Areligious Republican  wrote:
> Is the memory bus visible at all to allow the addition
> of (unlimited) I/O expansion?

No.

And it's not a memory bus these days, it would be an AXI master or slave
interface, or something like PCIe.  The Pi4 has PCIe, but you'd have to
desolder the USB chip and tap into the wiring - there's no external PCIe
slots. Somebody has hacked it (of course):
https://www.hackster.io/news/pci-express-on-the-raspberry-pi-4-9b03c59f7a04
but not a solution for most people.

Theo

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