Internal RAM and then ZXpand or Chroma Interface?

[msknight]

Here's a question.

If I have a zeddy on which I perform an internal 16k mod, and then hook up a ZXpand+ or a Chroma Interface which have extra RAM on board... what happens?

[mrtinb]

You have conflict in the address space. I'm not an electronics expert, but I believe two different signal is comming to the bus at the same time, which makes it a coincident who is winning the signal competition. At best the Zeddy wont work. At worst you destoy one of the chips. Anyone please correct me if I'm wrong.

[msknight]

This is the interesting thing. I have read that inserting external RAM packs, disable the on-board RAM... but that this is not always the case. ... and I don't know for certain if that statement is true or not.

And that set me to wondering whether this is, actually, the case and if so or not, what would that mean if the Lk1 was broken and Lk2 made.

[Moggy]

I think Martin is right re the memory address clash and I'm sure it's been addressed on the forum before, just knowing where to look.
If in the end you are aiming for a spand then I don't see the point of internal RAM upgrades especially the 32k variety that some favour, who the hell wants to wait 12 mins to load a program from tape if indeed should such a program exists?

[Moggy]

Found this,not sure of how useful though.

https://www.sinclairzxworld.com/viewtop ... and#p17118

[msknight]

Part of the problem is knowing what I'm aiming for. I'm also watching the Chroma Interface for when it comes on sale again.

If I'm waiting for these, does that mean I've got to keep a zeddy virgin, ready for the day I get these packs, or can I upgrade it to 16k internally and carry on. That's the ultimate question.

If I read this at its face value - https://www.sinclairzxworld.com/viewtop ... 882#p19882 ...

However, it does work with internal RAM and with modified RAM Paks.
When using internal RAM, take care about these two limitiations: Only a small
picture will fit into 1K memory (so the display procedure must increase
horizontal and/or vertical blanking times), and external RAM must be
disconnected (as it'd otherwise disable internal RAM).

...then whatever I do to the on-board RAM shouldn't matter, because external RAM will disable the on-board RAM.

But would it only disable the 1k on board, or would it disable a whole 16K if I had added it in.

[msknight]

Found this,not sure of how useful though.
https://www.sinclairzxworld.com/viewtop ... and#p17118

Very useful! Good find!

[msknight]

Thanks Moggy, that forum post explains the mechanism.

As long as my internal expansion works by listening to the /RAMCS line, then when an external RAM pack is added, that external RAM pack will deliberately hold /RAMCS high... so the internal RAM will never activate... no matter whether it's 1k or 16k.

This also explains why I was reading about some earlier RAM packs which only had 15K on board, and still used the 1K system RAM. But I believe that it was only early RAM packs that did this. At least, from what I'm reading.

That explains the whole thing! Many thanks.

[1024MAK]

As long as my internal expansion works by listening to the /RAMCS line, then when an external RAM pack is added, that external RAM pack will deliberately hold /RAMCS high... so the internal RAM will never activate... no matter whether it's 1k or 16k.

Correct.

This also explains why I was reading about some earlier RAM packs which only had 15K on board, and still used the 1K system RAM. But I believe that it was only early RAM packs that did this. At least, from what I'm reading.

RAM packs which only had 15K on board? I'm not convinced. The size of the RAM expansions was VERY tightly aligned with the available RAM chips.

The early Sinclair RAM packs for the ZX80 or ZX81 used small 1K SRAM chips. Hence they could expand the existing RAM by 1K (total of 2K), 2K (total of 3K) or 3K (total of 4K).

The next step up for SRAM chips was a 2K chip. Then the next one was 8K SRAM chips. But these were quite expensive at the time compared to the equivalent DRAM.

If you wanted even more RAM, DRAM was cheaper, and this was available in a 16K size. The 8K DRAM chip being a 'half' working 16K...

The point being that a 15K RAM expansion simply was too awkward. Of course, what you may have read may be that of someone who was confused. Or it may have been an OCR error or a typo.

In any event, the Sinclair offering was a 16K RAM pack and the vast majority of third party RAM packs were also 16K.

Mark