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Comment by tyingq | Hacker News Reader

Apparently the hardest part used to be the ULA chip, but now there are CPLD based replacements. This project used this one from Charlie Ingley: https://www.tindie.com/products/charlieingley/vla82-spectrum...

It uses a Xilinx XC95144 CPLD to emulate the ULA: https://docs.xilinx.com/v/u/en-US/ds067

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For anyone else who's unfamiliar, that's a "Complex programmable logic device"

https://en.wikipedia.org/wiki/Complex_programmable_logic_dev...

> A complex programmable logic device (CPLD) is a programmable logic device with complexity between that of PALs and FPGAs, and architectural features of both. The main building block of the CPLD is a macrocell, which contains logic implementing disjunctive normal form expressions and more specialized logic operations.

That’s pretty neat, the ULA is just a gate array after all.

[Edit] I forgot to mention I have speccy and ZX81 ULAs from Charlie and they are top notch and have been working flawlessly, and especially in the case of the ZX81, better than the original.

Chris Smiths’ book, “The ZX Spectrum ULA: How to design a microcomputer”, is an excellent read if you like to imagine yourself as a hardware guy back in the 80’s.

http://www.zxdesign.info/book/

There's also the superfo harlequin that disintegrates the ULA into 74 series logic:

https://www.bytedelight.com/?product_cat=harlequin128

I'm currently building the 48k version

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