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. GPIB, or General Purpose Interface Bus, instruments offer test and manufacturing engineers the widest selection of vendors and instruments for general-purpose to specialized vertical market test applications. GPIB instruments are often used as stand-alone benchtop instruments where measurements are taken by hand. You can automate these measurements by using a PC to control the GPIB instruments.

Fig.1 - Measurement and Automation Explorer

IEEE 488.1 contains information about electrical, mechanical, and functional specifications. 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 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 GPIB protocol categorizes devices as controllers, talkers, or listeners to determine which device has active control of the bus. Each device has a unique GPIB primary address between 0 and 30. The Controller defines the communication links, responds to devices that request service, sends GPIB commands, and passes/receives control of the bus. Controllers instruct Talkers to talk and to place data on the GPIB. You can address only onedevice at a time to talk. The Controller addresses the Listener to listen and to read data from the GPIB. You can address several devices to listen.

Data Transfer Termination

Termination informs listeners that all data has been transferred. You can terminate a GPIB data transfer in the following three ways:

• The GPIB includes an End Or Identify (EOI) hardware line that can be asserted with the last data byte. This is the preferred method.
  
• Place a specific end-of-string (EOS) character at the end of the data string itself. Some instruments use this method instead of or in addition to the EOI line assertion.
  
• The listener counts the bytes transferred by handshaking and stops reading when the listener reaches a byte count limit. This method is often a default termination method because the transfer stops on the logical OR of EOI, EOS (if used) in conjunction with the byte count. Asa precaution, the byte count on the listener is often set higher than the expected byte count so as not to miss any samples.
  

Data Transfer Rate

To achieve the high data transfer rate that the GPIB was designed for, you must limit the number of devices on the bus and the physical distance between devices.

You can obtain faster data rates with HS488 devices and controllers. HS488 is an extension to GPIB that most NI controllers support.