32K RAM Pack - Oddity?

[RWAP]

Louis Seidelmann has found an oddity with his 32K RAM pack (or at least it looks odd). I am having to use his direct English here unfortunately.... (he is from the Czech Repblic)

His 32k pack has RAM mapped in the address range 0x4000 - 0xFFFF. The range 0xC000 - 0xFFFF is a mirror of the first 16K RAM (0x4000 - 0x7FFF with display content out in M1 cycle giving the same as the standard 16k pack).

In the range 0x8000 - 0xBFFF is ROM selected out and replaced by other 16k RAM in read/write cycle (not in M1 cycle in my design).

The datasheet for 32k pack by Pokemon looks similar but he has not spotted a similar issue being mentioned with this.

OK, so what is the error...

You may need to enter POKE 16389,128: NEW, but that does not seem imperative.

Code: Select all

example 1:
10 POKE 32768,0
40 PRINT PEEK 32768

returns 0
 
example 2: 
10 POKE 32768,0
20 FOR A=1 to 1
30 NEXT A
40 PRINT PEEK 32768

returns 211
 
example 3:
enter the direct commands:
POKE 32768,0
PRINT PEEK 32768

returns 211
 
example 4:
enter the direct commands:
POKE 32773,0
PRINT PEEK 32773

returns 0

Any ideas?

[PokeMon]

The datasheet for 32k pack by Pokemon looks similar but he has not spotted a similar issue being mentioned with this.

Is it my ZX81XRAM or somewhat else ?

[RWAP]

From how he talks it is something else - did you sell one to Louis?

[RWAP]

I have checked - it is a 32K RAM pack he is trying to build himself

[PokeMon]

Well he could find schematics about it here:
viewtopic.php?f=6&t=1129&p=12157&hilit=ZX81XRAM#p11789

I use a diode and a resistor controlled with A15 to disable ROM.
The resistor is not needed in all cases but nicer than overdriving the ULA too hard.
So I pull /ROMCS not to +5V - more to +2V or 2.5V which is calculated with the onboard and the ZX81XRAM resistor.
I use a gate to drive this output (HCT would give enough power).
I also use some more signals/situations to switch off the internal ROM but for a traditional memory extension A15 would be sufficient.

[RWAP]

Having spoken some more with Louis, he is puzzled over this issue - Pokemon - could you try the same programming examples with your ZX81RAM interface to see what the results should be !!

[PokeMon]

You may need to enter POKE 16389,128: NEW, but that does not seem imperative.

Any ideas?

First he should POKE 16389,192 and then NEW.
After this, PEEK 16389 should give 192 again.

Maybe the address decoding is not correct for RAM or the "lift" off ROMCS when A15 high doesn't work.
He could provide schematics to proof.
And keep in mind, that RAM has to be enabled when either A14 is high OR A15 is high.

Is it for a ZX81 or ZX80 ? The ZX80 has no ROMCS on the edge connector by the way - if it is a ZX80CORE ROMCS can be activated for the edge connector with a jumper.

[RWAP]

Thanks Pokemon - Louis was looking at this thread and has now found the problem with his hardware and fixed it

It is not easy when English or German is not your first language - at least I could interpret for him!