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mahjongg2 / ZX81plus38

Mij latest ZX81 clone without ULA built from discrete pin through hole components
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Using DIP-24 ROM #13

Closed TankedThomas closed 1 year ago

TankedThomas commented 1 year ago

Hi Martin,

Awesome project, thanks so much.

Just a quick question: is it possible to use an original 24-pin ROM chip? A local seller has a bunch of new old stock parts so I thought it could be a bit of fun instead of flashing my own. Obviously it's not a drop-in replacement for a modern 28-pin EPROM though, so it would need an adapter.

Is it the same pinout as a DIP-24 to DIP-28 ROM adapter for the C64 or is it something that would need to be made from scratch? Something like this is what I'm referring to: https://github.com/SukkoPera/Open2327RomAdapter

mahjongg2 commented 1 year ago

Dear Stephano,

a bit late, and was a bit confused about your question.

My ZX81+39 board supports the use of the original 24 pin ROM, just change the applicable jumpers for a 24 PIN ROM, look in the schematics. and place the 24-pin ROM in the lower part of the socket, so do not use pin 1 , 2, 28 and 27

If you source new old stock ROM’s make sure you buy a ROM that contains the code for a ZX-81, not any other ROM, there must be thousands of old ROM’s that contain something else than the ZX-81 code.

the adapter you are talking about is so you can replace a 24-pin ROM with a 27Cxxx style EPROM, and this isn’t relevant in this case

best regards, martin de jong.

Op 25 jul. 2023, om 02:59 heeft Thomas @.***> het volgende geschreven:

Hi Martin,

Awesome project, thanks so much.

Just a quick question: is it possible to use an original 24-pin ROM chip? A local seller has a bunch of new old stock parts so I thought it could be a bit of fun instead of flashing my own. Obviously it's not a drop-in replacement for a modern 28-pin EPROM though, so it would need an adapter.

Is it the same pinout as a DIP-24 to DIP-28 ROM adapter for the C64 or is it something that would need to be made from scratch? Something like this is what I'm referring to: https://github.com/SukkoPera/Open2327RomAdapter https://github.com/SukkoPera/Open2327RomAdapter — Reply to this email directly, view it on GitHub https://github.com/mahjongg2/ZX81plus38/issues/13, or unsubscribe https://github.com/notifications/unsubscribe-auth/ACAHA4GSO5HL27H5QCRGCKTXR4LAPANCNFSM6AAAAAA2WKJOFU. You are receiving this because you are subscribed to this thread.

TankedThomas commented 1 year ago

Thanks Martin, I'll have a look at the jumpers. I assumed I'd need an adapter instead as the board defaults to 27C-style ROMs.

The documentation for them is a bit vague and I just had a burnout from trying to use the 32K RAM jumper, so I'm not in a hurry to use the rest.

I did, however, install jumpers to use a 27C64. Based on my knowledge of the ROMs, it should be enabled by using jumpers JP9 and JP10 to pull the necessary address lines high.

EDIT: This isn't correct, a 27C64 works stock. Not sure I understand why, but oh well. The 27C256 with the ROM flashed in all four banks didn't work properly for me (about half the characters were garbled), but the 27C64 worked totally fine without any jumpers. Also, I was getting no power with even just the two aforementioned jumpers, let alone the RAM one, so disconnecting them all allowed it to work. Hopefully this info helps someone else in the future.

FYI, the new old stock ROM I got is a genuine Sinclair ZX-81 original.