I derived a pinout for the old Mac IIfx/IIsi/IIci ROM SIMM from some info found at Gamba's place together with a specsheet for the Atmel chips he used on his custom SIMM.

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Are you trying to fabricate replica SIMMs?

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I was thinking about it but time and resources currently do not allow for that. A socketed ROM SIMM would be cool because you could use it for all sorts of cool ROM hacking projects. Just pull the ROMs out, reprogram, stick them back in. Currently there's no easy way to modify ROM in any Mac that I'm aware of.

I'm thinking it would be interesting to hear a Mac IIfx belt out the first generation Power Mac startup sound when first powered on. Although this would be much easier on a Mac that already has a sampled audio recording for the startup sound.