unsigned int EchoPin = 2;           // connect Pin 2(Arduino digital io) to Echo/RX at US-100
unsigned int TrigPin = 3;           // connect Pin 3(Arduino digital io) to Trig/TX at US-100
unsigned long Time_Echo_us = 0;
unsigned long Len_mm  = 0;
void setup()
{  //Initialize
     Serial.begin(9600);                        //Serial: output result to Serial monitor
     pinMode(EchoPin, INPUT);                    //Set EchoPin as input, to receive measure result from US-100
     pinMode(TrigPin, OUTPUT);                   //Set TrigPin as output, used to send high pusle to trig measurement (>10us)
}

void loop()
{
     digitalWrite(TrigPin, HIGH);              //begin to send a high pulse, then US-100 begin to measure the distance
     delayMicroseconds(50);                    //set this high pulse width as 50us (>10us)
     digitalWrite(TrigPin, LOW);               //end this high pulse
      
     Time_Echo_us = pulseIn(EchoPin, HIGH);               //calculate the pulse width at EchoPin,  
     if((Time_Echo_us < 60000) && (Time_Echo_us > 1))     //a valid pulse width should be between (1, 60000).
     {
       Len_mm = (Time_Echo_us*34/100)/2;      //calculate the distance by pulse width, Len_mm = (Time_Echo_us * 0.34mm/us) / 2 (mm)
       Serial.print("Present Distance is: ");  //output result to Serial monitor
       Serial.print(Len_mm, DEC);            //output result to Serial monitor
       Serial.println("mm");                 //output result to Serial monitor
     }
     delay(1000);                            //take a measurement every second (1000ms)
}

unsigned int HighLen = 0;
unsigned int LowLen  = 0;
unsigned int Len_mm  = 0;
void setup()
{   //connect RX (Pin 0 of Arduino digital IO) to Echo/Rx (US-100), TX (Pin 1 of Arduino digital IO) to Trig/Tx (US-100)  
     Serial.begin(9600);  //set baudrate as 9600bps.
}

void loop()
{
     Serial.flush();     // clear receive buffer of serial port
     Serial.write(0X55); // trig US-100 begin to measure the distance
     delay(500);          //delay 500ms to wait result
     if(Serial.available() >= 2)                    //when receive 2 bytes  
     {
         HighLen = Serial.read();                   //High byte of distance
         LowLen  = Serial.read();                   //Low byte of distance
         Len_mm  = HighLen*256 + LowLen;             //Calculate the distance
         if((Len_mm > 1) && (Len_mm < 10000))        //normal distance should between 1mm and 10000mm (1mm, 10m)
         {
             Serial.print("Present Length is: ");   //output the result to serial monitor
             Serial.print(Len_mm, DEC);             //output the result to serial monitor
             Serial.println("mm");                  //output the result to serial monitor
         }
     }
     delay(500);                    //wait 500ms
}