32k 62256 mem mod help

[Mike3]

Hello .. I have begun to upgrade my ZX81 to 32k and have run into a problem.. I did the 16k mod first and when I run MEM-TEST I only get 4K , , I have checked the wiring and it seems fine,, I don't want to move on to the 32k untill I solve this,, has anyone else had this problem? .. any Ideas?


Thanks Mike

[Mike3]

I had the diodes mixed up I thought they were 1-8 across left to right. after getting a diagram I found that they were 4-5-6 = 1-3-5 ,, but after rewiring I now only get 2K???? .. now I am really stumped.. could I have somehow damaged the chip by wiring it wrong? .. Help!

Regards,, Mike

[Mike3]

actually the diodes are 1-8 left to right,, after tracing out D1-D3-D5 to the A11-A12-A13 on the cpu.. the Schematic dosn't show them this way .. confusing for someone doing this 1st time,,, after rewiring and finishing to 32K,, when I run MEM-TEST ,, I only get 16K? ... do I need some other prg. to test? .. am I still doing something wrong???

Regards,, Mike

[nama]

Well, at least it's you got more memory this time. I was half expecting you to say that you upgraded to 32k, but it now only shows 1k!!!
Anyway, I can't help on the technical side, but I quickly tester this memory test prog in an emulator set to 32k, and it reports the correct amount:

http://www.emuasylum.com/index.php?page ... t%20(19xx)(-)

I'm not sure if this is the same program you are using, if it is then it still looks like you have a problem somewhere.

P

[siggi]

Hi Mike,
you could use the ram-test routine of the rom itself: when you switch on the Zeddy the rom tests the ram from 16K to 32K and sets RAMTOP above the last ram adress (normally 32K). But it does not test above 32K, so even if you have 32K ram installed, only 16K is tested after power-on.

It you set RAMTOP to a higher value (e. g. POKE 16389,255) and then enter "NEW", then the rom tests the ram up to the new RAMTOP and sets RAMTOP again above the last found ram address. So if PEEK 16389 then returns 128, the rom found ram form 16K to 32K. If it returns 192, the rom found ram from 16K to 48K.

HTH Siggi

[Mike3]

I tried both Nama's program. and Siggi's Poke/peek and I still have 16K? I only had one 74LS00 ,, and maybe it is bad.. need to order some more,, as I have checked and rechecked my wiring and it is now correct..

I know NAMA I was really scratching my head when I kept losing memory but now I understand what was happening,,

Thanks for your replies...

Mike

[siggi]

Perhaps you should post the schema (or a link to it) of the memory mod you made. Depending on the address decoder the 32K chip can be mapped as one big block (16K to 48K) or 2 schmaller blocks (16K to 32K and 48K to 64K) into the z80 address space. The rom memory check routine can handle only the 1-block variant of memory.

Siggi

[XavSnap]

HI,

I had to read all suggestions ... but...
http://zx81.ordi5.free.fr/brico/memoire/64K.gif

Only 16k will be read by the Ram/CS without the little decoder placed before the ROM (ROM/CS)!

[Mike3]

Thanks here is the link...

http://www.zx81.de/english/32k-rame.htm

I am temperaraly out of 74ls00 can I use another type.. as I am thinking also that is the problem,,

Regards,, Mike

[XavSnap]

It's Peter Liebert-Adelt's 32k tweek project:
But, my link is dead...
HTTP://home.t-online.de/home/p.liebert/32k-rame.htm
Yours is the same page !!!


The Ram circuitery for 32k by Peter Liebert-Adelt seem to use :
4000h-7FFFh (16k ram pack storage)
and C000h to FFFFh !
Both segment are actived by the RAM/CS.
Without Rom/Cs tweek, You had to move the secound 16k bloc to a highier address bloc... (the 16k ram pack, mirrored at the top of the memory!)

The ROM/Cs tweem, in my shem, is used to clean all Rom/Cs signals mirrored at on the secound 32k memory bloc.

Note:
With the Rom/Cs tweek, you can remove the 74LS00 and wirre the A14 to ... A14.
And the 32k bloc will be at 4000h to BFFFh ! (16k rom +32k ram)
We only keep 8k at the start of memory to use the ROM.
We had to remove the R28 pin, only the ULA side one.
We had to get the ULA Rom/Cs, clean all signal above 8k and throw this signal to R28 (ROM SIDE)...
The rom won't be mirrored on the upper 32k bloc memory, and the boot test will scan the memory up to 7FFFh, assigned to Ram now...

Use a 74LS32:
New Rom/Cs = (A13 or A15) or (old ROM/CS)

The Zx81 rom can't scan up to 7FFFh... at 8000h, it's a rom signal !
You 16k memory is located at C000h to FFFFh... try to Poke or Peek on this bloc ! it work !
Have Fun !