Sinclair ZX80 / ZX81 Forums

[siggi]

For /M1 the Z80 data sheet may help: http://tlienhard.com/zx81/z80um.pdf

And: your zeddy won't run with your last i/o card version, because the ROM is always disabled!
You must disable the ROM only during access to your i/o card. So you have to invert your enable signals for latch and input port (they are active low) to get the highl level needed to drive /ROMCS (via the diode) high!

Siggi

[XavSnap]

Hi Siggi,

True, the upper memory bloc is hard to read on FFFFh !
I will try to use a 8 bits decoder to set the address on the used VERSION basic variable.
And remove te /Romcs diode.

Many thanks !

[siggi]

I still recommend to use only addresses in the ROM address range 0-16K or 32-48K!
See Wilf's circuit how to decode an address at 9990:
http://www.user.dccnet.com/wrigter/index_files/HS3.htm

If you use the address 16393 (VERSN), every LOAD and SAVE will PEEK and POKE your card, because that byte is read to be saved on tape and is written back during LOAD. And if a non-zero value (coming from your i/o card during SAVE->PEEK) is saved as VERSN to tape, you will not be able to LOAD that program back!

Siggi