James F. Mayer wrote:
How do I get an HP 8568 spectrum analyzer to reply to an HPIB command in
VB4? What command do I use? I am using an NI GPIB/TNT card.

If you have an NI card, you are allowed to download the full suite of
NI VISA & GPIB software and run Measurement & Automation Explorer. You
should be able to type commands to the instrument and see responses.
Don't know if the 8568 will respond to a standard *IDN? query, but
that's one thing to try.

Like Ken said make sure you are sending a newline \n, ascii 10 or hex
0a or control J at the end of your command or query. You also have to
configure your computer to use 0x0a as the terminating character on
receives otherwise it will keep waiting for characters until you hit
the timeout.

I use VB6 with the visa32 library for GPIB, and key points are the same
as for using the manual window in measurment & automation. The visa32
documentation has an example that worked well for me.

This assumes your cable and adapter are good and that you have set a
valid address on the analyzer.