HP Interface Bus

HP Interface Bus. The HP Interface Bus, originally developed by Hewlett-Packard in the late 1960s, was a pioneering digital communications standard for interconnecting electronic test and measurement instruments with controllers like minicomputers. Formally standardized as IEEE 488 by the Institute of Electrical and Electronics Engineers in 1975, it became ubiquitous in laboratories and industrial automation, enabling the creation of automated test systems. Its design philosophy emphasized simplicity and reliability, fostering a new era of programmable instrumentation that moved beyond manual control.

Overview and History

The development of the HP Interface Bus, later known as HP-IB, was driven by the growing complexity of electronic measurement tasks in the 1960s. Engineers at Hewlett-Packard, led by figures involved in the company's renowned instrument division, sought a standardized method to connect devices like digital voltmeters, frequency counters, and power supplies to emerging computing controllers. This effort paralleled other early interconnection schemes but was distinguished by its focus on the test bench environment. Following its successful internal adoption, Hewlett-Packard advocated for its standardization, leading to its approval as IEEE 488 in 1975, a move endorsed by organizations like the American National Standards Institute. The standard's publication catalyzed its adoption across the electronics industry, with manufacturers like Tektronix, Fluke Corporation, and Keithley Instruments implementing it in their products, solidifying its role in automated test equipment (ATE).

Technical Specifications

The HP Interface Bus is a parallel, byte-serial digital interface that operates over a 24-conductor cable and connector system. It defines 16 signal lines: eight are used for bidirectional data transfer, three for handshake control, and five for general interface management. The bus supports a daisy chain topology, allowing up to 15 devices to be connected within a total cable length limit of 20 meters. Electrical specifications originally utilized TTL logic levels, with data transfer rates typically reaching up to 1 megabyte per second. The physical layer was designed for robustness in laboratory settings, with connectors featuring a secure locking mechanism. The bus employs a party-line structure where all devices share the same set of signal lines, with one device acting as the system controller, often a computer like the HP 9830 or later the HP 9000 series.

Operation and Protocol

Communication on the HP Interface Bus is governed by a strict protocol that defines devices as Listeners, Talkers, or Controllers. A device, such as a signal generator, might be addressed to Listen to receive programming commands, while a digital multimeter might be addressed to Talk to send measurement data. The system controller, typically a computer running software like HP BASIC, manages these addresses and coordinates data flow using the three-wire handshaking protocol. This asynchronous handshake, involving lines like NRFD (Not Ready For Data) and NDAC (Not Data Accepted), ensures reliable byte transfer between devices of differing speeds. The protocol allows for both uniline and multiline commands, enabling complex instrument setups and data acquisition sequences, which were often programmed in languages like BASIC or later C for systems involving DEC PDP-11 or IBM PC controllers.

Applications and Impact

The HP Interface Bus found its primary application in automated test and measurement systems, revolutionizing fields like electronics manufacturing, aerospace avionics testing, and telecommunications research. It enabled the construction of sophisticated ATE racks for testing complex assemblies like printed circuit boards and satellite components. Its influence extended into scientific research, connecting instruments in particle physics experiments and materials science laboratories. The standard's success demonstrated the value of open instrumentation connectivity, paving the way for subsequent standards like VXIbus and PXI. It also spurred the development of early computer-aided engineering (CAE) software and supported calibration systems at institutions like the National Institute of Standards and Technology (NIST). The bus was integral to the operation of pioneering workstations in engineering and was featured in products from Agilent Technologies after the split of Hewlett-Packard.

Variants and Related Standards

Several enhanced and specialized variants of the HP Interface Bus emerged over time. IEEE 488.1 refined the mechanical and electrical specifications, while IEEE 488.2 (1987) standardized command syntax and data structures, improving interoperability. The HS-488 protocol increased effective data transfer speeds. For higher-performance needs, the VXIbus standard essentially embedded the IEEE 488 protocol as a command layer within a modular VMEbus chassis. Similarly, the PXI standard leveraged PCI architecture for measurement systems. While largely supplanted in new designs by faster, simpler interfaces like USB, LAN (e.g., LXI), and PCI Express, the GPIB (General Purpose Interface Bus) incarnation remains in use for maintaining legacy test systems. Related contemporary standards include SCPI (Standard Commands for Programmable Instruments), which grew out of the command sets used with this bus.

Category:Computer buses Category:IEEE standards Category:Measuring instruments Category:Hewlett-Packard