#define pinA  2
#define pinB  4

volatile int counts = 0;

void setup(){
  pinMode(pinA, INPUT);
  pinMode(pinB, INPUT);
  digitalWrite(pinA, HIGH);       // turn on pull-up resistor
  digitalWrite(pinB, HIGH);       // turn on pull-up resistor

  attachInterrupt(0, doEncoder, RISING);  // encoder pin on interrupt 0 - pin 2
  Serial.begin(9600);
  Serial.println("start");                // a personal quirk
}

void loop(){
  Serial.println(counts, DEC);
}

void doEncoder(){
  if(digitalRead(pinB)){
    counts++;
  }else{
    counts--;
  }
}