© National Instruments Corporation A-1 NI-488.2 User Manual
AGPIB Basics
The ANSI/IEEE Standard 488.1-2003, also known as the General Purpose
Interface Bus (GPIB), describes a standard interface for communication
between instruments and controllers from various vendors. It contains
information about electrical, mechanical, and functional specifications.
GPIB is a digital, 8-bit parallel communications interface that supports
both interlocked and noninterlocked handshaking. The interlocked
handshake, also known as three-wire handshake, allows for data transfer
rates of 1 Mbyte/s and higher. The noninterlocked handshake, also known
as HS488, allows for data transfer rates up to 8 Mbytes/s. The bus supports
one System Controller, usually a computer, and up to 14 additional
instruments. The ANSI/IEEE Standard 488.2-1992 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 system
has a National Instruments GPIB interface and 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 be either active or inactive (standby). Control can pass from the current
CIC to an idle Controller, but only the System Controller, usually a GPIB
interface, can make itself the CIC.