Sourcing a ZX81 ROM or replacement

[Orac]

Hi,

Where could I find a ZX81 ROM or a more modern replacement or plug in circuit board with a modern chip that would do the same job but without the expense of having to buy a programmer and all that hassle for one chip?

Is there anything out there that can do the job?

[Lardo Boffin]

There are various options:

1) Original ROMs come up on eBay from time to time
2) I think SellMyRetro.com sometimes has ROMs
3) People on here may be able to program / sell you one
4) Erm on here sometimes sells the ROMel which contains multiple ROM images, including faster ones and Forth (see viewtopic.php?p=42478#p42478)
5) Mutant Caterpillar Games can program them

Whereabouts in the world are you?

[Orac]

I’m in the UK.

The reason I ask is I can’t see anyone offering anything right now, wondered if I just wasn’t looking in the right places.

[Lardo Boffin]

I’m in the UK.

The reason I ask is I can’t see anyone offering anything right now, wondered if I just wasn’t looking in the right places.

The global shortage of chips and covid has affected many hobby creators. It may be worth messaging Erm to see if he is making any more ROMels?

[1024MAK]

Which version of the ZX81 ROM do you want? There are now known to be three versions (for more information, start here).
The first ROM version with the maths bug is the most likely version that you will find being sold in the pre-owned market places.

With some minor wiring alterations, it’s possible to use either a 2764, a 27128, a 27256 or a 27C256 EPROM. If you want to go this route, you get to choose which ROM image(s) gets programmed (burned) into the EPROM. You don’t need to get a programmer (burner). Just ask on these forums and it’s very likely that a member will be willing to programme (burn) the EPROM for you.

Note that with a 27128, a 27256 or a 27C256 EPROM, you can have more than one ROM image. A simple switch can be wired up to choose between two different ROM images. So you could have the last official version from Sinclair and have say, the ZX81X2 Big Bang ROM (see here) as well.

Mark

[steev]

There was a case on hackaday recently (https://hackaday.com/2021/10/15/heres-a ... placement/) where someone created a pin-compatible FPGA version of the 6502 chip - the same idea could be used to replicate the ROM. (In theory it's simpler, since it's only returning data and not processing anything.)

[mrtinb]

There was a case on hackaday recently (https://hackaday.com/2021/10/15/heres-a ... placement/) where someone created a pin-compatible FPGA version of the 6502 chip - the same idea could be used to replicate the ROM.

Using a FPGA would be much more expensive than using an EEPROM. Mapping from EEPROM to ROM is not rocket science, and has been done with the ROM-el. It costs €30 from eBay.

I've also just hacked a stripboard with an EEPROM, to get my Lambda 8300 working.

[1024MAK]

A One Time Programmable EPROM, 32K x 8bit, is available brand new for £2.61 + VAT. This is the price from a large distributor, which does not include postage and handling charges.

Mark

[mrtinb]

I referenced an EEPROM, as they can be programmed with an Arduino, and be erased the same way. I think the EPROM will require a special burner with different voltages, and erasing them requires UV light.

[1024MAK]

I referenced an EEPROM, as they can be programmed with an Arduino, and be erased the same way. I think the EPROM will require a special burner with different voltages, and erasing them requires UV light.

Yes, an EPROM needs to be programmed in a suitable programmer, but as I suggested earlier, there are members who may be able to burn it for them.

Mark