Do you need to send a termination character such as a line feed for carriage return along with the command of [SN] ?
You have a lot of coercion dots on your block diagram unnecessarily because of using wrong datatypes. It don't think it is causing your issue, but it shows you are new at LabVIEW and don't know the easy way of creating Constants, Controls, and Indicators of the correct data types.
Disconnect any wire with a red dot on it. Right click on the terminals of the subVI Serial Configure, and pick Create Constant. Now you'll have the correct data type.
For the VISA REad and Write functions, you have them going to a Double Indicator even though the wire is blue, an integer. Delete those indicators. Right-click on the functions output terminal and pick Create Indicator. Voila! An indicator with the datatype that matches the function and no red coercion dot!
Don't forget to wire the Error wires on those VISA funtions. Are any of them giving you a timeout error?
Have you checked your parity settings? (Hard to know what your parity is since it is a constant rather than an enum). Is the RS-232 cable good?
