int speed1=11;
int dir1=10;
int dir2=9;
int Mspeed;
int jval;
int jpin=A0;
void setup() {
  // put your setup code here, to run once:
pinMode(speed1,OUTPUT);
pinMode(dir1 ,OUTPUT);
pinMode(dir2, OUTPUT);
pinMode(jval,INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
if (jval<512) {
digitalWrite(dir1, LOW);
digitalWrite(dir2,HIGH);
Mspeed=-255./512.*jval+255.;
analogWrite(speed1,Mspeed);
}
if (jval>=512){
digitalWrite(dir1, HIGH);
digitalWrite(dir2,LOW);
Mspeed=(255./512.)*jval-255.;
analogWrite(speed1,Mspeed);
}
}