With regard to your issue, have you installed the NI-488.2
driver? Installing the driver must take care of this issue. If you
already have the driver installed, please make sure that it is the
latest version. You can find our versions <a
href="http://www.ni.com/support/gpib/versions.htm">here</a>.
Hope this helps! Please let us know if you the problem persists.
Thanks and Regards
ShekharS
Applications Engineer
National Instruments