const int PLUS = 4;
const int ENABLE = 3;
const int MINUS = 2;


void setup()
{
  pinMode(PLUS, OUTPUT);
  pinMode(ENABLE, OUTPUT);
  pinMode(MINUS, OUTPUT);

  digitalWrite(ENABLE, LOW);
  digitalWrite(PLUS, HIGH);
  digitalWrite(MINUS, LOW);
  analogWrite(ENABLE, HIGH);
}

void loop()
{
  for (int i = 0; i < 256; i++)
  {
    forward(i);
    delay(10);
  }
  for (int i = 255; i > 0; i--)
  {
    forward(i);
    delay(10);
  }
  for (int i = 0; i < 256; i++)
  {
    backward(i);
    delay(10);
  }
  for(int i = 255; i > 0; i--)
  {
    backward(i);
    delay(10);
  }
}

void forward(int rate)
{
  digitalWrite(ENABLE, LOW);
  digitalWrite(PLUS, HIGH);
  digitalWrite(MINUS, LOW);
  analogWrite(ENABLE, rate);
}

void backward(int rate)
{
  digitalWrite(ENABLE, LOW);
  digitalWrite(PLUS, LOW);
  digitalWrite(MINUS, HIGH);
  analogWrite(ENABLE, rate);
}