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The Future for New Bus Technologies in Instrument Control and Connectivity

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Overview
The Basics of GPIB
The Benefits of New Bus Technologies
Preserve Your Investment with Bridge Products
Creating a Flexible Software Architecture
Conclusion
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Overview

For more than 20 years, scientists and engineers have widely used the IEEE 488 GPIB for automating instrumentation systems. As popular computer technology enters the test and measurement arena and buses such as USB, Ethernet, and IEEE 1394 are considered for instrument connectivity, questions arise about the future of GPIB as the preferred bus for instrument control. Because of its robustness and the large existing user base, GPIB will be around for many years to come; however, the instrument control industry will most likely begin to see a world comprising mixed I/O systems. This paper studies the future for instrumentation systems when GPIB and other buses are combined and the importance of software in maintaining compatibility with previous developments and investments in instrument control.

The Basics of GPIB

GPIB was designed specifically for instrument control applications. In the 1970s, IEEE Standard 488-1975 delineated the electrical, mechanical, and functional specifications for GPIB. ANSI/IEEE Standard 488.2-1987 strengthened the original standard by defining precisely how controllers and instruments communicate through GPIB. GPIB is a digital, 8-bit parallel communications interface with data transfer rates of up to 8 MB/s. The bus provides one system controller for up to 14 instruments, and cabling is limited to less than 20 m. Users can overcome both of these limitations by using GPIB expanders and extenders. GPIB cables and connectors are versatile and industrially graded for use in any environment.

The Benefits of New Bus Technologies

Recently, computers that previously included serial (RS232) and parallel ports are now providing Ethernet, USB, and sometimes IEEE 1394 ports. These new buses are attractive for a variety of reasons – ease of use (USB), connectivity (Ethernet), and high speed (IEEE 1394).

USB

USB was designed primarily to connect PC peripheral devices, such as keyboards, scanners, and disk drives. Over the past two years, the number of devices that incorporate USB connectivity has increased dramatically in the computer industry, led by Apple Computer's adoption of USB in 1998 as its only serial bus.

With the current USB 1.1 specification, throughput reaches a maximum of 1.5 MB/s. The next generation of USB products will incorporate the USB 2.0 specification, further increasing the bus throughput up to 60 MB/s. The USB 2.0 specification ensures compatibility with USB 1.1 devices and even uses the same connector. Because the USB is a plug-and-play technology, the USB host automatically detects when a new device has been added, queries the device for its identification, and configures the device drivers appropriately. Up to 127 devices can run concurrently on one port. For Windows OSs, USB connectivity is currently implemented only in Windows Vista/XP/2000/Me/98.

USB delivers an inexpensive and easy-to-use connection between devices and PCs. Furthermore, USB improves conventional serial port technology by providing faster performance, "hot plug" functionality, built-in OS configuration, multiple device connectivity from one port, and thin, flexible cabling.

Although USB has many attractive benefits, there are some drawbacks in its use for instrument control. First, USB cables are not industrial graded, which potentially allows data loss in noisy environments. Moreover, there is no latching mechanism for USB cables -- the cables can be pulled out of the PC or instrument relatively easily. The maximum cable length in USB systems is 30 m, including the use of inline repeaters. Finally, there is no industry-standard protocol designed for instrument control over USB; controlling USB instruments would require individual implementation from the instrument manufacturer.

Despite the shortcomings of USB, the wide availability of ports on today’s computers and the promise of high-speed with USB 2.0 place it as a forerunner in the future options for instrument control. Currently, there are very few instruments that offer a USB option for control. Users can take advantage of USB connectivity in their instrument control applications today through the use of bridge products – products that enable users to bridge between USB and GPIB. Bridge products are further discussed later in this paper.

Ethernet

Recently, instrument vendors have started to include Ethernet as an alternative communication interface on stand-alone instruments. Although Ethernet is new to instrument control applications, it is a mature technology that is widely used for measurement systems in many other capacities. With more than 100 million Ethernet capable computers worldwide, the argument for Ethernet as an instrument control solution is compelling.

