© National Instruments Corp.
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NI-488.2M UM for Windows NT
Chapter 1
Introduction
This chapter gives an overview of GPIB and the NI-488.2M software.
GPIB Overview
The ANSI/IEEE Standard 488.1-1987, also known as GPIB (General Purpose Interface
Bus), describes a standard interface for communication between instruments and
controllers from various vendors. It contains information about electrical, mechanical,
and functional specifications. The GPIB is a digital, 8-bit parallel communications
interface with data transfer rates of 1 Mbytes/s and above, using a 3-wire handshake. The
bus supports one System Controller, usually a computer, and up to 14 additional
instruments. The ANSI/IEEE Standard 488.2-1987 extends IEEE 488.1 by defining a bus
communication protocol, a common set of data codes and formats, and a generic set of
common device commands.
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 personal computer has a National
Instruments GPIB interface board and NI-488.2M software installed, it can function as a
Talker, Listener, and Controller.
Controller-In-Charge and System Controller
You can have multiple Controllers on the GPIB, but only one Controller at a time can be
the active Controller, or Controller-In-Charge (CIC). The CIC can either be active or
inactive (Standby) Controller. Control can pass from the current CIC to an idle
Controller, but only the System Controller, usually a GPIB interface board, can make
itself the CIC.
