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©National Instruments Corp. 1-1 GPIB-PC User Manual
Section One - Operation of the GPIB
Communication between interconnected devices is achieved by passing
messages through the interface system.
Types of Messages
The GPIB carries two types of messages — device-dependent messages
and interface messages.
• Device-dependent messages, often called data or data messages,
contain device-specific information such as programming
instructions, measurement results, machine status, and data files.
• Interface messages manage the bus itself. They are usually
called commands or command messages. Interface messages
perform such functions as initializing the bus, addressing and
unaddressing devices, and setting device modes for remote or
local programming.
The term command as used here should not be confused with some
device instructions which can also be called commands. Such device-
specific instructions are actually data messages.
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
computer's CPU, but a better analogy is to the switching center of a city
telephone system.