Instrument control applications over Ethernet can take advantage of the unique characteristics of the bus, including remote control of instruments, facilitated sharing of instruments among users, and easy publication of the resulting data. Moreover, users gain the advantage of existing, extensive Ethernet networks in their companies and laboratories. However, this advantage may pose a problem in some companies because it will force the involvement of network administrators in traditional engineering applications.

Other factors to consider when examining Ethernet for instrument control are transfer rate, determinism, and security. Most common Ethernet networks today are 10BASE-T or 100BASE-TX, transferring data at 10 Mb/s or 100 Mb/s, respectively. However, these transfer rates are rarely realized because of other network traffic, overhead, and inefficient data transfer. Moreover, because of the uncertainty of transfer rates, determinism is not assured in communicating across Ethernet. Finally, for sensitive data, users must take additional security measures to ensure data integrity and privacy.

IEEE 1394

IEEE Standard 1394-1995 is a high-performance serial bus originally developed by Apple in the 1980s. Currently, IEEE 1394 can achieve throughput rates up to 50 MB/s; however, the IEEE 1394 trade association is revising the specification to increase this data transfer rate to 400 MB/s. While devices must be within 4.5 m of the bus socket to conform to the 1394 specification, up to 16 devices can be daisy chained together for a maximum length of 72 m.

The 1394 bus offers great potential for high-speed applications. Many digital cameras and other consumer electronics products already include IEEE 1394 ports for transferring data. The high bandwidth involved with multimedia applications makes 1394 a viable solution. IEEE 1394 does hold an advantage over USB in that there is an existing protocol defined for controlling instruments over the bus. However, there are currently very few instruments available with 1394 ports.

While IEEE 1394 has a number of benefits, namely bandwidth, for instrument control, several factors prevent its immediate adoption. The main shortcoming for IEEE 1394 is that 1394 ports are not built into Intel PC peripheral chip sets. (All Macintosh computers have built-in 1394 ports.) Thus, Intel PC users must acquire a separate 1394 controller, typically a PCI board, to communicate with a 1394 device. Although the cables for IEEE 1394 are thin and flexible, they are not industrial graded, which may permit data loss in some test and measurement applications.

Current Adoption of New Bus Technologies

Only a handful of instrument manufacturers include built-in instrument control options for USB, Ethernet, or IEEE 1394 today. The slow integration of these buses by manufacturers can be attributed to a lack of dominance by one particular bus in the instrument control industry. The widespread availability of USB or the existence of large Ethernet networks may provide the motivation for designers to begin including a separate communication bus for instrument control. While instrument manufacturers debate the viability of each bus option, users who want to implement the latest bus technology into their existing test systems now have an option with bridge products.

Preserve Your Investment with Bridge Products

Due to the slow adoption of new bus technology and the uncertainty in the industry over a predominant bus, bridge products are emerging as a viable solution for instrument control and connectivity. Using bridge products, users can transparently convert from one bus type to a different bus type, thus taking advantage of the latest technology while maintaining backward compatibility. For example, one end of a bridge product may plug into an Ethernet, USB, or 1394 port in your computer or system, while the other end connects to traditional instruments through GPIB or serial ports. Users gain the advantage of plug-and-play capabilities, ease of use, and the wide availability of these new buses on most new computers. Moreover, by maintaining instrument control software compatibility users can minimize their investment in software and development time.

Bridge products are designed as a transparent solution for users’ applications. For example, a user can take the code written for a GPIB plug-in controller and reuse it without any modifications when replacing that controller with an Ethernet-to-GPIB bridge product. The importance of software in instrument control systems is discussed below.

National Instruments offers a wide variety of GPIB external controllers to get users up and running quickly with low-cost, high-performance solutions. With the NI GPIB-ENET/100 (the Ethernet-to-GPIB controller from NI), users can control and share instruments located anywhere in the world via a TCP/IP network. Moreover, users can adopt the code written for any other NI GPIB interface unmodified to use with their GPIB-ENET/100.

