Guide to instrument and control buses

Vanessa Trujillo, Instrument Connectivity Manager, National Instruments, Austin, TX -- Plant Engineering, 6/1/2001

Key concepts

• GPIB and serial buses are the most common instrument interfaces
• Bridge products convert equipment from one bus type to another
• The future of instrument control will be that of a mixed I/O world.

General purpose interface buses (GPIBs), also called (IEEE 488), and serial buses have been around for more than 20 yr and are the most common types present in instruments. While computer technology has increased exponentially in this timeframe, buses have changed little. Recently, however, new buses, such as IEEE 1394, Ethernet, and the Universal Serial Bus (USB), have emerged as candidates for instrument interfaces.

GPIB was created specifically for instrument control applications. While IEEE 488.1 focused on the electrical, mechanical, and functional specifications, IEEE 488.2 defined how controllers and instruments communicate and strengthened the original GPIB standard .

GPIB is a digital 8-bit parallel communications interface with data transfers higher than 1.5 Mbytes/sec. The bus supports one system controller and up to 14 additional instruments, with more instruments being added through expanders. GPIB cables and connectors are robust and industrial graded. Cabling is limited to less than 20 m, but with expanders can serve instruments up to 2-km away.

Currently, there is a proposal to improve the performance of GPIB from 1.5 Mbytes/sec to 8 Mbytes/sec undergoing standardization approval in IEEE. Instruments capable of high-speed GPIB transfers and compatible with existing instrumentation systems are based on the original GPIB standard.

The RS-232 serial specification most commonly controls modems and printers and usually consists of a computer interface connected to a device to be controlled. The interface is built into most computers and is very popular for instrument control applications. However, unlike GPIB, the RS-232 inter- face can only be connected to one device at a time. Furthermore, the maximum cable length without any expanders is 15 m and has data rates of less than 20 Kbytes/sec.

IEEE 1394, also known as FireWire, is a high-performance serial bus developed by Apple computer in the early 1990s. The standard handles throughput rates of 100, 200, and 400 Mbits/sec. However, the IEEE 1394 trade association is revising the specification to increase this transfer rate to 3.2 Gbits/sec.

IEEE 1394 offers great potential for high-speed applications. Furthermore, Intel currently does not include 1394 support in its PC peripheral chip sets. This situation means that in order to use a 1394 bus in an application, both the 1394 device and the 1394 controller must be acquired.

Other features make 1394 ideal for many types of instrument control applications. While devices must be within 4.5 m of the bus socket to conform to the specification, up to 16 devices can be daisy-chained together for a maximum run of 72 m. FireWire also features "hot pluggable" technology that enables devices to be connected and disconnected while the system is powered. However, there are few instruments with 1394 ports available.

Instrument control applications over Ethernet take advantage of several characteristics of the bus, such as remote control of instruments, sharing among different users at different locations, and easier integration and publication of the resulting data. Moreover, users can take advantage of the already existing knowledge within their company on Ethernet-based networks.

Furthermore, instrument vendors have recently started to include Ethernet as an alternative communication interface on stand-alone instruments. Although Ethernet is somewhat new to instrument control applications, it is a mature technology that is widely used for measurement systems. With more than 100 million TCP/IP-capable computers worldwide, the shear ubiquity of Ethernet for controlling instruments is compelling.

Some factors to consider when deciding to use Ethernet for instrument control are transfer rate, determinism, and security. Most common Ethernet networks today are 10BaseT or 100BaseTX, transferring data at 10 Mbits/sec or 100 Mbits/sec, respectively. However, these transfer rates do not imply that your application achieves the same performance.

The nature of Ethernet — with its overhead, carrier sense, collision detection, and bus sharing by multiple devices — not only does not allow you to achieve the theoretical transfer rates, but it cannot be deterministic in assuring data transfer, depending on topology. In addition, if you have a very sensitive instrumentation system based on Ethernet, you need to take additional security measures to ensure its integrity (for example, firewalls).

Created in 1995, VXI is a standard for controlling instrumentation over Ethernet (see "Bus schedule" for explanations of acronyms). This protocol was created through the VXI plug and play consortium and defines GPIB-style communication over Ethernet. Some virtual instrument software architecture (VISA) software supports control of Ethernet-based instrumentation, and it is easy to use VXI-11 compatible instruments since you can reuse your VISA code written for other buses (GPIB, Serial, or VXI) virtually unchanged.

