The General Purpose Interface Bus (GPIB)
The interface described by the IEEE 488 standard, which we will refer to as GPIB in this chapter, is used to connect instruments to test and measurement systems. Examples of such instruments are digital voltmeters, storage oscilloscopes, printers, and plotters. In general, these instruments are called GPIB devices. These devices operate under the coordination of a controller. Most modern systems consist of a group of such devices connected to one or more computers. In such a system, one of the computers will become the controller. Historically, the interface was developed by Hewlett-Packard in 1965. At the time, the interface was called HPIB, and there was no general standard. In 1975, it was formulated as IEEE 488 and called the IEEE Standard Digital Interface for Programmable Instrumentation. The standard specified the electrical, mechanical and hardware aspects, that is, the signals, their operation and purpose. Instrument manufacturers used the interface freely without adhering to a standard protocol for communicating with the instruments. Instruments intended for the same purpose, but made by different manufacturers, required widely varying commands. Some instruments made measurements in response to a command, while other instruments of a similar type made measurements without any command.
In addition, there were no agreed data formats between the instruments sending data and the instruments receiving data. This situation led to the development of an extension to the IEEE 488 standard. The new standard was published in 1987 and was IEEE 488.2 compliant devices must present data via the data formats and codes specified in the standard. The standard also specifies a minimum set of mandatory control commands or sequences and suggests some other optional commands. It also provides a standard status reporting template for instrument manufacturers to implement to make instrument status determination easier for instrument programmers. Although not yet a standard, the Standard Commands for Programmable Instrumentation (SCPI) drafted in 1990 agree on a standard set of commands for various categories of instruments. Consequently, all digital voltmeters made by different manufacturers will respond to the same GPIB command. This section describes the electrical and mechanical specifications of the GPIB interface, as well as the description of the signal and its purpose.
All GPIB devices use a special cable with each end connected to the male and female ends of the connector. This allows piggyback connections of cables. Devices can be daisy-chained (i.e. device B connected to device A, device C connected to device B, etc.) or in a star configuration (i.e. device A, B, C, etc. connected to a common node). A maximum of 15 devices can be connected to the bus. The maximum separation between two devices is 4m with an average separation of no more than 2m. At least two thirds of the connected devices must be turned on.
The GPIB cable consists of 24 wires. Eight of these lines are data lines, while three lines are used for the handshake. Another five lines are used for interface management and the remaining eight lines are ground lines. Between the ground lines are a cable shield line, a signal ground line, three ground return lines for the handshake signals, and another three ground return lines for three of the interface management lines. . All signals used are standard TTL signal levels with negative logic.
