HP 8753E Programming And Command Reference Manual page 29

This mode allows the analyzer to control peripherals directly in a
stand-alone environment (without an external controller). This mode
can only be selected manually from the analyzer's front panel. It can
only be used if no active computer or instrument controller is connected
to the system via HP-IB. If an attempt is made to set the network
analyzer to the system-controller mode when another controller is
connected to the interface, the following message is displayed on the
analyzer's display screen:
The analyzer must be set to the system-controller mode in order to
access peripherals from the front panel. In this mode, the analyzer can
1-22 HP-IB Progmmnu 'ng and Command Reference Guide