How to decide the port of RS232 ?

[whuili]

Hi, I am using RS-232 port to control several instruments and I have a
card on which there are more than 3 RS-232 ports. Each port connects
with one instruments. When I program in Labiew, how can I decide which
port is for which instrument? Moreover, can any one provide an example
Labview program for RS-232? Thanks a lot.