Using an external EPROM

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benthepoet
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Joined: Tue Aug 02, 2022 10:50 pm

Using an external EPROM

Post by benthepoet »

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".
IMG_20230101_120645.jpg
This is the startup prompt.
IMG_20230101_120706.jpg
Last edited by benthepoet on Sun Jan 01, 2023 6:45 pm, edited 1 time in total.
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1024MAK
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Re: Using an external EPROM

Post by 1024MAK »

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.

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Moggy
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Re: Using an external EPROM

Post by Moggy »

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.
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marste
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Re: Using an external EPROM

Post by marste »

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!)
Mustermann
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Re: Using an external EPROM

Post by Mustermann »

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.
ZX81 issue 1 near to original state
ZX81 issue 1 56k internal ram with battery backup, UDG, CHR$128 and WRX enabled
Minstrel 3 ZX81 clone 64k with battery backup, UDG, CHR$128 and WRX enabled
Mustermann
Posts: 43
Joined: Sun Dec 22, 2019 12:09 pm
Location: Augsburg, Germany

Re: Using an external EPROM

Post by Mustermann »

Might be, this is doing what you want:
https://ws-nbg.de/zx81/main.php
ZX81 issue 1 near to original state
ZX81 issue 1 56k internal ram with battery backup, UDG, CHR$128 and WRX enabled
Minstrel 3 ZX81 clone 64k with battery backup, UDG, CHR$128 and WRX enabled
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