Sinclair ZX80 / ZX81 / Z88 Forums

I've been breadboarding a project to try and replace the Timex 1000 internal ROM with an external EPROM (by way of the edge connector). Has anyone been able to successfully accomplish this? I'm just using a 27c128 and driving the /OE and /CE with A14 (LOW) AND /MREQ (LOW). /ROMCS is also pulled high to disable the internal ROM.

With this configuration, code is executing from the EPROM as it displays a prompt and responds to keypresses. However, it doesn't seem to be displaying any characters correctly. I tried factoring in the /RFSH (LOW) signal as well to no avail. Tried with a TreeForth ROM and exhibited the same behavior.

The 27c128 also works correctly if I plug it directly into the socket on the board (using a 2364 adapter).

This is the result of running PRINT "HELLO". This is the startup prompt.

Due to the way that a standard ZX81/TS1000 generates the display, it’s normally only possible for the display system to get the pixel data from an internal ROM (EEPROM/EPROM).

If the internal ROM cannot be accessed, the display system can’t get the required pixel data from the character set data.

Mark

I believe that's the way ZXpand works, uses it's own code to run the show as it were but uses the character table from the internal ROM for display.

Not sure if this website can help: https://ws-nbg.de/zx81/main.php

Seems to offer schemas for an external EPROM to run (e.g. the v3.0 seems very simple circuit!)

It takes a lot of effort to display characters from external ROM.

If characters in internal and external ROM are the same and at the same locations you can enable internal ROM in refresh cycle and external ROM in all other states.

Might be, this is doing what you want:
https://ws-nbg.de/zx81/main.php