Review on Advanced Ultrasonic Distance Rangefinder Sensor by Virtuabotix by Shawn Quick

I have bought some ultrasonic parallax sensors that only have three pins. But they have four, one pin for the trigger and the other for the echo. My Arduino sketches were based on parallax sensors and I had to modify them to make Trig and Echo use an extra pin. At first my program didn't work, but then I realized that at least one of the pins must be on the PWM port. I believe this is an echo pin but I haven't tested it yet. Because currently I am using two pins on the ADK Mega board and two PWM ports. When I only had them on two data ports I got nothing. There is also a description and a sketch on the Arduino site that talks about how to connect two pins together and only use one port. If you really need a pin on a PWM connector, that means you need to connect them to the PWM if they are connected together. I hope this doesn't confuse anyone as I'm still a bit confused but I think I get it.