Replacement ROM

[msknight]

I like to understand these things

OK - I've toned out a socket. A12 on the 24 pin, is linked to pin 2 on the 28 pin... which happens to be an A12 to A12 map. So far so good.

By linking those two pins to ground, that's CE (chip enable) being kept low, which by the bar above CE, is what it should be... but you're keeping A11 low as well.

So what's acting instead of A11? That's the only part of the puzzle I don't understand. And as the ROM is a full 8K image, it's basically kneecapping it? I'm obviously missing something here. There is one pin on the socket... pin 27 of the 28 pin map, which I haven't managed to trace yet.

[Moggy]

This diagram supplied by Mark may make things clearer.

[Moggy]

As I read it lifted pin 23 is picking up A11 from socket position 20 via the wire link and A12 from socket position 2 normally unused with a 24 pin ROM.
It was assumed that Sinclair may have used 28 pin EPROMs during early testing and the board layout tends to support this in my opinion.

Pins 26-28 on the socket are all 5v of which pins 27-28 on the EPROM get tied to 5v for it to work, pin 26 is not connected internally on the EPROM.

[msknight]

Ahhhh.... got it. I looked at the picture again. I thought A11 was also wired to ground... but it isn't! It's wired into the socket beneath it.

Got it!!!!

[Moggy]

Just found the thread in which Mark explains the wiring etc.

https://www.sinclairzxworld.com/viewtop ... ing#p45083

[msknight]

That does bring me to the ultimate question of whether I should be using eeproms at all and instead look at eproms. From what I'm reading, eeproms have about 10 to 15 years before they start to forget the data on them.

I'm using eeproms because it was the easiest way to program them, at the time, on Linux.

[Moggy]

To be honest I think it's horses for courses.

I only use EPROMs because I have a ZX Spectrum powered 21/25v programmer no other reason and they have been totally reliable for me as far as a 30 odd year old chip can be and could be read when I first got them so that kind of age is not a stretch I would have thought. If you have a programmer that handless EEPROMS ok then I would say the 28C64 is the way to go for you, no hassle programming it, same pin outs as the 2764 and subsequent wiring plus newer and more reliable it has to be hoped.

[msknight]

Just reading discussions about various programmers. There is talk about the Fairy YL23 but I'm going to have to look deeper because it's not coming up in the usual places for me.

I'll sit back and see what people recommend for Linux use to program eproms.

[1024MAK]

Yes, EEPROMs E²PROMs / Flash ROMs can be used. As these have different pin-outs, I need to know what type you want to use.

I am busy at the moment, so please be patient.

Mark

[msknight]

There is no rush at all. I'm continuing to find more posts and to do more reading. Just discovered the double speed basic rom!

I'm still kicking around the benefits/drawbacks of eeproms and eproms.

eeproms for the BBC are not an issue because there are no changes to the pins. They can be put back in the programmer and blown again. I'm using the AT28C256 which I think is about ten years.

In the case of the zeddy, there are pins and soldering to consider; reversing that in ten years time is a bit of a pain.

So I can see how eproms are probably the better way forward for zeddy roms, but obviously my arduino based eeprom programmer won't be able to program those.

What I'm going to need is advice on a programmer that will work on Linux. It'll only be used for the chips for the zeddy. If it also works on chips for the BBC, that would be a bonus... and also then a suitable chip... again if it also works for the BBC, that would be a bonus.