I've got a sick 8300 here, it doesn't respond to keyboard inputs. After swapping chips with a friend's machine, I'm now 95% certain it's the ULA that's at fault (certainly, it works perfectly with a different ULA). I know the ULA is different to the ZX81's - and also not something I'm likely to find a replacement for. But since this thread started, there's been some great work done on ULA replacements, such as vLA 80. Is it possible the same techniques could be used to create a ULA replacement for the Lambda too?
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However, the Lambda 8300 ULA is significantly different to the ZX81 ULA. The ZX81 ULA does not contain the pixel data for the character set, as in the ZX81, this is contained in the ROM.
But in a Lambda, the ULA contains the pixel data for the character set. So a larger capacity modern logic array chip would be needed compared to a ZX81 ULA replacement.
There may well be other differences.
In order to recreate the logic and functionality of a chip, you have to know intimate details of it. Either by reverse engineering (with a chip, that means decapping it and using photography to see the circuitry on the silicon chip). Or by trying to work out how it works by carefully monitoring what it does for every possible combination of inputs.
We know a lot about the ZX81 and it’s ULA, because over the years many people have studied it. Often each person learning more about it. It’s also an evolution based on the ZX80 and we have the circuit schematic for that.
Less is known about the Lambda 8300 ULA. Although enough is known for emulator writers to include it in emulators (Eightyone for example). I don’t know if the available information is enough for someone to put the effort into putting the appropriate logic into a modern logic array chip.
There are also some other difficulties. The ZX81 uses a +5V supply. The Lambda 8300 ULA uses a much higher supply voltage (9V if I remember correctly). Even in a ZX81, a ULA replacement is actually a module (PCB containing the modern logic array chip plus support components). Because modern logic array chips don’t handle analogue type signals. The same would apply to a Lambda 8300 ULA replacement. So physical size and fit have to be worked out.
And finally, the availability of 5V compatible modern logic array chips has got worse, plus prices have gone up due to global supply issues.
So it would almost certainly be cheaper and quicker to buy another Lambda 8300 computer. If only for the ULA.
http://blog.tynemouthsoftware.co.uk/201 ... clone.html
So with this information I think it would be possible to create a ULA with built-in ROM. However as I'm a software developer, and not an electrical engineer, it's not a task for me.
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube