Sinclair ZX80 / ZX81 Forums

[siggi]

You 16k memory is located at C000h to FFFFh... try to Poke or Peek on this bloc ! it work !

Indeed: as written on Peter's page: "Step 2: full 32K-RAM from 16K to 32K and 48K to 64K".
The rom detects only the first ram block from 16 to 32K. The other block thus cannot be used by a BASIC program, but can hold data poked there ...

Siggi

[Mike3]

"Step 2: full 32K-RAM from 16K to 32K and 48K to 64K".

thanks, you know I read that ,, but didn't really pay attention...
XavSnap... and I will do the circuit you posted,, try again

Thanks again I am kinda new at this you can probably tell ,, still learning

Regards,, Mike

[XavSnap]

hi,

The 64kb decoder !

[siggi]

Wilf uses a HC251 as ram address decoder, which also generates a /ROMCS to disable the rom. See the schema (upper part) in

http://www.user.dccnet.com/wrigter/index_files/ZXRAMIO.htm

Siggi

[siggi]

Here is the memory part only:

32K%20newdecoder.gif (8.15 KiB) Viewed 6283 times

And for 64K:

64KINTRAM.gif (9.52 KiB) Viewed 6299 times

Siggi

[Mike3]

Fantastic,,, you guys are great!... I knew the Guru's here would come to the resue....one thing tho I only have access to 74LS251's without ordering .. I see they are pin compatable.. would these work OK in the circuit?

Regards,,,
Mike

[siggi]

Yes. Wilf uses HC or HCT chips to save some power.

Siggi

[iturbez]

siggi:

I want to update to 32K mem with an 74hc251, but I can´t find M1 signal (pin 12 of 74hc251). Is this MI signal (pin 27 of Z80)?

Thanks in advance.

[Mike3]

thats correct.. pin 10 on the ULA and pin 27 on the Z80.. i DID THE Same 32K using the 74HC251.. remember to download the ZX81 schematic if you havn't already and make sure you bend up all the other pins to connect the address lines..(the ones not already connected via the socket) in the diagram this isn't that apparent...if you run into trouble let me know.. I just did the mod...

Regards.. Mike

[iturbez]

Thanks, Mike.

I will let you know how this mod is made as soon as I go on with it.