The ZX Spectrum Reverse Engineering and Clone Desgin Blog

The Sinclair ZX Spectrum ULA Book

A site dedicated to the reverse engineering of the ZX Spectrum and related projects.

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ULA Schematic Back-Annotation

Jun 22, 2009

A huge amount of work has been carried out on the back-annotation of the ULA over the last few months, and the schematics are huge. I've split the ULA die into quarters and have translated each into its component transistors and resistors, which was both tricky and time consuming, taking between 12 and 16 hours per quarter. Each of the four schematics measure 85 x 78 cm, which, with the interface circuits, gives a total schematic size of 210 x 196 cm. All of the work so far has been on paper due to the size of the schematic, as I find it easier to work from a physical drawing.

The second phase has been to convert each transistor/resistor based schematic into an equivalent logic gate schematic. This is easiest done from the transistor schematic and not directly from the ULA die image, because you can read the transistor schematic and begin to understand how it is connected. You cannot do that from the die image.

I now have two schematics, one at the transistor level and one at the logic gate level, both retaining the layout of the ULA. I am about to start phase 3, which is to redraw the logic schematic at the functional level, with flip-flops and latches represented as single entities instead of as their component NOR gates. This schematic will be slightly rearranged for clarity, and will be much smaller than the other two.

Below are a number of photographs showing the work in progress. The first shows a tiny section of the ULA image. As I translate each detail I colour it in red, so I know where I've been. The second photo shows a closeup of the transistor level schematic. The third and forth photos show my drawing board, illustrating the size of a quarter schematic.

I pretty much understand how the ULA works now, so the functional schematic will be straightforward. It will then be a matter of fleshing out the appropriate chapters of the book.

Book Example Pages

I've put together a selection of six pages from the book so far, downloadable from here. It is just a taster, as the real detail has yet to be compiled into a presentable form.