Sinclair ZX80 / ZX81 / Z88 Forums

Might be a silly question, but would a 128K ROM work in a 48K Spectrum? Not expecting to use 128K games, but would the 48K option work?

I’ve got one on the way for a future Harlequin 128K build and would like to test it at least functions….


Could I drop it into a Retroleum plug in card instead of the supplied diag ROM?

Yes and no.

The ROM area in all ZX Spectrum computers is 16K bytes.

The ROM chip in a 128K (toastrack, +2 grey) is 32K bytes. The top address line is used to bank switch between each half. One half is the 128K menu, 128K full screen editor and the 128K basic. The other half is the “48K” BASIC.

So if you modify the 48K Spectrum (*), and apply the appropriate logic level to address pin A14, yes it should work.

(*) the ROM used in a 16K/48K/+ ZX Spectrum has two /CS (/CE) control inputs and a /OE control input (total of three), but 128K ROM and 27256 (or 27C256) EPROM chips only have a total of two. Plus the different ROM manufacturers used slightly different pin-outs for the control signals. Hence a small amount of rewiring is needed.

Mark

Thanks Mark,

I think I understand, but forgive me, if the Retroleum diag board uses an EPROM, would this not have the required pinouts for a 128K+ ROM?

Sorry, must have been tired when I read your previous post, hence I may have answered a question you didn’t ask

Yes, the Retroleum ROM board can take the ROM chip from a 128K machine (toastrack, +2, +2A, +2B, +3). These ROMs have the same pin-out as a 27256 or 27C256 EPROM. Which will work in a socket designed for a 27512 / 27C512 EPROM.

Mark

Thanks Mark,

I'll give it a try when it arrives

Although I should add that the DIP switches must be set correctly.

Mark

1024MAK wrote: ↑Wed Nov 15, 2023 1:04 pm Although I should add that the DIP switches must be set correctly.

Mark

to make sure the lower half is used?

For a 27256/27C256/32K byte ROM
Pin 1 should be +5V (A15 on a 27512, VPP on a 27256)
Pin 27 (A14) should be +5V to select the top half (which is where the “48K” ROM is IIRC).

Mark

I'll check on the instructions to see how to set the switches to use the upper half

ROM arrived today, put it in the Retroleum card, switches set for 2nd 16K set out of 4. Connected to my creaky series 2 with the internal ROM removed (to make sure it was the external ROM being used) and it booted fine.

Very happy for just over a tenner. Hopefully Santa can bring me a Harlequin 128K kit