const int DriverA_1 = 6;
const int DriverA_2 = 7;
const int DriverB_1 = 8;
const int DriverB_2 = 9;
char key;
char S = 's';
char C = 'c';

void setup() {
  pinMode(DriverA_1, OUTPUT);
  pinMode(DriverA_2, OUTPUT);
  pinMode(DriverB_1, OUTPUT);
  pinMode(DriverB_2, OUTPUT);
  Serial.begin(115200);
}

void loop() {
  if (Serial.available()) {
    key = Serial.read(); 
    if(key == S){
      Serial.print("Start!");
      Serial.println("");
      motor(HIGH, LOW, 3000);
    
    } else if(key == C){
      Serial.print("Stop!");
      Serial.println("");
      motor(LOW, LOW, 3000);
    
    }
  }
}

void motor(int a1, int a2, int d){    
  digitalWrite(DriverA_1, a1);
  digitalWrite(DriverA_2, a2);
  digitalWrite(DriverB_1, a1);
  digitalWrite(DriverB_2, a2);
  delay(d);
}