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Active controller
Device
Listener
Talker
There is always one system controller in a GPIB system. The system
controller is designated at system initialization either through the use of
hardware switches or by some type of configuration software, and is not
changed. The system controller can be the same controller as the one which
is the current active controller or an entirely different one. Note that if a
controller is both a system controller and the active controller and it passes
control to another controller, the system controller capability is not passed
along with it.
The active controller is the controller which has the ability to mediate all
communications which occur over the bus. In other words, the active
controller designates (addresses) which device is to talk and which devices
are to listen. The active controller is also capable of relinquishing its
position as active controller and designating another controller to become
the active controller.
A device is any IEEE-488 instrument which is not a system controller or
active controller. It can be idle or act as a talker and/or listener when
addressed or unaddressed by the active controller.
A listener is any device which is able to receive data when properly
addressed. There can be up to 14 active listeners on the bus concurrently.
Some devices can also be a talker or controller; however, only one of these
functions can be performed at a time.
A talker is a device which can transmit data over the bus when properly
addressed. Only one device can transmit at a time. Some devices can also
be a listener or controller; however, only one of these functions can be
performed at a time.
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