Sinclair ZX80 / ZX81 / Z88 Forums

Hi all, apologies for reviving such an old thread!

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?

Is it possible? Yes.

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.

Mark

It seems the Minstrel ZX80/ZX81 clone can be modified to work as Lambda 8300 hardware.

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.

My Lambda 8300 chipset:

CPU: NEC Japan D780C-1 8338X5
ULA: C4005 2783
ROM: NEC Japan D2764D (modified which include some modifications to the board)

My guess is that it’s just the “original” ROM that is replaced by the “upgraded” ROM. Since the PROM has a different pinout compared to an EPROM, some wires makes it work anyway.

I don’t know the hardware so well, to know if my guess is right.

However having two different ROMs made it more difficult to program machine code for the machine. Therefore some software would only work with either version of the ROM.

Well, a mask ROM needs less package pins compared to an EPROM of the same memory capacity. Also although most of the signal pins are the same, not all of them are. Hence why the modifications were needed if you replace the mask ROM chip (with 24 pins) with a EPROM (that has 28 pins)…

Mark

Hi Martin and Mark.

Thanks for your comments.
I thought the modified ROM and board modifications was applied by a previous owner, but as i read your comments, modifications is done by the manufactorer?
To me, the handwritten yellow label with the year 1985, seems pretty DIY style.

Anyway, my intention is to bring the unit alive again.
Until now all DC supplies to the IC’s is checked and the crystal is generating a pulse to the CPU. But that where the dynamics ends - no beep when started, and no output to monitor or TV.

I have been using this schematic during my search for errors:
download/file.php?id=1418&sid=5270d4b89 ... 55b1a3b451

Do you know if more detailed scematics are available?

I think the schematic you have is the best there is.

I might not have been clear in my answer.

Let's assume your Lambda has a serial number below 15031. Then it was shipped with "original" ROM. The previous owner could have made this modification if he had found a copy of the "updated" ROM. Then again Fred Nachbaur made a "ZX81 compatible" ROM, so this may also be what the previous owner have installed.

Ok

With a seral number 017150 on my Lambda, the “new” ROM must be manufactor-fitted.

Martin: Seems you have been into the details with this old machine.
When i have got me another Lambda, i will try swabbing IC’s to detect the faulty one. Everything else on the board seems okay.

Until now, thx for replies…