exxos's Atari, Amiga & retro forum

Here's a pinout comparison between 27c256 & 28c256


Looking at this the reason that the 28c256 needs the rom writing to the upper half is that Vpp which is normally on pin 1 of a 27C256 is A14 on the 28c256 so presumably will be supplied with vcc on the beeb? (it's much higher for programming).

I think you just need to make sure Pin 27 (a14 on the 27c256) is tied high to get the upper address range? (lift leg and solder blob to pin 28, or just check 27 with a multimeter first - worst case it won't boot if it's grounded or floating)

Hopefully someone with more of a clue will be along shortly to ratify/debunk this :lol:

@BennehBoy Yeah, A14 I have been told must be tied high, which isn't a problem and now I have the mask ROM and 27C256 side by side on a bit of paper, the only other difference is pin 1.

The mask ROM is NC, but the 27C256 is Vpp. Everything else is identical. I guess I could try with both A14 & pin 1 tied high.

Vpp shows as 5.5v on the datasheet, so should be fine.

In the socket /WE is high so it doesn't matter to an (E)EPROM what Vpp does. On the Beeb that's high too.

So, A14 on the 27C256 will be high, Vpp is too low to matter. If in doubt lift pin 1 on your 27C256. Just write the image in the upper half of the chip, or write two copies, one in each half.

stephen_usher wrote: 03 Jun 2021 21:05 In the socket /WE is high so it doesn't matter to an (E)EPROM what Vpp does. On the Beeb that's high too.

So, A14 on the 27C256 will be high, Vpp is too low to matter. If in doubt lift pin 1 on your 27C256. Just write the image in the upper half of the chip, or write two copies, one in each half.

Thanks for that info, good to know, I'll give them a try based on that.

Top marks @BennehBoy and good info @stephen_usher

I dropped the diag ROM in and it worked like a charm, no mods or adapter needed :) Thanks to both of you for the assistance.

Of course this is also good for me, because being an ST fellow, I have quite a few spare 27C256s lying around that could find their way into the Beeb if needed. I have some 27C512s as well so I assume that if I write 2 copies to each half of the chip this would also work.

Either way, nice one, both the diag and OS ROMs worked perfectly :)

Nice.

A batch of 10 28c256's just dropped from China, and I'm surprised to find they're all genuine Atmel parts. I'm going to try get my head round kicad so I can modify the schematic for the 2732 to 27256 adapter so it will also fit 28256's, should just need another trace and resistor so it can operate without jumpers I hope.

BennehBoy wrote: 05 Jun 2021 15:26 Nice.

A batch of 10 28c256's just dropped from China, and I'm surprised to find they're all genuine Atmel parts. I'm going to try get my head round kicad so I can modify the schematic for the 2732 to 27256 adapter so it will also fit 28256's, should just need another trace and resistor so it can operate without jumpers I hope.

Cool, be good to hear how you get on. I'm currently playing games on the Beeb.


And I'm now using the RGB cable, so good to know that works too.

:dualthumbup:

I know the kbd surround is a bit nasty, but someone on stardot is trying to get replacements made.

The case colour doesn't bother me too much, and my other one is a much better color.

When you've got the hang of the analogue joysticks, my favourite memories are of Elite (obviously!), Aviator & Revs...others might not have aged quite so well (but Defender, Magic Mushrooms, Monsters, Arcadians & Chuckie Egg to mention just a few others) were all played to death.

Might have to fire up the Beeb emulator later tonight... ;)