External board for SPonZY (and other ZX81 ROMs)

[kolbeck]

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.

zx81_proto.png (259.95 KiB) Viewed 3955 times

Based on this I've created a small PCB

[mrtinb]

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.)

[larry2zx81]

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.

[larry2zx81]

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

[1024MAK]

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