Replacement ROM

[msknight]

Someone's going to point out an easier way of doing this, or else explain why my thought pattern is wrong, but I'm expecting this

After thinking I would try and buy a ROM or two for spare, or replace an older ROM if I encounter one, I looked in the usual places to buy a couple of replacements. I didn't find anything.

Now... I have the equipment to put an image on a 32k eprom for use in a BBC Micro. I wondered how I could utilise this to write an eprom that I could use in the ZX81. Address pin A11 on the 32k chip is A12 on the zeddy. CS is actually A11 on the zeddy. With a bit of re-wiring a socket, I could get it to work. A13 would be high anyway because that's 5v on the zeddy, and A14 would have to be tied high also. (easier than tying it to ground when there's a 5v pin opposite it)

The only thing to solve would be the A11 and A12 swap, which would just mean swapping around a few parts of the ROM image, and then putting it in the highest 8K of the eprom.

Yes?

[1024MAK]

Unless you want to mess around reorganising the data, it's far easier rerouting the signals as shown elsewhere on these forums. For A13, A14 etc., tie them to either +5V or 0V.

Then "burn" the 8K byte ROM image in as many times as there is space (twice for a 16K byte EPROM, four times for a 32K byte EPROM).

Mark

[msknight]

I can't find posts with that as my generic key words are either turning up nothing, or loads. Grateful for any pointers.

The eeprom programmer I'm using for the BBC eproms is an arduino unit, so I'm not sure whether it would work with ROMs for the zeddy.

As a Linux user, I'd be grateful for pointers to appropriate eeproms and a programmer that I can use on Linux if anyone has any knowledge on that.

[msknight]

I'm starting to wonder if it's perhaps a better idea to make a "conversion socket" to put an 8k eeprom chip into the programmer I have...
https://www.tindie.com/products/oddblk/ ... uino-mega/
..and program it that way?

[Moggy]

The method I and some others use is to seek out a 2764 (21v) or the lower program voltage 2c764. (12v) they are easy enough to obtain, they were fitted to all the early ZXpands which folk were happy to buy even though they were a relatively old chip so you should have no problem sourcing and using them them, plus no faffing around with sockets and the rewiring is but two pins lifted and rewired as per the picture.

[msknight]

My eeprom programmer won't get up to those kind of voltages. Is there an equivalent eeprom please?

I've tried using the Mouser eeprom searcher and I'm coming up with the wrong sized chips each time!

[msknight]

The only one I can find, I'm back to the 28 pin eeproms ...
https://www.mouser.co.uk/ProductDetail/ ... cLow%3D%3D
...and they've got that thing with A11 being OE.

[msknight]

The obvious question I have about that picture, is if pin 20 is tied to ground, then how is the zeddy addressing A11?

[Moggy]

The 28c64 looks to have the same pin out as the 2764 so I wouldn't have thought there was a problem, just wire it the 28 the same as the 27 no?

[Moggy]

The obvious question I have about that picture, is if pin 20 is tied to ground, then how is the zeddy addressing A11?

With things like this I tend not to overthink or get tied up in knots over, it's a tried and trusted method that works for many of us that's all I know.
No doubt Mark will know and understand the detail whereas I'm just a dumb user who's happy that it works at all!