Dell Pentium II and Pentium III era desktops — including the OptiPlex GX1, GX110, Dimension 4100, and Precision 410 — used a 20-pin Molex Mini-Fit Jr connector (Molex 39-01-2200 at the cable, Molex 39-29-9202 at the motherboard) that is physically identical to the standard ATX 20-pin connector but carries radically different voltage and signal assignments. Because the housing is physically interchangeable, connecting a standard ATX power supply to one of these motherboards — or vice versa — will immediately destroy both the PSU and the motherboard. Dell returned to the standard ATX pinout for Socket 478 Pentium 4 systems, making the proprietary connector specific to the late 1990s to early 2001 product generation. There is no industry standard governing this variant; Dell deviated unilaterally from the Intel ATX Power Supply Design Guide.
20 pin MOLEX 39-29-9202 connector at the motherboard.
20 pin MOLEX 39-01-2200 connector at the cable.
| Color | Signal | Pin | Pin | Signal | Color |
|---|---|---|---|---|---|
| grey | /PS_On | 11 | 1 | +5V | red |
| black | GND | 12 | 2 | GND | black |
| black | GND | 13 | 3 | +5V | red |
| black | GND | 14 | 4 | GND | black |
| white | -5V | 15 | 5 | Power_Good | orange |
| red | +5V | 16 | 6 | +5VSB (standby) | purple |
| red | +5V | 17 | 7 | +12V | yellow |
| red | +5V | 18 | 8 | -12V | blue |
| KEY (blank) | - | 19 | 9 | GND | black |
| red | +5V | 20 | 10 | GND | black |
Signals
- /PS_On — power-supply enable; active-LOW (pull to ground to switch on), functionally equivalent to PS_ON# in ATX but located on a different pin.
- Power_Good — power-good signal driven HIGH by the PSU once all rails are stable; used by the motherboard to release the processor reset.
- +5VSB (standby) — 5 V standby rail, present whenever mains power is connected regardless of system state; powers wake-on-LAN and similar circuits.
- +12V — 12 V rail; note the position differs from standard ATX 20-pin, making cross-connection destructive.
- -12V — negative 12 V rail for legacy RS-232 level shifting.
- -5V — negative 5 V rail for legacy ISA bus devices.
- GND — common ground return.
- KEY (blank) — pin 19 position is physically blocked (absent pin) to provide a mechanical keying feature preventing reversal; no electrical connection.
Notes
- 18 AWG is recommended for all wires except pin 11 (/PS_On), which should be 22 AWG.
- For 300 W configurations 16 AWG is recommended.
- The connector housing is physically identical to the standard ATX 20-pin Molex Mini-Fit Jr — cross-connection with a standard ATX PSU will cause immediate and irreversible damage to both the PSU and the motherboard.
- Affected systems include the Dell OptiPlex GX1, GX110, Dimension 4100, and Precision 410; the Dell Dimension 8100 uses a separate dual-connector (24-pin + 16-pin) proprietary arrangement.
- Dell reverted to standard ATX pinout beginning with Socket 478 Pentium 4 systems.
References
- Compaq Deskpro EN 6550/6400 Service & Reference Manual (Doc 133756-004) — Connector Pin Assignments — Compaq Computer Corporation; documents OEM proprietary Molex 39-01-2200 connector pinouts contemporaneous with Dell’s proprietary ATX-physical variant.