I'm using a Hagisonic ultrasonic sensor and need help taking readings from it. The data sheet is found here (http://www.robotshop.us/content/PDF/hagisonic-anibat-data-sheet.pdf). I would like to be able to code a program in C or C++ and tell the arduino to ask for a reading from one of the 4 sensors I plan on having attached. I understand that 2 digital pins are needed per sensor, 1 for trigger and 1 for echo. How do I go about this? Sorry I'm a complete beginner! I have a standard Arduino board like the one seen here (http://www.sparkfun.com/commerce/product_info.php?products_id=666). I just need help with getting the initial reading. THANKS!
That's probably the least helpful datasheet I think I've ever seen (and I've seen a lot).
It tells you an awful lot, yet tells it in a very unuseful manner.
You need a couple of pins (lets say 2 & 3).
Declare 2 as an output, and 3 as an input.
Initialise the output pin LOW.
In "loop()", send 2 HIGH, record the time using "micros" and then wait for 2 milliseconds, then send pin 2 LOW.
Wait until pin 3 goes high, and record the time again using "micros".
Subtract the start time from the end time, and voila, a time proportional to the range.
I'd say AWOL summed it up well. I suggest setting pin 2 low after the time measurement is done. Because the transmission pulse is driven by a positive leading edge of pin 2, you can go ahead and begin the timer and worry about resetting the pin after the measurement is made. Setting it low again before starting the timer means you will be wasting processing time setting the pin low... meanwhile the ping is already in flight. Thus you will have an offset in the range measurement the magnitude of which being the time it takes to set the pin low. Of course this is a speed of sound application (slow) and setting a pin state takes nanoseconds... so maybe I'm being to anal... 
Thanks guys, that makes a lot of sense and helps clear up the problem a lot. Is there any good projects online that use the Arduino the way I would like to, or any good documentation of commands? Thanks for all the help already I'm understanding this project more than I thought I would!
With respect to the Hagisonic sensor only? There's a great example in the examples>sensors>ping sketch that comes with the Arduino IDE. But it's for the Ping sensor...
This sensor works the exact same way my sensor does, so this example is perfect. Thank you very much. I'm guessing that the host machine communicates via the USB connection? It doesn't say specifically, but is that correct. Also does the code from this example get loaded and ran on the Arduino? If so is there a way to call the program to start and stop?