void setup() {
  Serial.begin(115200);
  pinMode(3, OUTPUT); 
  pinMode(5, OUTPUT); 
}

void loop() {
  int c;
  c = Serial.read();
  if (c == 'P') {
   if (Serial.read() == 'C') {
     c = Serial.read();
     if (c == 'S')  {
       int val = Serial.parseInt();
       if (val == 0) {
         analogWrite(3, 255);
         analogWrite(5, 255);
       }
       else {
         int pwm = abs(val) * 255 / 100;
         if (val < 0) {
           analogWrite(3, 0);
           analogWrite(5, pwm);
         }
         else {
           analogWrite(5, 0);
           analogWrite(3, pwm);
         }
       }
     }
   }
  }
  delay(100);
}