int a = 0;
int spd=500; //0~511の値にする

void setup(){
  Serial.begin(9600);
  pinMode(1,OUTPUT); //信号用ピン
  pinMode(2,OUTPUT); //信号用ピン
  motor_stop();
}

void motor_stop(){
  digitalWrite(1,LOW);
  digitalWrite(2,LOW);
  analogWrite(3,0);
  digitalWrite(13,HIGH);
}

void motor_go(){
  digitalWrite(1,LOW);
  digitalWrite(2,HIGH);
  analogWrite(3,spd);
}

void motor_go2(){
  digitalWrite(1,LOW);
  digitalWrite(2,HIGH);
  analogWrite(3,spd/2);
}

void loop(){
  if(Serial.available()>0){//データの読み込み
    a=Serial.read();
    switch(a){
      case 'a':
        motor_go();
        delay(1000);
        break;

      case 'b':
        motor_go2();
        delay(1000);
        break;
    }
  }
}