National Instruments GPIB-120A - Manual (Page 20)

Operation of the GPIB Appendix A
GPIB-120A User Manual A-2 © National Instruments Corporation
Talkers, Listeners, and Controller
There are three types of GPIB communicators: Talkers, Listeners, and Controllers. A Talker
sends data messages to one or more Listeners. The Controller manages the flow of information
on the GPIB by sending commands to all devices.
Devices can be Listeners, Talkers, and/or Controllers. A digital voltmeter, for example, is a
Talker and may be a Listener as well.
The GPIB is a bus like an ordinary 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 CPU of a computer, 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 addresses a Talker and a Listener before the Talker can send its message to the
Listener. After the message is transmitted, the Controller may unaddress both devices.
Some bus configurations do not require a Controller. For example, one device may always 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.
With a GPIB interface board and its software, your personal computer plays all three roles.
• Controller – to manage the GPIB
• Talker – to send data
• Listener – to receive data
The Controller-In-Charge and System Controller
Although there can be multiple Controllers on the GPIB, only one Controller at a time is active
or Controller-In-Charge (CIC). Active control can be passed from the current CIC to an idle
Controller. Only one device on the bus, the System Controller, can make itself the CIC. A GPIB
interface board in a computer is usually the System Controller.