Power Mac G3 - 20 pin Power Supply Connector
The Power Mac G3 Blue and White (1999) uses a 20-pin power supply connector that is mechanically identical to the standard ATX 20-pin Molex housing, but carries a different pin assignment that makes it electrically incompatible with commodity ATX power supplies. Pin 8 provides a second +3.3 V rail (rather than the ATX PWR_OK signal), and pin 18 is ground (rather than ATX −5 V); fitting an ATX supply will cause damage. The same 20-pin assignment was carried forward to the Power Mac G4 AGP/PCI Graphics variant before the G4 line moved to 22-pin and then 24-pin proprietary connectors in later generations.
| Pin | Signal | Color |
|---|---|---|
| 1 | +3.3V | Orange |
| 2 | +3.3V | Orange |
| 3 | GND | Black |
| 4 | +5V | Red |
| 5 | GND | Black |
| 6 | +5V | Red |
| 7 | GND | Black |
| 8 | +3.3V | Orange |
| 9 | +5Vsb | Purple |
| 10 | +12V | Yellow |
| 11 | +3.3V | Orange |
| 12 | -12V | Blue |
| 13 | GND | Black |
| 14 | Power On | Green |
| 15 | GND | Black |
| 16 | GND | Black |
| 17 | GND | Black |
| 18 | GND | Black |
| 19 | +5V | Red |
| 20 | +5V | Red |
Notes
The 20-pin housing is a standard Molex connector, physically interchangeable with ATX, but the voltage assignments differ in two critical positions. Do not substitute a commodity ATX power supply without a wiring adapter: pin 8 carries +3.3 V on the G3 (ATX uses this position for PWR_OK), and pin 18 is ground on the G3 (ATX carries −5 V here). The trickle supply (+5Vsb on pin 9) remains live whenever the machine is plugged in, supplying standby power to PCI slots. Probing live voltages is easiest by inserting meter probes into the rear of each crimped wire connection rather than disconnecting the plug.
Signals
- +3.3V — main 3.3 V regulated rail; present on pins 1, 2, 8, and 11
- +5V — main 5 V regulated rail; present on pins 4, 6, 19, and 20
- +12V — main 12 V regulated rail; drives motors and legacy circuitry
- -12V — negative 12 V rail; used by serial-port drivers and legacy analogue circuits
- GND — common ground return; pins 3, 5, 7, 13, 15, 16, 17, and 18
- +5Vsb — 5 V standby (trickle); remains live while the machine is connected to mains, powering PCI slots and wake-on-LAN logic
- Power On — active-low control signal (pin 14); the system asserts this line to command the power supply to switch on; equivalent to ATX PS_ON#
References
- Apple PowerMac G4 Service Source Manual (Apple Service Source K) — confirming 20-pin connector pinout shared with G3 Blue and White
- Power Macintosh G3 (Blue and White) and Power Mac G4: Power Supply Voltage Test — Apple Knowledge Base article 58561 (no longer live; documents voltage test procedure for the 20-pin connector)