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avatar_orzel

Which chip for internal rom replacement

Started by orzel, 13:08, 04 July 25

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orzel

Hello,

Say i want to change the ROM chips in a amstrad cpc6128. Either the main 32k one, or the smallest (16k?) used by AMSDOS. I don't mean ROM expansions here.

Is there any modern, compatible chip i could use like a eeprom ? Or can't I avoid using a one-time-only PROM or (even worse) a windowed EPROM ?

Rabs

Quote from: orzel on 13:08, 04 July 25Hello,

Say i want to change the ROM chips in a amstrad cpc6128. Either the main 32k one, or the smallest (16k?) used by AMSDOS. I don't mean ROM expansions here.

Is there any modern, compatible chip i could use like a eeprom ? Or can't I avoid using a one-time-only PROM or (even worse) a windowed EPROM ?
There is a 28c256 to 27c256 adapter on github, see aDIPters. May be suitable as a 23256 replacement.

But I must stress, I have not built or used the project. However, I would be interested to see if it is suitable.

orzel

Quote from: Rabs on 07:41, 05 July 25There is a 28c256 to 27c256 adapter on github, see aDIPters. May be suitable as a 23256 replacement.
So basically, "no".
There's really not much difference between 28c256 and 27c256 pinouts... I wonder if they did it on purpose.

Thanks for the link, might give it a try. It's nicely done.

Rabs

Quote from: orzel on 12:24, 05 July 25
Quote from: Rabs on 07:41, 05 July 25There is a 28c256 to 27c256 adapter on github, see aDIPters. May be suitable as a 23256 replacement.

Thanks for the link, might give it a try. It's nicely done.
I may give it a try myself, but generally I don't mind using the EPROMs as I have a UV EPROM Eraser.

The PCB is available on PCBWay, see 28C256 to 27C256 and I can see the 28C256 SOIC in stock on DigiKey, see AT28C256-15SU

orzel

What about flash chips like  AT29C256 ? they are not easily sourced, but still a lot more recent than amstrad-period ROMs. They are 5v and the pinout seems ok... ?

Rabs

Quote from: orzel on 17:27, 05 July 25What about flash chips like  AT29C256 ? they are not easily sourced, but still a lot more recent than amstrad-period ROMs. They are 5v and the pinout seems ok... ?
I guess that's the problem, sourcing them.

Rabs

Not a very elegant solution but you can still source W27C512 ICs. Indeed I recently purchased 5 from eBay for another project.

But you need to handle pin 1 (A15) as this is not connected on the motherboard. 

I must stress I have only done a simple boot test while pulling pin 1 to GND but my 464 boots ok.

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eto

What's the reason to avoid EPROMs? They are easy to source and cheap. I always use EPROMs to replace the internal ROMs. 

orzel

Quote from: Rabs on 15:44, 06 July 25Not a very elegant solution but you can still source W27C512 ICs. 

Ah, good to know, thx !

Quote from: eto on 16:13, 06 July 25What's the reason to avoid EPROMs? They are easy to source and cheap. I always use EPROMs to replace the internal ROMs.

I've seen some on aliexpress, but i'm not confident enough. I assumed those old chips were not available elsewhere. There's none on lcsc for exemple. I can't find any 27c128 anywhere.

For the 27c256, there are indeed stocks on digikey/mouser. But their shipping is very expensive.

Thanks for the tip !

Rabs

#9
Quote from: orzel on 22:07, 06 July 25I can't find any 27c128 anywhere.

May help your decision but, I just used a 27256 and doubled the ROM (i.e. copy AMSDOS.ROM/b+AMSDOS.ROM/b AMSDOS_32K.ROM/b) to ensure that the ROM code sits in the top 16K.

orzel

Quote from: Rabs on 17:39, 07 July 25May help your decision but, I just used a 27256 and doubled the ROM (i.e. copy AMSDOS.ROM/b+AMSDOS.ROM/b AMSDOS_32K.ROM/b) to ensure that the ROM code sits in the top 16K.

That's what i did with my 29c256 for the 40015 chip.
But I haven't been able to test it yet.... my 40025 still doesn't work.

McArti0

CPC 6128, Whole 6128 and Only 6128, with .....
NewPAL v3 for use all 128kB RAM by CRTC as VRAM
One chip driver for 512kB(to640) extRAM 6128
TYPICAL :) TV Funai 22FL532/10 with VGA-RGB-in.

orzel

The CPC is ok. I'm trying to replace roms with 29c256, but without success so far.
https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/rom-replacements-6128/

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