The gray 128K Spectrum that appeared later could use the same ULA as the 128K “toastrack.” ## How These Documents Appeared on eBay Earlier this year, a channel follower named Pabs sent the creator an intriguing message: he had some Sinclair-related Ferranti documents that he had acquired on eBay and was willing to lend them to the creator. The eBay listing Pabs found was for “a collection of rare original ZX Sinclair test diagnostic sheets,” with a Buy It Now price of £10,000. He made a very low offer and ultimately secured them for £110. A few months later, the creator went to London to see The Offspring in concert and made a detour to Pabs’s home to collect a very heavy box packed with documents. The folders in the box each covered a different subject, including: - Spectrum Micro Drive - 2C210 ZX81 (about the ULA) - 6C001 circuits (48K ULA) - 128K Spectrum reports and memos - Spectrum hardware - 7K010 drawings (128K ULA) - Spectrum ULA drawings - Chemston DFS for Spectrum - An intriguing unnamed folder - Spectrum ULA 5C112 (48K ULA) - ZX81 software - ZX81 memory ULA 6C functional test box - ZN41 text test box (TV ULA) - ZN410 datasheet and test specifications (a chip used in some kind of drilling rig) All these documents came from Ferranti’s integrated circuit applications manager, who was clearly deeply involved in the Sinclair projects. The creator does not know whether this person preserved and stored the documents or was the one who eventually sold them. The contents went far beyond the test documentation suggested by the listing: ULA schematics and logic diagrams, many drawn by hand; hand-drawn Spectrum schematics; documentation for prototypes from before the Spectrum’s release; photographs of oscilloscope waveforms taken during testing; reports; internal memos; correspondence between Sinclair, Timex, and Ferranti; code; and designs for ULA test boxes. ## Intellectual Property: Negotiating with Sky’s Lawyers The first hurdle was intellectual property. The documents were full of personal information, and the IP belonged neither to Sinclair nor to Amstrad, but to Sky—because Sky had acquired Amstrad. The creator carried out extensive cataloging: describing every document, noting the personal information it contained, and assigning a sensible naming convention. He then sent the catalog to Sky. Sky’s lawyers were very helpful and understood the documents’ historical significance. They replied to confirm: “Yes, we own the IP in these documents, and you are free to share them, provided you have carefully redacted all personal information and have confirmed that they contain no other IP we need to be aware of.” The conditions for sharing were that the documents were for educational and historical interest only, for noncommercial use, and could not be used for profit or to manufacture products for sale. They could only be shared through the channel. They could be used for personal projects, learning, and exploring history, but not commercially. Every document has a watermark, and a disclaimer on the catalog’s first page explains all of this. ## Digitization: The Laborious Way, Without AI Digitization took an extremely long time. Scanning everything would have required an enormous flatbed scanner, and some documents were A0 size—the creator had not even known A0 existed. These drawings were absolutely huge. Sending them to a professional scanning company was also prohibitively expensive. The eventual solution was to use a good macro lens with a flat field of focus, position the camera high above the documents, and photograph one A4-sized area at a time. Documents larger than A0 were stitched together manually in Photoshop. The creator explicitly states that he used no AI—he did not want AI hallucinations or errors. Once stitched together, the images were converted to PDFs, personal information was redacted, an “Educational and Historical Interest Only” watermark was added, and the files were organized into an online catalog. The JPEG files totaled several gigabytes, while the PDFs were a more manageable size and could be hosted on Google Drive. A spreadsheet was also created listing links to all the documents. ## Further Research to Look Forward To After receiving a box full of ULA logic diagrams, the creator contacted Chris Smith, author of *The ZX Spectrum ULA*. They spent an hour talking and looking through the documents. The creator plans to make in-depth analysis videos in the future. Some interesting highlights identified so far include: - **Hand-drawn diagrams explaining how the Micro Drive ULA works**—the creator admits he does not yet fully understand them, but they will feature in future in-depth videos. - **Drawings for the ZX81’s 2C184 ULA**—this is the version from before the “back porch fix.” There are also drawings for the 2C210 ULA, and comparing the two should make for a very interesting video. Looking closely, one chain of blocks is likely a pixel counter. The creator says he is unsure, but promises to investigate it thoroughly someday. - **Logic diagrams for the ZX Spectrum Mark 7 ULA**—these are clearly much more complex than the ZX81 diagrams, with many blocks whose internal logic is described in separate drawings, also found in the box. Zooming in reveals how the additional delays were implemented. All the documents are available through the links in the description. Feel free to explore them yourself. Source: The Ferranti files: 40 years of Sinclair secrets unearthed (https://youtu.be/jiW3P1rJRMc)