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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        │
└────────────────────┘