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Indy Power Supplies

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SGI Indy with a Sony Power Supply

Overview

SGI Indy workstations shipped with two different power supply variants, both rated at approximately 170W. Early Indys received a Nidec power supply, while later machines were outfitted with a Sony model with similar output specifications. The Nidec models have long been considered the less reliable PSU variant, although as of 2019 Sony PSU failures appeared to becoming more common. All Indy power supplies also contain the system's internal speaker as well as power and reset switches connected to a small logic board.

Both power supplies included the Indy's sole cooling fan, a 92mm exhaust fan. The fan, which operates at variable speed, is mounted at an angle and pulls air through the machine and power supply before blowing it out the rear of the unit. At least for Sony PSUs the fan was a Panaflo FBA09A12H 12-volt fan. In the Nidec variant, the fan is always on although its speed increases when the computer becomes hot. The fan in the Sony, however, comes to a complete stop at low system temperatures. This might contribute to failures of the Sony PSU when the machine is operated without the cover, thereby allowing the CPU and other components to shed heat through convective cooling, which can keep the PSU fan from operating at all, while temps inside the closed PSU increase.

Due to the failure rate among Indy PSUs, as of mid-2019 user "Elf" of the Silicon Graphics Users Group began to explore detailed operation of Indy power supplies and prepare to produce modern replacements using off-the-shelf DC-DC converters.

Output Specifications

Sony PSU with top cover removed, showing internal fan, speaker, power connector, and auxiliary connector.

Both Sony and Nidec PSUs are rated at approximately 170 Watts (171 and 170 respectively). While this is a seemingly low output, both power supplies are rated to deliver substantial power on the +5V rail (25A/125W), which is uncommon for modern ATX PSUs, even those rated for much greater total output power.

Power Supply Type +3.3V +5V +5V Standby +12V -12V
Sony 7A 25A .02A 4.5A .75A
Nidec 7A 25A .001A 4.5A .75A

Based on the very low Nidec +5V standby rating and considering that both PSUs are interchangeable on all Indy systems, it is likely that nothing substantial on the motherboard is powered during standby. It has been speculated that at most extremely minimal logic in combination with the power button is all that remains powered when the system is off.

Connectors and Pin-outs

The Indy PSU delivers power to the motherboard via a 20-pin Molex mini-fit Jr. type connector. The PSU has the female connector, consistent with electrical norms. Power delivery pinouts corresponding to the mechanical drawing are listed in the below table

Indy PSU Power Connector
Pin # Wire Color Function
1 White +3.3V
2 White +3.3V
3 Red +5V
4 Red +5V
5 Red +5V
6 Red +5V
7 Red +5V
8 Red +5V
9 Green +5V Standby
10 Light Blue -12V
11 Black 0V
12 Black 0V
13 Black 0V
14 Black 0V
15 Black 0V
16 Black 0V
17 Yellow +12V
18 Black 0V
19 Black 0V
20 Black 0V

Auxiliary connections, including power switches, LEDs, speaker connections, etc., are provided by a 20 position 0.1" spacing (2x10) pin header, with the motherboard also housing the male connector. Pin 3 controls operation of the power supply; when the normal +5V signal on this pin is grounded the power supply turns off. Pinouts, corresponding to the mechanical drawing to the right, are listed in the below table.

Indy PSU Auxiliary Connector
Pin # Wire Color Function
1 No Connection
2 Brown Temperature sensor control voltage (0-5V)
3 Red/White Run (ground to stop)
4 Orange Power good signal
5 No Connection
6 No Connection
7 No Connection
8 No Connection
9 Gray Power switch (other leg to switch common)
10 White Volume up switch (other leg to switch common)
11 Blue/White Volume down switch (other leg to switch common)
12 Periwinkle Reset switch (other leg to switch common)
13 Black Switch common
14 Red LED common (+/anode)
15 Violet Red LED (-/cathode)
16 Violet/White Green LED (-/cathode)
17 Orange/White Speaker terminal 1
18 Green/White Speaker terminal 2
19 No Connection
20 No Connection

Temperature and Fan Curves

The temperature signal for the Indy PSU appears to originate from a thermistor on the Indy CPU module. The PSU uses the variable voltage from the thermistor and associated circuitry to change the fan speed by changing the drive voltage (i.e. not/not by using a pulse width modulation signal common in modern variable speed computer fans). Empirical observations of one sample each of a Sony and Nidec power supply produced the attached graph of temperature signal voltage vs. fan voltage.

Indy PSU Temperature Signal vs. Fan Voltage


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