ZX81 ROM burning

Discussions about Sinclair ZX80 and ZX81 Hardware
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AmintaSpectrum
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Joined: Tue Jul 04, 2017 12:12 am

ZX81 ROM burning

Post by AmintaSpectrum »

Hi all!

I've bought a TL866II Plus Eprom programmer (https://it.aliexpress.com/item/32992209 ... 4c4d2HKIbP).

What kind of Eprom I need to burn Zx81 Rom? Do I need an adapter?

And what about a Rom Switcher? There is an open source project? What Eprom I have to buy to host multiple roms?


Many thanks!

Davide
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1024MAK
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Re: ZX81 ROM burning

Post by 1024MAK »

For the ZX81 ROM content, you need a 64K bit (8K bytes) EPROM such as a 2764. But these have been obsolete for many years.
You can also use a 128K bit (16K bytes) EPROM such as a 27128 or 27C128. Alas these are also now obsolete. You can however have the ZX81 ROM code in the first 8K bytes and your own machine code in the remaining 8K bytes. You may be able to find some on Internet trading sites, or from suppliers that still have old stock.

The smallest currently available (from the main electronic suppliers) EPROM is a one time programmable [OTP] (you can ‘burn’ them, but there is no window to erase them) 256K bit (32K bytes) EPROM such as a 27C256. But only half (16K bytes) can be seen by the ZX81. You could wire up a switch to manually change over to the other half, but you would have to have a copy of the ZX81 ROM code in both half’s, or an equivalent ROM that the ZX81 can start with, as you will need to reset or power down/up the computer after switching as it will almost certainly crash.

All of the above will require some wiring modifications, as the layout of the pads on the board is for a mask ROM chip which has a some of the signals in different places.

Some types of EEPROM chips or flash RAM chips can also be used. They also require wiring modifications.

Mark
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AmintaSpectrum
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Joined: Tue Jul 04, 2017 12:12 am

Re: ZX81 ROM burning

Post by AmintaSpectrum »

Thanks Mark!

And what about a 2364 a 2764 EPROM adapter using 2764 for burning Rom (I have many 2764 ROM).
Moggy
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Re: ZX81 ROM burning

Post by Moggy »

I use quite a few 2764's in my zeddies but was guilty of using an over elaborate rewiring system for the chip until i was educated by the forum member found here..
https://www.sinclairzxworld.com/viewtop ... =22&t=3725

I think you will find his first post most illuminating with a very easy quick wiring job and no need for adaptors

Hope this is useful to you.
Moggy
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Re: ZX81 ROM burning

Post by Moggy »

You may also find this useful, a double speed BASIC, ROM image found here...

https://www.sinclairzxworld.com/viewtop ... lit=zx81x2
Last edited by Moggy on Sat Jul 11, 2020 10:51 pm, edited 1 time in total.
AmintaSpectrum
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Joined: Tue Jul 04, 2017 12:12 am

Re: ZX81 ROM burning

Post by AmintaSpectrum »

Thanks!

I’ll read the post and I’ll make the wiring. However, because I’ve ordered several adaptors, do you think they’ll work with Zx81 without modifications?

Thanks!
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 ROM burning

Post by Moggy »

I don't see why they shouldn't work from an electrical point of view if they are designed to make the 2764 replace the original chip, the one point to consider is will the cases go back together with the adaptor fitted as there is not much clearance between the ROM chip and the top case and any adaptor will make the EPROM stand slightly taller.
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