Re-Programmable ROM Replacement

[David G]

New project boards almost exactly the same size as an original ROM (24-pin low profile). As I understand it, the ZX81 uses a 3216-type 24-pin 8K ROM, either:
* Motorola ZCM38818P
* NEC D2364C (different speeds available)

2364 2332 2316 Re-Programmable ROM Replacement board

advantages over other replacement ROMs:
* no modifying the ZX81 required
* looks nearly original
* the parts are cheap (flash chip < $1/1000 or $4 to $20 for quantity one)
* can purchase an assembled board from PCBWay (price ?)

On the bottom of the board (hidden from view) is a 1Mbit TSOP-32 SST39SF010A flash chip, and chip-select select jumpers (can be used as 2K, 4K or 8K ROM)

disadvantages:
* Kit requires soldering a few components
* single-quantity chips are already obsoleted
* requires a programmer T48/TL866 programmer which costs about $65

Video: 2364 2332 2316 Re-Programmable ROM Replacement

Programming: he has a programming adapter on PCBWAY to use with 32-pin programmers: 2364 2332 2316 Re-Programmable ROM Replacement

[David G]

What are your thoughts on this project? I haven't tried it, but thought it was interesting enough to bring to the forum's attention. The project is from James Harmony and the video is by The Retro Dungeon

An advantage of using a modern flash-based ROM replacement over an older EPROM/EEPROM is simpler programming hardware, however ... at $65 for an XGECU programmer it may be a moot point. Are there less expensive programmers that can be used with 32-pin DIP type flash memory? 
 

[Moggy]

Being an old dinosaur I tend to stick with the boatload of 2764s I have, two pins lifted and two wires soldered, job done.
As for the programmer I paid a fiver for one which does all the old 25v jobbies it's powered by my ZX Spectrum and is far simpler than any modern programmer so going into minor debt for something such as this doesn't really appeal.

All my Zeddies have big bang EPROMS fitted which were blown by the Speccy so not much else to do programming wise and I also would think that most here are already sorted ROM wise.


Just my thoughts on it as asked and thanks for bringing it to the forum.

[1024MAK]

It’s a nice idea.

EEPROMs (E2PROMs) and flash memory should be easier. But as these in DIP/DIL parts get discontinued, it’s no longer as easy.

28 pin EEPROMs (E2PROMs) and flash memory that only need a single +5V supply can be programmed in some systems. But because of the way the ZX81 works, that’s a bit more tricky than it is on say, a ZX Spectrum or a BBC Micro (and even with these it’s not as simple as you would think).

I agree with Moggy, if you have to spend that much on a programmer, it’s just as easy to program an EPROM. One Time Programable 64k byte EPROMs are still being made (by Microchip) and the older smaller capacity types are available on eBay and elsewhere. Yes, these also require a programmer. But some here will already have one.

There are ways to program any of these devices with a microcontroller, but that’s a bit of a sharp learning curve if all you want is a “new” ROM.

Mark

[David G]

Thanks for the thoughts

[1024MAK]

Oh, one other thing I want to mention. In most computers, as long as the chip physically fits, you can often use a larger capacity memory chip. It’s the address decoding that determines the amount of memory that the processor can see in the chip.

The “extra” higher address lines do however have to be tied to either 0V/GND or to +5V/VCC so that they are at a valid logic level. And the ROM image has to be programmed in the correct address in the memory chip taking into account of the wiring of the higher address lines.

Mark