Controller - HP 8753E Programming And Command Reference Manual

The network analyzer is a listener when it is controlled over the bus by
a system controller.

Controller

A controller is defined as a device capable of:
1. managing the operation of the bus
2. addressing talkers and listeners
There can be only one active controller on the interface at any time.
Examples of controllers include desktop computers, minicomputers,
workstations, and the network analyzer. In a multiple-controller
system, active control can be passed between controllers, but there can
only be one system controller connected to the interface. The system
controller acts as the master and can regain active control at any time.
The analyzer is an active controller when it plots, prints, or stores to
an external disk drive in the pass-control mode. The analyzer is also a
system controller when it is operating in the system controller mode.
HP-IB Programmu 'ng and Command Reference Guide