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In Apple schematics for at least 2011 to 2015 boards, the "ISL Version" of "S0 Rail PGOOD Circuitry" is usually no-stuff (everything with BOM option S0PGOOD_ISL should be absent from a production board).

Instead, the "BJT Version" is used, as well as the bunch of resistors to the right of "ISL Version" (without BOM option S0PGOOD_ISL).

For example on 820-3332 (pictured here), ALL_SYS_PWRGD is pulled up by PP3V3_S0_PWRCTL (which is PP3V3_S0), and Q7950 can pull down ALL_SYS_PWRGD if one of its input signal is missing. Any of the other PGOOD signal going through R7963 to R7968 can also pull it down.

As for Q7950, all of the bases of the BJT (left part of Q7950) must be 0.6V-0.7V in order to pull low the gate of the MOSFET (right part of Q7950) so that it doesn't pull ALL_SYS_PWRGD to ground.

ALL_SYS_PWRGD on 820-3332