The Compaq Deskpro EP and EN desktop systems of the late 1990s and early 2000s used a proprietary 20-pin power connector built on a Molex 39-01-2200 housing — physically identical to the standard ATX 20-pin connector but carrying a substantially different signal map. The critical incompatibility lies in the power-on control signal: Compaq labels this pin ON/STBY and drives it active-HIGH, whereas the equivalent ATX PS_ON# signal is active-LOW; mixing up the two without a signal inverter will prevent the system from starting or can damage hardware. Deskpro EN models introduce additional deviations at pins 1, 8, and 14 (3.3V/RS sense, RTN/RS, and Aux COM respectively), confirming that this connector family is not interchangeable with standard ATX power supplies.
20 pin MOLEX 39-01-2200 connector at the PSU cable.
| Pin | Name | Description |
|---|---|---|
| 11 | +3.3V | |
| 1 | +3.3V | +3.3V/RS in Deskpro EN |
| 12 | COM | |
| 2 | +5V | |
| 13 | COM | |
| 3 | +5V | |
| 14 | COM | Aux COM in Deskpro EN |
| 4 | Fan OFF | may be +3.3VSB in some server PSU |
| 15 | +5VSB | |
| 5 | +12V | |
| 16 | +3.3V | |
| 6 | -12V | |
| 17 | COM | |
| 7 | PS_ON | |
| 18 | COM | |
| 8 | COM | RTN/RS in Deskpro EN |
| 19 | COM | |
| 9 | -5V | |
| 20 | +5V | |
| 10 | +5V |
Notes
- The Molex 39-01-2200 housing is physically identical to the standard ATX 20-pin connector; a standard ATX power supply will mate with a Deskpro motherboard without resistance, but the signal mis-match can immediately destroy both the power supply and the motherboard.
- The ON/STBY power-on control signal (pin 7, labelled PS_ON in the table above) is active-HIGH on Compaq PSUs. Standard ATX PS_ON# is active-LOW. To use a Compaq PSU with a standard ATX motherboard, a signal inverter (e.g. a 74LS04 or 7404 hex inverter) is required on this line.
- Deskpro EN models deviate further from the Deskpro EP baseline: pin 1 carries a 3.3V remote-sense (RS) function, pin 8 is a RTN/RS return for that sense line, and pin 14 serves as an auxiliary COM rather than a standard COM.
- Pin 4 (Fan OFF) may be reassigned to +3.3VSB on some Compaq server power supplies used in related configurations.
- Compaq also used a 14-pin variant of this proprietary connector (Molex 39-01-2140) on Small Form Factor Deskpro EP/EN systems; that connector is documented separately.
Signals
- +3.3V — 3.3 V supply rail; pin 1 additionally carries a remote-sense function on Deskpro EN models (labelled 3.3V/RS).
- +5V — 5 V supply rail (pins 2, 3, 10, 20).
- +5VSB — 5 V standby supply; remains live when the system is in soft-off state to allow wake events.
- +12V — 12 V supply rail.
- -12V — Negative 12 V rail, used for legacy serial-port signalling.
- -5V — Negative 5 V rail; present for ISA bus compatibility on older platforms.
- COM — Signal ground / power return. Pin 8 also serves as RTN/RS (remote-sense return) on Deskpro EN; pin 14 is designated Aux COM on Deskpro EN.
- Fan OFF — Fan control signal used by the Compaq power management circuit to disable the PSU fan under low-load conditions; may be reassigned to +3.3VSB on certain server PSU variants.
- PS_ON — Power-on/standby control signal. Active-HIGH on Compaq PSUs (ON/STBY), the inverse of the active-LOW ATX PS_ON# convention.