You might try the set of examples located in the Developer Exchange for the GPIB-232CT. They can be found by typing in GPIB-232CT in the search window on the left of the Developer Exchange web area. In the serach pulldown, select NI Developer Zone. One of the results should be "GPIB-232CT and GPIB-485CT configurator and testor." This link contains three vi's that test whether or not communication is working with your instrument in either G or S mode (and it was written in 5.1).
Hope this helps!
Luke Schreier
National Instruments