USB was primarily designed to connect peripheral devices, such as keyboards and mice, with PCs. The USB 1.1 specification has a maximum throughput of 12 Mbits/sec. However, future USB devices will have a throughput of 480 Mbits/sec, thanks to the USB 2.0 specification.

The USB host automatically detects when a new device has been added, queries the device for its identification, and configures the drivers appropriately. The topology of the bus allows up to 127 devices to run concurrently on one port.

USB ports also come standard on today's PCs, which means that you do not have to purchase a dedicated controller to interface to a USB-based device. The standard delivers an inexpensive, yet easy-to-use, connection between devices and PCs and provides a "step up" from conventional serial port technology. The setup features faster performance, "hot pluggable" functionality, built-in operating system configuration, and thin and flexible cabling, which allows you to connect multiple devices from the same port.

Although USB has many attractive benefits, there are some less convenient factors when comparing it to other buses. USB cables are not industrially graded, which can degrade the data in noisy environments; and there is no robust mechanism to prevent them from accidentally being detached. In addition, the USB system topology allows for a maximum distance of 30-m between the controller and the device, after using repeaters.

Finally, even though there is a great potential for USB instrumentation, especially with the April 2000 release of the USB 2.0 specification, currently there is no standard protocol available for controlling instruments over USB. In addition, there are few instruments with USB ports available.

Due to the slow shift of instrument manufacturers to implement new buses, bridge products are emerging as a viable solution. Bridge products allow you to convert from one bus type to another one. For example, one end of a bridge product may plug into an Ethernet, USB, or FireWire port in your computer or system, while the other end connects to traditional GPIB or Serial ports in your instruments .

With bridge products, you can take advantage of the plug-and-play capabilities, ease of use, and wide availability of controllers that practically come free with new computers, while building applications by conserving your investment in hardware, software, and time. Bridge products should be designed as a transparent solution for the application.

For example, you should be able to take the code written for a GPIB plug-in controller and reuse it without any modifications if you decide to replace the GPIB plug-in controller with an Ethernet-to-GPIB bridge product.

Bridge products solve the integration of legacy equipment into systems with new buses incorporated into the computer. However, in the future, systems will consist not only of legacy equipment, but also of new instrumentation equipment with new communication buses on them. Moreover, it is possible that more than one of the buses discussed here, or even some other bus, will be present in the same instrumentation system.

This scenario leaves engineers with the task of implementing mixed I/O systems. And the best way to design such systems is to base them on the appropriate software architecture capable of handling multiple buses without adding complexity to the equation. VISA has this capability.

The virtual instrument software architecture (VISA) specification was created by the VXI plug-and-play Systems Alliance in 1993 to serve as a common application programming interface (API) to control VXI, GPIB, and Serial instruments (Fig. 3). VISA then became the preferred API used in the development of instrument drivers with thousands written in different development environments. With VISA, users could be more productive since they only needed to learn one API instead of three separate, very different ones.

Today, with the potential for new instrument control buses to be part of systems, there is an even higher demand for a common software API. Users could create code compatible with the traditional connectivity buses and upgrade their equipment to new control buses without having to modify their software. This situation helps preserve their hardware and software investments, while eliminating the learning curve of a different API. Currently, one implementation of the VISA specification supports GPIB, serial, and VXI interfaces, as well as Ethernet and Peripheral Component Interconnect eXtensions for Instrumentation, the latest, most popular connectivity options for instrumentation systems.

The new VISA design separates the support for connectivity buses from the core VISA library, which contains the popular high-level VISA API. With this model, each different bus requires a passport to connect to the core VISA engine. With this plug-in architecture, support for new buses can be added very easily without disturbing the existing interfaces.

Several buses, such as Ethernet, USB, and IEEE 1394, have great potential to be standard bus interfaces for test and measurement applications. As technology evolves, these and other buses, such as Bluetooth, will be considered for instrument control applications. On the other hand, equipment with traditional control buses will be present in instrumentation systems for many years to come.

The future of instrument control will be that of a mixed I/O world. The key relies on having a system with the adequate structure to leverage existing hardware and software investments, while at the same time incorporate and take advantage of the latest technologies.

Edited by Jack Smith, Senior Editor, 630-320-7147, jsmith@cahners.com