Guts of my Hot Rodded 3D Printer

// by Les Hall
// started Mon Nov 17 2014
//

int controlPin = 2;
int coilPins[] = {0, 1, 4, 3};
int fullStep[][4] = {  // two bit Gray code
    {0, 1, 0, 1},  // - + - +
    {0, 1, 1, 0},  // - + + -
    {1, 0, 1, 0},  // + - + -
    {1, 0, 0, 1}}; // + - - +
int steps = 0;
long pos = 0;
long command = 0;


void setup()
{
  pinMode(controlPin, INPUT);
  for(int i=0; i<sizeof(coilPins); ++i)
    pinMode(coilPins[i], OUTPUT);

  // reset sets pos to zero
  pos = 0;
  command = 0;
}


void loop()
{
  // read the high and low pulse durations
  unsigned long pulseHigh = pulseIn(controlPin, HIGH, 20000);
  unsigned long pulseLow = pulseIn(controlPin, LOW, 20000);

  // set commanded position if the control period matches
  unsigned long T = pulseHigh + pulseLow;
  if ( (T > 9500) && (T < 10500) )  // check for frequency
    command = pulseHigh - pulseLow;

  // take a step toward the commanded position if not there yet
  if (pos < (command - 25) )
  {
    --steps;
    if (steps < 0) steps = 3;
    pos += 10;
  }
  else if (pos > (command + 25) )
  {
    ++steps;
    steps %= 4;
    pos -= 10;
  }

  // execute the next step
  for (int i=0; i<4; ++i)
    if (fullStep[steps][i] == 1)
      digitalWrite(coilPins[i], LOW);
    else
      digitalWrite(coilPins[i], HIGH);
}