Thanks! (pity it couldn't just be done with a single POKE, though.. ) Another little query..... any easy way of deactivating the zxpand RAM and both the zx ROM and zxpand ROM , so it could be fitted 'permanently' in my Fuller keyboard case, and any other external devices could deactivate the RAM and/or ROM as if it was a 'normal' ZX81???
I don't think it's easily possible to control the ROM overlay externally, that said I don't know of any other devices which disable the internal ROM. I'm sure people will enlighten me though
Ditto the RAM. It would entail listening to and obeying RAMCS - which would be tough as it also needs to control it. As ROM and RAM cs lines are designed to let external devices control the internal chips it's not really possible to have multiple external devices all mastering the same line. If you see what I mean.
The way I was thinking of doing it was adding a resistor of about 680ohm in series with the CS lines of the RAM and ROM chips of the zxpand, and feeding those points out to an 'external' expansion connector through diodes(cathode to chip CS), so if pulled to +5v would de-activate the ZXpand chips( but would it confuse the ZXpand processor? ), that's how it appears to be done in the ZX80 or ZX81 itself... as for deactivating the main ZX ROM (and RAM) as well, maybe the same way, fit a resistor in series with the CS line from the ZXpand and diode from this to the external ROM CS..??