This optional sideband signal functions as a “Power Good” indicator from the Add-in Card to the PSU. When this signal is asserted, the Add-in Card is indicating that local power rails on the Add-in Card are within their operating limits. This signal can provide a fault detection from the Add-in Cards to the PSU, which can allow the PSU a protection opportunity.
If this signal is used by the PSU, the signal will come from the Add-in Card as an open collector / open-drain fashion. In the Add-in Card, this signal will be tied to a 100kΩ pull-down resistor to ground. When implemented, the signal must be tied to a 4.7kΩ pull-up resistor to +3.3V at the power supply. When the PSU or system monitors the state of the CARD_PWR_STABLE signal it must be done with a high impedance 3.3V logic compatible device input.
The Add-in Card will set this signal to Open (high impedance) whenever its local power rails that are critical to correct operation are within their operating limits. When the Add-in Card directly drives this signal low (0 / Ground), any of its local power rails are outside of their operating limits results in a fault for the Add-in Card. The Add-in Cards must drive this signal low for at least 100 ms or as long as the input power stays outside of the voltage specification detailed in Table 3-5: Auxiliary Power Connectors Power Supply Rail Requirements - +12V Only in Section Section 3.2 of this document. When the voltage returns to within correct operating ranges and the fault is done, the signal will change to open (high impedance).
Support for this optional sideband signal does not rely on any of the other sideband signals defined for the 12VHPWR connector and can be implemented independently of other of these sideband signals.