SRF04 Ultrasonic Rangefinder Unresponsive

[the401tauntaun]

we are using the Devantech SRF04 Ultrasonic Rangefinder. We have sent a 5v pulse to the input pin, and verified this with a voltmeter. The power supply is working. We are not getting any feedback. The voltage from the "echo output" pin stays at zero. The code we are using to send this signal folows:

if(p1_sw_trig == 0)
{
rc_dig_out02 = 0;
}
else
{
rc_dig_out02 = 1;
}

We are reading the output from the rangefinder using both a voltmeter and printfs, and neither one shows any output. We expected a pulse at the falling edge of our signal, to signify that it was "listening" for echoes. Is there something obvious we are missing here? These rangefinders are a few years old, so it's possible (though unlikely) that they are just broken.