The ATX (Advanced Technology eXtended) power supply standard, originally developed by Intel in 1995, defines not only the form factor of PC power supplies but also the electrical characteristics and connector pinouts used to deliver stable power to computer components. This standard ensures compatibility between motherboards, power supplies, and peripherals across different vendors.

Understanding ATX Power Supply Pinout and Specifications

The ATX (Advanced Technology eXtended) power supply standard, originally developed by Intel in 1995, defines not only the form factor of PC power supplies but also the electrical characteristics and connector pinouts used to deliver stable power to computer components. This standard ensures compatibility between motherboards, power supplies, and peripherals across different vendors.

ATX Power Supply Basics

An ATX power supply converts AC mains voltage (typically 110/220V depending on the region) into regulated DC voltages required by computer systems. The main output rails are:

• +3.3V: Used primarily for logic circuits and memory.
• +5V: Historically used for logic circuits, USB, and some peripherals.
• +12V: Supplies CPUs, GPUs, fans, and drives.
• -12V: Legacy rail, rarely used in modern systems (some network cards and COM ports).
• +5VSB (Standby): Keeps certain circuits powered when the PC is “off” for wake-on-LAN, USB charging, and soft power-on.

ATX Connector Pinout

The most recognized connector is the 20-pin or 24-pin ATX main power connector, which links the PSU to the motherboard. Key signals include:

• PS_ON# (Pin 16 on 24-pin): A logic signal that turns the PSU on when pulled low by the motherboard.
• PWR_OK (Pin 8): A status signal indicating stable power output.
• Ground pins: Multiple pins to ensure stable return paths and reduce electrical noise.

Typical 24-Pin ATX Connector Layout (simplified)

Other connectors include:

• 4/8-pin CPU power connectors (EPS12V).
• 6/8-pin PCIe connectors for GPUs.
• SATA and Molex connectors for drives and peripherals.

Signaling and Control

• Soft Power-On: Unlike older AT supplies, ATX allows the motherboard to control power state via PS_ON#.
• Power Good: Ensures system stability by only allowing the CPU to boot after voltages are within tolerance.
• Standby Power: Enables modern features like remote wake and low-power states.

Engineering Considerations

From an electrical engineering standpoint, the ATX specification ensures:

• Standardized voltage regulation tolerances (e.g., ±5% on 5V and 3.3V).
• Defined current capacities per rail.
• Protection circuits for over-voltage, over-current, and short circuits.

These aspects are critical to system stability and component safety. The standard has evolved (ATX12V, EPS, SFX), but the fundamental pinout principles remain consistent.

References for Further Reading

• ATX Standard – Wikipedia
• Power Supply Unit (Computer) – Wikipedia
• Institute of Electrical and Electronics Engineers (IEEE) – for broader standards in electronics engineering
• PCI-SIG – relevant for power delivery in PCIe devices

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