exploring Ping Ultrasonic Range Finder

Hi all;

I successfully uploaded the code for Ping US sensor posted on the Arduino site, but I lose my serial port connection when I plug the SIG out on the sensor to Digital in #7 on the board. Any ideas around why this might be happening? It doesn’t happen with gnd or 5v, only SIG… it is serial port:

/dev/cu.usbserial-A6008m8d