Using Particle Photon to control an arcade game

Troubleshooting
3

While the JAMMA docs are a bit unclear here

I suppose @indraastra is right and it should be "GROUND signal".

So you'd rather want to pull active LOW and since the Photon GPIO HIGH level is 3.3V I would suggest to set the pin high impedance (INPUT) when not triggered.

void setup()
{
  ...
  pinMode(D6, INPUT);
}

int credit(String args)
{
  Serial.println(args);
  pinMode(D6, OUTPUT);
  digitalWrite(D6, LOW);
  delay(200);
  pinMode(D6, INPUT);
  return 1;  
}
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