Saturday, March 12, 2005

General Purpose Interface Bus (GPIB)

The ANSI/IEEE Standard 488.1-1987, also known as General Purpose Interface Bus (GPIB), describes a standard interface for communication between instruments and controllers from various vendors, such as scanners and film recorders. It contains information about electrical, mechanical, and functional specifications. GPIB is a digital, 8-bit parallel communication interface with data transfer rates of 1 Mbyte/s and higher, using a three-wire handshake. 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.
GPIB instruments offer test and manufacturing engineers the widest selection of vendors and instruments for general-purpose to specialized vertical market test applications. GPIB instruments have traditionally been used as stand-alone benchtop instruments where measurements are taken by hand.
The GPIB is a 24-conductor parallel bus that consists of eight data lines, five bus management lines (ATN, EOI, IFC, REN, and SRQ), three handshake lines, and eight ground lines. The GPIB uses a byte-serial, asynchronous data transfer scheme. This means that whole bytes are sequentially handshaked across the bus at a speed that the slowest participant in the transfer determines. Because the unit of data on the GPIB is a byte, the messages transferred are frequently encoded as ASCII character strings.

All GPIB devices and interfaces must have a unique GPIB address between 0 and 30. Address 0 is normally assigned to the GPIB interface. The instruments on the GPIB can use addresses 1 through 30. 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. The GPIB VIs automatically handle the addressing and most other bus management functions.
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