Consider the GND, 3.3 volt and 9 volt wires for psx. The 9 volt from the psx goes to Vin on the regulator, Vout yields 5 volts to Vcc of the P360. Now you connect the GND for the regulator, P360 and pad together.
Add a DC pad into the mix. Now you connect the GND of DC to the GND of psx, and the same with the signal wires. But the power wires need a special way to merge, diodes. Now you have to go back and put a diode on the 5 volt right after the regulator. You’ll need a second one(part #: 1n4001 or 1n4004) for the DC. You’ll need to cut the 5 volt wire(blue for stock sega pads) and insert the diode there.
The way to wire it up is this:
-cut the 5 volt wire on the DC cord, put diode in series.
-install a diode on the output of the regulator, in series.
-now the free ends of the diodes get connected together and anything that needs 5 volt power taps into this point(P360 and DC pad).
that way both pads work when one pad is plugged in, but the P360 is off when only the DC pad plugged in. Hazzah, I just designed a smart stick for you.
BTW, diodes prevent curent from flowing backwards, so they must be installed in the correct direction. They all have a white or black strip off center on their body, the stripe points to GND i think…oops, not sure anymore of the polarity. You’re gonna have to try it one way before soldering.