Hi,
First , since it is a VISA error , are you sure your instrument
responds to "*idn?" in the correct way or "freq?"
If ok, with AG33120A, for frequency sweeping between 100 and 1000, you
can output on the GPIB bus a string argument of the following type:
FREQ:START 100;STOP 500;MODE SWEEP;:TRIG:SOUR IMM;:FREQ?
But if you are not familiar with the 33120A, try to set it up manually
and send a simple command TRIG:SOUR IMM;:FREQ? read the response and
see the effect ... Since waveform configuration is a subtle chapter of
the manual.
When it does what you want, try to find which string to output with the
manual.
To send the commands conveniently, you can use a direct dialog tool,
either from NI if you are familiar with "ib" functions or from the url:
www.ieee-488.com/ni_gpib0 ( here you will have to change temporarily
your gpib address from 10 to 1 with hp33120A I/O menu and back when
finished).
Good play ;=)
Regards,
FG