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[smilesdavis]

Apple officially sold the G4 mini as having “no PCI slots,” yet internally it has a 33 MHz PCI host and Apple routed that PCI bus right to a removable expansion module.

https://leopard-adc.pepas.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-G4/MacMiniG4/2Architecture/q88_archi.html

- the mini already contains a real conventional PCI bus
- The Intrepid I/O controller has a 32-bit PCI bus with a bus clock speed of 33 MHz.
- The connector for the AirPort Extreme Card uses the PCI bus.
- The mini's host bus is exactly the sort of bus your older Digi cards expect: 32-bit PCI at 33 MHz.
- Apple also used the same generation AirPort Extreme card across many G4/G5 Macs. Available Apple schematic material for that interface shows essentially the conventional PCI ingredients: AD address/data lines, C/BE, FRAME, IRDY, TRDY, DEVSEL, STOP, clock, reset, interrupt, REQ/GNT, IDSEL, etc. (https://www.informaticanapoli.it/download/SCHEMIELETTRICI/APPLE/Q45-G5-IMG5-051-6482-080304-RC.pdf)
-


Mac mini G4
    │
AirPort PCI
    │
custom PCBway interposer* for CardBus
  │
Magma Chassis
    │
 ┌─┼───────────────┐
PCI cards.....

a theoretical adapter should have its own regulated PCI power supply

Mac mini AirPort
  PCI SIGNALS ──────────────────┐
  GND ──────────────────────────┤
                               ▼
                         PCI connector
                               ▲
external PSU:
 +3.3 V ────────────────────────┤
 +5 V ──────────────────────────┤
 +12 V ─────────────────────────┤
 -12 V if required ─────────────┘

Grounds obviously have to be common.

G4 minis don't put the AirPort card directly onto the main logic board. They have a removable mezzanine board (820-1660-06) which can simply be removed from the motherboard, and replacement boards exist as Apple parts 922-6671 / 922-6893. The late 2005 1.33/1.5 GHz revision is less attractive, because Apple revised the mezzanine and integrated the wireless hardware more tightly. Earlier 1.25/1.42-era hardware gives us the nicest physical starting point.

That means I would not touch the Mac mini motherboard initially.

Mac mini motherboard
       │
 original mezzanine connector
       │
 donor mezzanine PCB
       │
 [remove AirPort connector /
  attach interposer here]
       │
 normal PCI

┌────────────────────┐
│    Mac mini G4     │
│                    │
│ Intrepid PCI       │
│      │             │
│ AirPort connector  │
└──────┼─────────────┘
       │
       │ custom PCB
       ▼
┌────────────────────┐
│  32-bit PCI slot   │
│  external power    │
└──────┬─────────────┘
       │
    Magma
   host card
       │
       ║
       ║ Magma cable
       ║
┌──────▼─────────────┐
│ MAGMA 4-slot PCI   │
│                    │
│ MIX Core           │
│ MIX Farm           │
│ DSP Farm II        │
└────────────────────┘

[laulandn]

This sounds plausible and possible, and something I have wondered about also.

The trick will be if the firmware will query roms on the PCI bus that the airport card uses.  If it does, you're gold.  If it doesn't some work will be needed. 

On the other hand, if you don't need cards on the new PCI bus at boot time, the drivers can just be loaded as needed.