How can you tell if a pcb is common ground?

I have a random 3rd party gamecube pcb i want to use for tvc but i dont know if its common ground or not. Trying to dual mod with my Cthulhu board (also need guidance on). thanks

Generally: look at the traces. A common ground pcb will have one strip of copper that’s connected to every other button + direction as well.

To be sure: take a wire and touch it to the ground wire where the plug connects to (should be a black wire, usually). Touch that wire to the “hot” sides of the buttons, and the buttons should trigger.

Take a multimeter, put it into continuity mode. It should beep audibly when you touch the two leads together. You can use the ohm setting if your multimeter doesn’t have an audible continuity mode.

Pin 3 is ground.

Use the continuity test to find the point where pin 3 connects to the PCB. This is ground on the PCB side. Now use that point on the PCB in continuity tests to find out if all of the button and directional inputs’ grounds are connected to it. At each point where a button/dpad direction is touched on the PCB, there will be a signal and a ground that are joined when the button is pressed. For a common ground PCB you should find a ground at each of these points that has continuity with your test point from pin 3.

Look up the pinout for that console, and pin the pin labelled Ground on the pinout. (on most, including the GC, the outermost metal shielding will be ground along with one of the pins.)

Use a multimeter to check for continuity between that ground on the end of the cable, and the pads on the pcb. If it’s common ground, one half of each button and direction is directly connected to ground.

Why not just add a Gamecube cable to you Cthulhu? If yours isn’t a multi-console version, the upgrade chip is pretty cheap and easy to swap in. Overall, less headache and work, and better support for playing on an arcade stick.

Edit: shakes fist at robokrikit

so the ground connects to the light green pcb part?

scratch that. Ok so i just busted out the multimeter and saw that when connected to pin 3, one side of the button goes beyond the omega sign and the other side of the button stops before 1. Would that mean its common?

also, where can i find just the mc chip for my cthulhu?

Click on the Cthulhu thread in Toodles signature. On the first post you can get the upgrade kit directly from him. I believe that it is just an IC and a set of diodes.

Basically, one side of every button and directional input should go past that omega (or infinite) sign, and the other half should either give off zero or slightly over. If more than half of them don’t, then your pad isn’t common ground.