arduino due + parallax Ping-sensor

hallo
hab seit einigen tagen einen arduino due.

  1. tests ok
    da ich von parallax-propeller-mc komme, habe ich einige sensoren, unteranderem einen
    ping - sensor. mit 3 pins (+,gnd,signal)
    heute wollte ich mal mein 1. sketch fuer den ping schreiben (ist uebertrieben, eher copy and paste)
    compile sketch ok
    run: mit seriellen monitor auch ok, aber output ist 0in, 0cm
* Ping))) Sensor
 * dd. 03.05.2013
 * name: sketch-due-ping1.txt
 * 
 * Ref. it's for the arduinoDue
 *      changing from the orig.digitalPin 07 to
 *      due digitalPin22
  
   This sketch reads a PING))) ultrasonic rangefinder and returns the
   distance to the closest object in range. To do this, it sends a pulse
   to the sensor to initiate a reading, then listens for a pulse 
   to return.  The length of the returning pulse is proportional to 
   the distance of the object from the sensor.
     
   The circuit:
	* +V connection of the PING))) attached to +5V
	* GND connection of the PING))) attached to ground
	* SIG connection of the PING))) attached to digital pin 22

   http://www.arduino.cc/en/Tutorial/Ping
   
   created 3 Nov 2008
   by David A. Mellis
   modified 30 Aug 2011
   by Tom Igoe
 
   This example code is in the public domain.

 */

// this constant won't change.  It's the pin number
// of the sensor's output:
const int pingPin = 22 // old:7;

void setup() {
  // initialize serial communication:
  Serial.begin(9600);
}

void loop()
{
  // establish variables for duration of the ping, 
  // and the distance result in inches and centimeters:
  long duration, inches, cm;

  // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);

  // The same pin is used to read the signal from the PING))): a HIGH
  // pulse whose duration is the time (in microseconds) from the sending
  // of the ping to the reception of its echo off of an object.
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  // convert the time into a distance
  inches = microsecondsToInches(duration);
  cm = microsecondsToCentimeters(duration);
  
  Serial.print(inches);
  Serial.print("in, ");
  Serial.print(cm);
  Serial.print("cm");
  Serial.println();
  
  delay(100);
}

long microsecondsToInches(long microseconds)
{
  // According to Parallax's datasheet for the PING))), there are
  // 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
  // second).  This gives the distance travelled by the ping, outbound
  // and return, so we divide by 2 to get the distance of the obstacle.
  // See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
  return microseconds / 74 / 2;
}

long microsecondsToCentimeters(long microseconds)
{
  // The speed of sound is 340 m/s or 29 microseconds per centimeter.
  // The ping travels out and back, so to find the distance of the
  // object we take half of the distance travelled.
  return microseconds / 29 / 2;
}

2.sketch

const int signal=22;
int distance;
unsigned long pulseduration=0;

void setup()
{
 pinMode(signal, OUTPUT);
 Serial.begin(9600);
}

void measureDistance()
{
 // set pin as output so we can send a pulse
 pinMode(signal, OUTPUT);

// set output to LOW
 digitalWrite(signal, LOW);
 delayMicroseconds(5);

 // now send the 5uS pulse out to activate Ping)))
 digitalWrite(signal, HIGH);
 delayMicroseconds(5);
 digitalWrite(signal, LOW);

 // now we need to change the digital pin
 // to input to read the incoming pulse
 pinMode(signal, INPUT);

 // finally, measure the length of the incoming pulse
 pulseduration=pulseIn(signal, HIGH);
}

void loop()
{
 // get the raw measurement data from Ping)))
 measureDistance();

 // divide the pulse length by half
 pulseduration=pulseduration/2; 

 // now convert to centimetres. We're metric here people...
 distance = int(pulseduration/29);

 // Display on serial monitor
 Serial.print("Distance - ");
 Serial.print(distance);
 Serial.println(" cm");
 delay(500);
}

beide sketches laufen, hab im augenblick keine ahnung warum der output immer 0 ist?
fuer hinweise und hilfe waere ich dankbar
gruesse nomad