Sinclair ZX80 / ZX81 / Z88 Forums

Based on my prototype I've created a small simple PCB in order to run SPonZY external on a ZX81. You can see the protoype in this post

The prototype chaos turned out to be working anyway

Interface supports W27C512 - a 64KiB EEPROM and can operate in two modes: 1) 8 banks of 8KiB each or 2) 4 banks of 16KiB as needed by SPonZY. The active "bank" is selected by using the DIP-switch. For now this is able to run the different ZX81 roms and the SPonZY without having to replace the internal ROM. Note that SPonZY requires 16KiB WRX compatible RAM - and the internal ROM is still required if you want to run other ZX81 roms, such as Shoulders Giants, in order for the display system to work properly.
Based on this I've created a small PCB

That look great.

I also designed an internal board to shift between ROMs on the Lambda 8300 (Original ROM, Color ROM and ZX81 compatible ROM). However I have never tested it, as I have more ideas than time to implement them.

(I need to get my ZX81 inside Minitel project finished, and I'm also working on a Pascal subset compiler.)

I build the external rom circuit using bread board, hand wired and finally had the external rom pcb made. zx81 wont boot, I had found early on that if I removed the romcs\ line from the circuit it would boot of course. I have four zx81/ts1000. No difference. Pcb version I removed all the chips, only switch, transistor and resistors remained. Same
Any help here?

I have internal 16k ram installed.
ZX-Wespi for video out.

I need to add I used the Thomas heckmann bd. github.com/thomasheckmann external rom

An external board that has a ROM (EPROM/EEPROM/E2PROM/flash) chip needs to drive the /ROMCS line on the expansion port/edge connector high (to +5V) to disable the internal ROM. But it should only do this when the processor is reading a ROM address 0 through 8191 (0x0000 through 0x1FFF) (although this could be up to 16383 (0x3FFF) if wanted.

The /ROMCS line should never be driven low.

The external board that has a ROM (EPROM/EEPROM/E2PROM/flash) chip must then respond to this address range itself.

Mark