#define a1 5
#define a2 6
#define b1 4
#define b2 3
void setup() {
  // put your setup code here, to run once:
  pinMode(a1,OUTPUT);
  pinMode(a2,OUTPUT);
  pinMode(b1,OUTPUT);
  pinMode(b2,OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  stepperDrive(200);
}

void stepperDrive(int T){
  digitalWrite(a1,HIGH);
  delayMicroseconds(T/4);
  digitalWrite(a2,HIGH);
  delayMicroseconds(T/4);
  digitalWrite(b1,HIGH);
  delayMicroseconds(T/4);
  digitalWrite(b2,HIGH);
  delayMicroseconds(T/4);
  digitalWrite(a1,LOW);
  delayMicroseconds(T/4);
  digitalWrite(a2,LOW);
  delayMicroseconds(T/4);
  digitalWrite(b1,LOW);
  delayMicroseconds(T/4);
  digitalWrite(b2,LOW);
  delay(T/4);
}