External Diagnostics ROM

[valentin]

Hello,

Is there any downside with this simple external ROM extension board? It would be much cheaper for diagnostics than the ZX DIAG CART.

I am assuming it'll work with ZX Spectrum 48K clones, not only with ZX Spectrum 48 / 128 / +2 / Timex 2048.

[1024MAK]

Hello,

Is there any downside with this simple external ROM extension board? It would be much cheaper for diagnostics than the ZX DIAG CART.

I am assuming it'll work with ZX Spectrum 48K clones, not only with ZX Spectrum 48 / 128 / +2 / Timex 2048.

Most of the diagnostic boards available are enhancements of the original design, which is on this web site. There was some discussion way back in 2014 on what extra features were to be included on WoS forums. But someone went further. Obviously as it gets more complex, the cost goes up.

You can use ANY interface which maps a 16k byte ROM (or a 16k byte area of a larger ROM) into the address space of the BASIC ROM. There are a number of expansions/interfaces that can do this.

As per your question, I don’t like that the linked to ROM board ignores the Z80 /RD control line (*1) even though there are THREE spare logic gates. Apart from ignoring standard Zilog Z80 design practice (*1), I don’t see any other problems.

Note *1: normally the Z80 /RD control line should go to a memory chip’s /OE input. Address decoding should be performed by suitable logic (gates or other suitable logic), then combined with the Z80 /MREQ control line with more logic. The output of this logic then controls the memory chip’s /CS or /CE pin.

If a clone or compatible computer has the exact same signal allocation on it’s expansion port / edge-connector AND taking the /ROMCS line logic high disables the on board (BASIC) ROM, yes, it should work.

Mark

[valentin]

Cheers, Mark.

And I understand your observation about the Z80 best practices.

I have ordered on eBay a similar board as the one linked in my original message - they are the same, the one I ordered has SMD components.

[1024MAK]

I have ordered on eBay a similar board as the one linked in my original message - they are the same, the one I ordered has SMD components.

Your link is incorrectly formed

Mark

[valentin]

Your link is incorrectly formed

Mark

Ooops. Edited.