Once instrumentation bus options are known, an app usually dictates the type of bus or buses best suited to its needs
BY CHRIS ARMSTRONG
Keithley Instruments
Cleveland, OH
http://www.keithley.com
Data acquisition and test and measurement system performance depends as much on the proper instrumentation bus as having the right hardware. With the multitude of buses available today, sometimes it’s difficult to know which one to choose for a given application.
The choice can be made simpler just by first looking at some of the most common instrumentation buses available today—PXI, LXI, GPIB, and USB. With a basic knowledge of these buses, users will generally find that the application will dictate its own bus preference or preferences. Indeed, systems with several busesso-called hybrid systemsmay provide the best solution.
Hybrid systems consisting of several buses are sometimes the best solution to a measurement challenge.
PXI
PXI (PCI eXtensions for Instrumentation) is based on the industry-standard PCI interface. PXI specifications for timing and triggering signals on the backplane enable synchronization of multiple devices without external connections. PXI modules can transfer data to an internal or embedded PC for analysis. Performance specifications for PXI bus interfaces include the number of channels and data transfer rates that, for serial communications, are typically expressed in megabytes per second.
There are a few key differences between PCI and PXI. For instance, PXI adds a trigger bus that enables tight coordination between the controller and peripherals. A 10-MHz clock signal can also be used to better synchronize peripheral operation. For applications with nanosecond-level trigger requirements, a star trigger controller can synchronize multiple modules. Finally, a local signal bus ensures that peripherals can share signals from slot to slot for added flexibility.
In general, data acquisition applications that demand high levels of synchronization are well suited to PXI. This includes many multichannel applications that require close time synchronization. For example, an aerospace application with simultaneous monitoring of hundreds of sensor channels requires a high level of trigger integration. The integrated PXI backplane provides the necessary trigger channels, as well as high data throughput.
There are also a wide variety of communication, image acquisition, and industrial modules available for PXI that make it ideal for industrial applications that mix measurements with other test activities.
LXI
LXI (or LAN eXtensions for Instrumentation) is an Ethernet-based standard for instrumentation communications. The LXI specification builds upon industry-standard technology to ensure interoperability, convenience, and ease of use.
The LXI standard defines three classes of device: Class A, B, and C. All three classes include at the very least a standardized Ethernet interface, built-in Web server with standardized Web pages, and an IVI instrument driver. Class B devices include some additional triggering, messaging, and synchronization features while Class A devices feature an LXI hardware trigger bus. LXI’s discovery feature lets host computers or other devices locate and identify any and all LXI devices present on a network subnet and obtain sufficient information about them to establish LAN-based communications.
On the application side, LXI takes advantage of the distributed nature of Ethernet. Ethernet is most efficient at transferring large packets of data, so batching commands and data together becomes an efficient way to architect test code. Also, being Ethernet-based makes LXI suitable for long-distance applications, as well as bench-top applications where the necessary data can be collected from within the Web page of the instrument.
Common Instrumentation Buses
As an example, LXI class B features IEEE-1588 timing synchronization. This allows instruments distributed over the LAN network to synchronize their timestamps to within a microsecond or better. For process applications where it is important to correlate data across a factory, this capability is a unique solution to a difficult testing problem. The additional advantage of using the inexpensive and widely used Ethernet protocol and cabling makes LXI an important bus for a majority of modern instrument-grade applications.
LXI’s embedded Web pages provide easy to use, zero installation getting-started application software that helps users make measurements, evaluate instruments, and develop and debug tests more quickly. These factors impact test system development time and can lead to significant time-to-market gains.
GPIB
The IEEE-488 bus, or GPIB (general-purpose interface bus), in existence since 1965, provides a standardized communication interface for an extremely wide range of laboratory instruments. The 8-bit parallel bus transmits data at up to one megabyte per second, and up to 15 devices can be wired on one bus in either a daisy chain or star configuration.
GPIB interfaces typically are not standard equipment on a PC, so a plug-in adapter board and appropriate driver software must be installed in the PC to communicate with GPIB instruments. These boards are available in USB, PCI, and Ethernet-to-GPIB versions, among others.
With a long history of development and support behind it, the GPIB interface and GPIB-based instruments have the advantage of an extremely large investment in many types of industrial and academic applications and are not likely to be replaced soon.
GPIB is best suited for sensitive measurements, including low signal levels such as picoamps, nanovolts, or micro-ohms, and high-power applications that require supplying amperes or hundreds of volts to a device under test. The large set of available products for these applications will make it an important bus for years to come.
USB
The Universal Serial Bus (USB), introduced in 1995, addresses a number of connectivity issues associated with existing serial communications standards. USB supports multiple devices and provides easier installation, faster transmission speeds, and simpler cabling requirements than conventional parallel or serial ports. USB is also designed to supply operating power directly to peripherals, eliminating the need for external power supplies in some cases.
USB peripherals can be attached or removed from an energized PC without damage and without the need to reboot. The protocol incorporates plug-and-play capability so an operating system can automatically recognize and reconfigure PC resources to handle the addition or removal of a USB peripheral. Currently, the availability of USB data acquisition hardware is growing as the demand for USB grows. Adapters are also available to mate conventional GPIB, serial, and parallel devices to USB.
A USB data acquisition system does have some potential grounding concerns. Unlike PCI boards, which have ground systems built into the PC backplane, USB modules have a long ground connection with active circuitry at both ends. If the USB module is not designed with proper isolation, this can cause system lockups, erratic behavior, and electromagnetic transients. Isolation, which is designed into some USB modules, protects the PC from damage and preserves data integrity by physically separating the electrical connections between circuits.
There are several advantages to using USB over other buses. For instance, USB modules are remote from sources of electrical noise, such as PC motherboards and power supplies, so there is less potential for noise to be introduced into measurements. USB modules are also compact and portable, which means they are ideal for field applications in test and measurement.
USB is ideal for data acquisition applications that don’t require synchronization. the bus also works best in low-power, low-resolution, and medium-speed applications like data acquisition. Making basic analog I/O measurements on a small number of channels can be done using USB with convenience and pricing benefits.
Hybrid systems
A hybrid test system is designed to include multiple interfaces or buses in a single application. Hybrid systems offer a mix of interface options, including PXI, LXI, GPIB, and USB. Hybrid systems grew from a realization that no single instrument interface is optimal for all testing needs. The best solution may involve multiple bus architectures for the differing requirements of each part of the system.
For multicapability tests, hybrid systems that take advantage of the strengths of different buses in a larger test stand are the best overall solution. It doesn’t make sense to do sensitive or power applications using PXI alone, because the test also calls for synchronized data acquisition. In production test applications, hybrid test systems provide a number of benefits. For example, they can help improve test times per part or overall test throughput, which is a critical parameter in any production system. A hybrid system can also improve development speed. Depending on the application, the industry, and the devices, there may be significant pressure to get a system up and running very quickly. This time-to-market pressure is important enough to influence test system architecture. Additionally, a hybrid test system can help decrease long-term ownership costs. Making the system more adaptable for future enhancements or maintenance is especially important in fast-moving or high-mix production.
Get more information on buses at http://electronicproducts-com-develop.go-vip.net/testmeasure.asp ■
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