The compact NI GPIB-USB-HS transforms any computer with a USB port into a full-function, plug-and-play, IEEE-488.2 controller for up to 14 programmable GPIB instruments. The GPIB-USB-HS takes advantage of Hi-Speed USB to provide superior performance of up to 1.8 MB/s with the standard IEEE 488 handshake and 7.2 MB/s with the high-speed IEEE 488 handshake (HS488).The small size and light weight of the GPIB-USB-HS make it ideal for portable applications using a laptop computer or benchtop applications where the computer has no available internal I/O slots.

Ethernet and USB have great potential as standard bus interfaces for test and measurement applications. As PC technology evolves, these and other future buses could be considered for instrument control applications. However, it is important to consider that PC technology changes rapidly, while test and measurement instruments have a much longer life span. Therefore, it is pertinent that the buses used for instrument control applications be available for an extended time. Proven success over time is one of the main reasons GPIB today is the bus most widely used for instrument control and connectivity. Unlike the emerging bus technologies, GPIB was designed specifically for instrument control, and it will continue to be present in test and measurement applications for years to come.

Creating a Flexible Software Architecture

Regardless of which bus is used in the future in conjunction with GPIB, software compatibility and integration will be the key to any user’s success in a mixed I/O world. Software that maintains backward compatibility, while seamlessly integrating new buses, will make the difference between spending three hours bringing a new instrument online and spending three months rewriting the application.

Because multivendor, multi-interface systems are becoming increasingly prevalent, you need to have in place a software architecture that can handle those systems with minimal effort and with maximum software reuse. Using industry software standards, such as IVI and VISA, users can maintain software investments when migrating to new buses by maintaining compatibility with previously written code and adhering to industry-standard communication protocols.

Maintain Software Investments with VISA

As a step toward industry-wide software compatibility, the VXIplug&play Systems Alliance developed one specification for I/O software – VISA. When the alliance was founded in 1993, many nonstandard commercial implementations of I/O software for VXI, GPIB, and serial interfaces existed. For these buses, VISA provides a common foundation for the development, delivery, and interoperability of high-level multivendor system software components, such as instrument drivers, soft front panels, and application software. Although the alliance defined VISA, individual vendors created different implementations of VISA.

Because VISA defines one API for instrument communication, users can preserve their software investment when they migrate to new interface buses or mixed I/O systems.

A problem with the previous model was that each vendor designed its VISA implementation to work with that vendor’s own controllers, and users could not use it with those from other vendors. In addition, to work with new interfaces, users had to install a complete VISA library. Sometimes it came from a different vendor and did not guarantee the preservation of existing interfaces.

To solve such problems, National Instruments has redesigned its VISA implementation using a "Passport" plug-in model, which defines a distinct communication port, or passport, for each different bus. The NI Passport model separates the specific communication mechanisms 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, so compatibility with new buses is added easily without disturbing the existing interfaces.

With this model, users can truly have multivendor, multi-interface systems. Unlike other solutions that rely on technologies such as component object model (COM), multiplatform ANSI C technology is still the basis of the Passport model. In addition to the interfaces with which VISA works today, National Instruments is committed to adding compatibility within VISA for any bus interface that becomes popular in test and measurement applications.

Find Versatility with IVI

The IVI Foundation is defining a standard for instrument drivers – software modules that circumvent low-level communication details – a standard that builds on VISA to provide a robust, high-performance, and easy-to-use instrument communication protocol. These instrument drivers, created according to the foundation standards, contain high-level functions, such as Configure Measurement or Read Waveform, which internally contain the low-level VISA read and write functions. Combined with VISA, IVI provides a great mechanism for delivering multivendor, multiplatform, mixed I/O test systems.

Conclusion

While the future of Ethernet and USB technology in instrument control systems is unclear, with bridge products, users can easily take advantage of new computer bus technology for their applications. Regardless of the bus of the future, users will demand compatibility with existing software and developed applications before any new technology is widely adopted.

The future of test systems will comprise instrumentation hardware with mixed I/O connectivity. The best way users can maintain their investment in software and hardware throughout the life of the system is to use a stable software architecture capable of working with multivendor, multi-interface, and multiplatform systems.

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