GPIB-Anwendungen

A bus system for Test and Measurement applications

The GPIB success story began at the end of the 1960's when Hewlett Packard invented the bus. The intention was to create a reliable bus system especially designed for connecting computers and instruments.

This networked system has all features that are required to create a measurement system. Remote control of instruments is one aspect of these features, but there are other more important features like data handshake for reliable operation and real-time response capability, only to name a few.

History

The original bus system, designed by Hewlett Packard (today the Test & Measurements divisions name is Agilent Technologies), was called HPIB, a short form for Hewlett Packard Interface Bus.

Because of its success and proven reliability, in 1973 the HPIB bus became an US standard, introduced by the IEEE. Since then, the name has been GPIB, for General Purpose Interface Bus. The standards number is IEEE 488.1.

In parallel, the International Electronic Commission (IEC), responsible for the world-wide standardization, approved the standard and called it IEC 625.1. Due to the introduction of a new naming scheme for all standards, it was renamed to IEC 60625.1 later.

There was a slight difference between the IEEE 488.1 and IEC 625.1: The IEC 625.1 standard used a 25 pin DSUB connector for the bus, the IEEE 488.1 standard favored a Centronics-like 24 pin connector. Today, the 24 pin connector is always used, but there are also adaptors available in case older instruments are equipped with a 25 pin DSUB connector.

For these historical reasons, there exist several names for the same interface.

Bus features

  • Up to 15 instruments, called devices, can be connected to a computer, usually called controller, because it is responsible for coordinating bus traffic.
  • Specified transfer rate: 1 MBytes per second
  • Cable length: 20m between controller and a one device or 2m between each device, whichever is less.
  • Data transfer mode: 8 bit parallel
  • Handshake: so called '3 wire handshake', reception of each data byte is acknowledged
  • Additional control lines for special needs exist:
    • SRQ - Service ReQuest is used for notification of the controller that a device needs servicing.
    • IFC - InterFace Clear provides a reset function.
    • REN - Remote ENable disables the device's front-panel operation. After assertion of REN, Device is controlled by controller
    • EOI - End Or Identify is primarily used as termination indicator in data transfers.
    • ATN - ATtentioN, is a means of the controller to distinguish between data transfer and control messages.

Related standards

The '.1' extension of IEEE 488.1 / IEC 60625.1 indicates that there are several layers of interface standards. In fact, there is a whole 'family' of standards:

  • IEEE 488.1 / IEC 60625.1 defines the physical layer of the bus system.
  • IEEE 488.2 / IEC 60625.2 is not a revision of the '.1' standard, but it extends its functionality: A command language (syntax) is defined and common properties of instruments are defined. Same command names result in similar actions. In contrast to the '.1' standard that defines physical means like cables, timing and so on, the '.2' standard focuses on the instrument model.
  • An application of IEEE 488.2 / IEC 60625.2 is IEEE 1174. It is currently adopted. To say short, it translates GPIB functionality to a serial RS232 line, of course without networking capability. It is intended for low cost instruments.

Todays status

Today, GPIB interfaces are found in nearly all instruments in the USD 1000$ class and above. Low cost instruments use RS232 interfaces. The GPIB standard has been established as the world standard in instrument connectivity. With millions of instruments in the field, the GPIB every day proves to meet the user's expectations for reliable and standardized Test and Measurement systems.

More information

More information on the standards IEC 60625.1 and IEC 60625.2 can be obtained directly at the IEC. The IEEE 488 standards are distributed by the IEEE Standards Association.