Agilent Technologies 54621A Programmer's Manual page 36

Programming over GPIB
This section describes the GPIB interface functions and some general
concepts. In general, these functions are defined by IEEE 488.1. They
deal with general interface management issues, as well as messages
which can be sent over the interface as interface commands.
For more information on connecting the controller to the oscilloscope,
see the documentation for the GPIB interface card you are using.
The optional Agilent N2757A GPIB Interface Module must be connected
to the oscilloscope to allow programming over GPIB.
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