This adapter cable connects a standard ATX 24-pin power supply — defined in Intel ATX Specification v2.2 to support 75 W PCI Express requirements — to the proprietary 24-pin motherboard connector used in Compaq Deskpro EP and EN full-tower systems from the Pentium II through early Pentium 4 era. Both connectors share the same Molex Mini-Fit Jr physical housing (Molex 39-01-2240), making them physically interchangeable but electrically incompatible; plugging a standard ATX supply directly into a Compaq Deskpro motherboard without this adapter will damage both units. The Compaq connector diverges from ATX in several critical ways: its power-on control signal (ON/STBY, pin 16) is active-HIGH rather than the active-LOW PS_ON# of ATX, pin 8 carries a Fan OFF signal instead of PWR_OK, and pin 12 carries a Fan CMD line rather than +3.3V. Note that the later HP Compaq dc7600–dc7900 SFF and CMT systems used a physically distinct Molex Micro-Fit 3.0 (3.0 mm pitch) 24-pin connector and require a different, incompatible adapter.
This schematic allows to connect a ATX Motherboard Power - 24 Pin (ver. 2.x) on a ATX Motherboard Power - 24 Pin - Compaq/HP.
| Color | Pin PS | Pins MB | Name | Description | Color | Pin PS | Pin MB | Name | Description |
|---|---|---|---|---|---|---|---|---|---|
| orange | 1 | 1, 13 | +3.3 VDC | Power +3.3V | orange | 13 | 23 | +3.3 VDC | Power +3.3V |
| orange | 2 | 2 | +3.3 VDC | Power +3.3V | blue | 14 | 14 | -12 VDC | Power -12V |
| black | 3 | 3 | GND | Ground | black | 15 | 15 | GND | Ground |
| red | 4 | 4 | +5 VDC | Power +5V | green | 16 | 16 | PS-ON | Remote Power ON/OFF |
| black | 5 | 5 | GND | Ground | black | 17 | 17 | GND | Ground |
| red | 6 | 6 | +5 VDC | Power +5V | black | 18 | 18 | GND | Ground |
| black | 7 | 7 | GND | Ground | black | 19 | 19 | GND | Ground |
| grey | 8 | PWROK | Power Good | 20 | 8 | ||||
| purple | 9 | 9 | +5 VSB | +5V Standby | red | 21 | 21 | +5 VDC | Power +5V |
| yellow | 10 | 10 | +12 VDC | Power +12V | red | 22 | 22 | +5 VDC | Power +5V |
| yellow | 11 | +12 VDC | Power +12V | red | 23 | 12 | +5 VDC | Power +5V | |
| orange | 12 | 11 | +3.3 VDC | Power +3.3V | black | 24 | 24 | GND | Ground |
Notes
- The standard ATX 24-pin connector (power supply side) follows Intel ATX Specification v2.2; Molex Mini-Fit Jr housing, cable connector Molex 39-01-2240, maximum 6 A per circuit.
- The Compaq Deskpro EP/EN 24-pin motherboard connector uses the same Molex Mini-Fit Jr housing and is therefore physically interchangeable with a standard ATX plug, but the signal assignments are entirely different and the two are electrically incompatible.
- The Compaq ON/STBY signal at pin 16 is active-HIGH; ATX PS_ON# at the same pin position is active-LOW. A signal inverter is required for correct power-on behaviour when cross-wiring these systems.
- ATX PS_ON# (pin 16, green wire) connects to Compaq ON/STBY (pin 16) but polarity is reversed; the adapter must account for this inversion to avoid holding the system permanently on or permanently off.
- ATX pin 8 (PWROK, grey) has no equivalent on the Compaq board (pin 8 = Fan OFF); it is left unconnected on the HP/Compaq motherboard side.
- ATX pin 11 (+12V, yellow) is left unconnected on the HP/Compaq motherboard side; Compaq pin 11 = +3.3V Aux, which is supplied instead by ATX pin 12 (+3.3V, orange) routed to HP MB pin 11.
- Compaq MB pin 20 carries −5V (a rail absent from standard ATX 24-pin, where pin 20 = NC); this pin is left unconnected on the ATX supply side of the adapter.
- Compaq MB pin 23 = +3.3V R/S (remote sense); ATX PS pin 13 (+3.3V, orange) is routed here.
- Compaq MB pin 24 = Fan Sink; this signal has no ATX equivalent and is left unconnected on the supply side.
Signals
- +3.3 VDC — +3.3 V regulated supply rail; orange wires (ATX pins 1, 2, 12, 13); supplies Compaq MB pins 1, 2, 11 (+3.3V Aux), 13, and 23 (+3.3V R/S remote sense).
- +5 VDC — +5 V regulated supply rail; red wires (ATX pins 4, 6, 21, 22, 23); supplies Compaq MB pins 4, 6, 12 (Fan CMD uses +5 V level), 21, 22.
- +12 VDC — +12 V regulated supply rail; yellow wires (ATX pins 10, 11); ATX pin 10 routes to Compaq MB pin 10; ATX pin 11 is unconnected on the Compaq MB side.
- −12 VDC — −12 V regulated supply rail; blue wire (ATX pin 14 → Compaq MB pin 14).
- +5 VSB — +5 V standby (always-on); purple wire (ATX pin 9 → Compaq MB pin 9).
- GND — Signal and power return; black wires (ATX pins 3, 5, 7, 15, 17, 18, 19, 24).
- PS-ON / ON/STBY — Power-on control; green wire (ATX pin 16). ATX PS_ON# is active-LOW; Compaq ON/STBY is active-HIGH — polarity is opposite, requiring an inverter in the adapter.
- PWROK — Power-good signal from supply; grey wire (ATX pin 8). Left unconnected on Compaq MB side (Compaq pin 8 = Fan OFF).
- Fan OFF — Compaq MB pin 8; fan control output with no ATX equivalent; unconnected on supply side.
- Fan CMD — Compaq MB pin 12; fan speed command signal; connected to the +5 V level from ATX pin 23.
- Fan Sink — Compaq MB pin 24; fan-sink control with no ATX equivalent; unconnected on supply side.
- +3.3V Aux — Compaq MB pin 11; auxiliary +3.3 V rail; supplied by ATX pin 12 (+3.3V, orange).
- +3.3V R/S — Compaq MB pin 23; remote-sense line for the +3.3 V rail; supplied by ATX pin 13 (+3.3V, orange).
- −5V — Compaq MB pin 20; −5 V rail absent from standard ATX 24-pin (ATX pin 20 = NC); left unconnected on supply side.