In a very general way, communication through RS-232 is quite simple:
1. Open and configure the port
2. Write a request to some external device
3. Wait the appropriate time for the device to answer to the request
4. Read the answer from the serial port or decide what to do in case a response is not arrived
5. At the end of all or at the end of the program close the serial port opened
Actual implementation differs depending on the development environment used (LV, CVI, Visual Basic, C++...): to be more specific we need to know which is yours.
Basic problem of communicating with the device is to study the protocol used by the device itsel: it's your program that must adapt to it, and if the syntax is incorrect either the response will be some wa
rning message or nothing at all.
Going a little further, for some instruments there are drivers already made by the manufacturer or third party.
Again, if we know the device you need to communicate to maybe someone can help you indicating a specific driver.
Roberto