#define Ap 10
#define Am 11
#define Bp 9
#define Bm 8

int stepCount = 0;
int stepsPerMove = 200;  // steps in one direction 400 steps for 360 rotation
int dir = 1;            // 1 = CW, -1 = CCW

void setup() {
  pinMode(Ap, OUTPUT);
  pinMode(Am, OUTPUT);
  pinMode(Bp, OUTPUT);
  pinMode(Bm, OUTPUT);
}

void loop() {
  int step = stepCount % 4;

  if(dir == 1)
    stepWave(step);         // CW
  else
    stepWave(3 - step);     // CCW (reverse order)

  stepCount++;
  delay(50);                // adjust speed

  if(stepCount >= stepsPerMove) {
    stepCount = 0;          // reset counter
    dir = -dir;             // reverse direction
  }
}

// Full-step wave drive (one coil at a time)
void stepWave(int step) {
  switch(step) {
    case 0: digitalWrite(Ap,HIGH); digitalWrite(Am,LOW);  digitalWrite(Bp,LOW); digitalWrite(Bm,LOW); break;
    case 1: digitalWrite(Ap,LOW);  digitalWrite(Am,LOW);  digitalWrite(Bp,HIGH); digitalWrite(Bm,LOW); break;
    case 2: digitalWrite(Ap,LOW);  digitalWrite(Am,HIGH); digitalWrite(Bp,LOW); digitalWrite(Bm,LOW); break;
    case 3: digitalWrite(Ap,LOW);  digitalWrite(Am,LOW);  digitalWrite(Bp,LOW); digitalWrite(Bm,HIGH); break;
  }
}