System Controller And Active Controller - National Instruments GPIB-MAC User Manual

November 1985
Devices can be Talkers, Listeners, and/or Controllers. A digital
multimeter, for example, is a Talker and may also be a Listener. A
printer or plotter is usually only a Listener. A computer on the
GPIB
will often combine all three roles to manage the bus and
communicate with other devices.
The GPIB is a bus like a typical computer bus except that the
computer has its circuit cards interconnected via a backplane bus
whereas the GPIB has standalone devices interconnected via a cable
bus.
The role of the GPIB Controller can also be compared to the role of
the computer's CPU, but a better analogy is to the switching center
of a city telephone system.
The switching center (Controller) monitors the communications
network (GPIB). When the center (Controller) notices that a party
(device) wants to make a call (send a data message), it connects the
caller (Talker) to the receiver (Listener).
The Controller usually addresses a Talker and a Listener before the
Talker can send its message to the Listener. After the message is
transmitted, the Controller usually unaddresses both devices.
Some bus configurations do not require a Controller. For example,
one device may only be a Talker (called a Talk-only device) and
there may be one or more Listen-only devices.
A Controller is necessary when the active or addressed Talker or
Listener must be changed The Controller function is usually
handled by a computer.

System Controller and Active Controller

Although there can be multiple Controllers on the GPIB, only one
Controller at a time is Active Controller or Controller-in-Charge
(CIC). Active control can be passed from the current Active
Controller to an idle Controller. Only one device on the bus, the
System Controller, can make itself the Active Controller.
108 APPENDIX D - OPERATION OF THE GPIF3