Connections
The PSU has an i2c port through which it reports status and takes commands. It contains two devices that responds on the i2c bus, the EEPROM and the PIC micro processor. Their addresses are set by the server by pulling up or down certain pins in the end connector. The EEPROM can be read and written using this bus. The MCU responds to commands sent to it. raplin (https://github.com/raplin/) has shown that it accepts fan speed commands, and reports fan speed back if queried. He has also documented several other commands and data.
The PSU has a double sided edge connector. The majority of which is taken up by the high current output tabs. There are also 6 smaller tabs on each side. These have various functions in turning on the PSU and selecting its i2c address.
The bottom part of the connector has pins 1 to 32.
The top one has 33 to 64.
Pinout:
1: +12V Power out
14: GND Power out
27, 28, 29: I2C slave address selection pins.
30: GND (for I2C and I2C slave address selection, I guess)
31: I2C SCL
32: I2C SDA
33: ENABLE#
34: LOAD SHARE
35: STATUS
36: PRESENT
37: +12V stand-by
38: PSALARM
The load share both both carry and read information. The output voltage is proportional to the output current. It reads information as other PSUs in parallell with it can pull this voltage up or down to signal how much help they want with power delivery. This can be utilized to regulate the current. The load share pin manipulates the output voltage to regulate current. It knows about the OVP limit as it does not increase voltage over that level.
3.2V means A-OK, 0V means error.
For a more in-depth look at the different pins' properties: (http://colintd.blogspot.com/2016/10/hacking-hp-common-slot-power-supplies.html)