Appendix A
GPIB Basics
GPIB-120B User Manual
A-2
ni.com
Talkers, Listeners, and Controllers
GPIB devices can be Talkers, Listeners, or Controllers. A Talker sends out
data messages. Listeners receive data messages. The Controller, usually a
computer, manages the flow of information on the bus. It defines the
communication links and sends GPIB commands to devices.
Some devices are capable of playing more than one role. A digital
voltmeter, for example, can be a Talker and a Listener. If your system has a
National Instruments GPIB interface and software installed, it can function
as a Talker, Listener, and Controller.
The GPIB is like a typical computer bus, except that the typical computer
has circuit cards interconnected via a backplane bus, whereas the GPIB has
standalone devices interconnected via a cable bus.
The role of the GPIB Controller is similar 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 the 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