exxos's Atari, Amiga & retro forum

Hi Folks - I’m upgrading my TOS to 1.04 and am modifying from a 2 to a 6 ROM setup. As such, I need 6 ROMs and wanted to ask if the ST27C256B would be useable? Or is the ‘B’ version going to be an issue?

Cheers

'B' is just the revision of the chip. They did some changes to the die in the chip. I think they changed the programming voltage needed a bit as well. But if you choose the right chip in your programmer it shouldn't make any difference. ST/M is the same company (STMicroelectronics). Use 120ns or faster. I think 150ns is fine as well.

What motherboard and revision do you have? Do you have the three solder blobs on the board?

What @DoG says goes, but it would be useful if you could add a photo of the board.

ST's generally have 3 jumpers to select between 1MB and 256KB ROM sizes and they are solder pads near the ROMs.

You'll need to remove the solder from the 1MB pads and replace with solder on the 256K side.

You may also need to install decoupling capacitors for the currently unpopulated ROM slots you're going to fill. Be very careful to ensure orientation is correct as I've seen somebody plug in ROMs 180 degrees out recently and that can kill the ROMs, which also need to be in the correct Lo/Hi slots.

Make sure you fit sockets as it will make for an easier time later.

Hi Folks - gave it a go with my EPROMS and Im now running TOS 1.04 - Result! Thanks everyone for all the great tips and advice!

Lamaman1971 wrote: 30 Jun 2026 01:53 Hi Folks - gave it a go with my EPROMS and Im now running TOS 1.04 - Result! Thanks everyone for all the great tips and advice!

Good stuff :)