Indigo 2 Power Supply Information

The SGI Indigo 2 power supply is a common failure point of this system with issues often traced to aging passives and semiconductor circuitry. This page serves to catalog the information originally aggregated by Elf on SGUG but later removed (and restored against his will). He has granted an exclusive license for TechPubs to use and publish his information.

Pinout (Non-IMPACT)

The Indigo 2 power supply used in non-IMPACT systems is a multi-board switch-mode unit built into a metal chassis with an integral cooling fan. It delivers regulated +5 V, +12 V, –12 V, and auxiliary standby voltages to the system backplane. The unit interfaces to the mainboard and peripherals through two primary connectors:

• Main power connector: 24-pin AMP/Tyco Mate-N-Lok style plug, keyed to prevent mis-insertion.
• Auxiliary connector: 6-pin Mini-Fit Jr. style plug used for standby and control signals.

The supply relies on “ground-to-enable” logic at specific pins for startup sequencing and protection. Failure to meet expected grounding or load conditions will prevent the system from powering up.

Pinout Table — Main Power Connector (Non-IMPACT Indigo²)

Pin	Signal	Notes
1	+5 V	Primary system rail, high current
2	+5 V
3	+5 V
4	+12 V	Disk, fan, and peripheral rail
5	–12 V	Serial and audio subsystems
6	GND	Common return
7	GND
8	GND
9	+5 V
10	+5 V
11	PWR-ON (GND)	Grounding this pin signals the PSU to start
12	GND
13	+5 V
14	+5 V
15	+12 V
16	–12 V
17	GND
18	GND
19	+5 V
20	+5 V
21	+5 V
22	+5 V
23	GND
24	GND

Auxiliary Control Connector (6-pin)

Pin	Signal	Notes
1	+5 V Standby	Always present when AC is connected
2	GND
3	PS-Good	Logic high indicates rails are in spec
4	Fan Tach	RPM feedback signal from PSU cooling fan
5	Remote Enable	System logic control for main power switching
6	GND

Pinout and Information on IMPACT systems

The Indigo 2 IMPACT systems, introduced alongside the IMPACT graphics architecture, required a higher-capacity power supply to handle increased current draw from the graphics subsystem and additional memory. Although mechanically similar to the non-IMPACT unit, the IMPACT power supply differs in both electrical capacity and connector arrangement.

Key Differences

• Higher Current Rating: The IMPACT PSU provides substantially more +5 V current, as this rail powers the expanded graphics pipelines and geometry engines.
• Connector Variations: The IMPACT version uses a 30-pin main connector (rather than the 24-pin of the non-IMPACT), with additional +5 V and ground lines to support heavier load balancing. The auxiliary 6-pin control connector remains present but has slight signal reassignments.
• Physical Compatibility: While both units occupy the same chassis slot and mount points, the connectors are not directly interchangeable; a non-IMPACT PSU cannot power an IMPACT system and vice versa.
• Startup Behavior: Like the earlier unit, the IMPACT PSU requires grounding of its designated “Power-On” pin to start, but its sequencing and monitoring are more stringent due to the higher load demands.
• Backplane Connector: See below section.

IMPACT Pinout (Main Connector)

Pin	Signal	Notes
1–12	+5 V	Multiple pins dedicated to high-current +5 V
13–16	+12 V	Disk and peripheral power, increased provision
17–18	–12 V	Legacy serial/audio support
19	PWR-ON (GND)	Grounding signals PSU start
20–30	GND	Extensive grounds for current return

(Layout simplified; many positions are parallel +5 V or GND to spread load.)

Auxiliary Connector (6-pin)

The auxiliary connector functions similarly to the non-IMPACT version, providing standby power, PSU “good” status, fan tachometer feedback, and remote enable. Signal placement differs slightly, but functional roles remain the same.

Backplane Connector

In addition to the main and auxiliary PSU harnesses, the Indigo IMPACT power supply also interfaces with the system backplane through a dedicated edge-style connector mounted on the rear of the PSU. This connection is critical because it distributes power directly to the motherboard, graphics boards, and option slots without requiring additional cabling.

Characteristics

• Form Factor: Custom multi-pin backplane connector integrated into the PSU housing.
• Function: Carries bulk +5 V, +12 V, and ground to the system plane, along with several control and sense lines.
• Load Sharing: Like the main harness, the backplane connector contains numerous parallel +5 V and ground contacts to accommodate the high current draw of Indigo² logic and graphics subsystems.
• Startup Dependence: The PSU will not energize without proper backplane connection, as several of the “power good” and enable signals are returned through this connector.

Repair Notes

Over time, these power supplies have passives and other parts that wear out. The following issues are most frequently observed:

Electrolytic Capacitors: Dried-out or leaking capacitors in the primary and secondary stages lead to unstable regulation, startup failures, or audible whining from the switching circuitry.

Startup Circuitry: The “ground to enable” startup logic (pin 11 on non-IMPACT units) is prone to failure when small control transistors or resistors drift out of tolerance. Symptoms include the PSU never starting or shutting down immediately after startup.

Power MOSFETs and Rectifiers: High-voltage MOSFETs in the switching stage and secondary rectifiers often fail short when stressed, particularly after capacitor degradation causes irregular loads.

Control ICs: Though less common, failures in the PWM and supervisory ICs result in unstable or absent output rails.

Repair Considerations

Proactive recapping (replacement of electrolytic capacitors) is strongly recommended before attempting to power on long-stored units.

Load testing on the +5 V rail should be performed after repair, as this rail carries the bulk of the Indigo² system load.