© National Instruments Corp.
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GPIB-140 User Manual
Appendix A
Operation of the GPIB
This appendix describes some basic concepts you should understand to operate the GPIB.
It also contains a description of the physical and electrical characteristics and the
configuration requirements of the GPIB.
Types of Messages
Communication among interconnected GPIB devices is achieved by passing messages
through the interface system. The GPIB carries device-dependent messages and interface
messages.
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Device-dependent messages, often called data or data messages, contain
device-specific information such as programming instructions, measurement results,
machine status, and data files.
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Interface messages manage the bus itself. They are usually called commands or
command messages. Interface messages perform such tasks 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 stand-alone 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).
