The functional test and data acquisition community has been instrumental in the development of many industry standards ranging from the definition of communication interfaces to instrumentation backplanes. These functional requirements are often tightly coupled to, and have evolved from, the application space that they serve. Applications may range from small systems to medium sized combinational systems to high-channel count, highly integrated implementations.
The general purpose interface bus (GPIB) is an example of an instrumentation interface that has addressed small systems, and some aspects of medium sized combinational systems, while VXI-based systems have dominated the high-channel count application market. However, GPIB is now being challenged by the need for ever increasing bandwidth and higher data transfer rates. Many other standards have been advertised as the next generation replacement for GPIB, such as the universal serial bus (USB), FireWire, CAN bus, PCI, and PXI, but none of these have met with wide industry acceptance.
As a result of this need, Agilent Technologies and VXI Technology, Inc. have combined engineering resources to develop the next generation instrumentation interface with the release of LAN eXtensions for Instrumentation (LXI™). LXI™ is based upon industry standard Ethernet technology and will provide the flexibility and performance commonplace on larger VXIbus systems, to small and medium size systems .
Ethernet: The Logical Choice
Ethernet is by far the most widely accepted communications interface in use today; nearly every computer is manufactured with an integrated Ethernet interface and networking hardware is becoming increasingly inexpensive. Many of the attributes that have made Ethernet so popular to the computer industry are also attractive to the instrumentation community.
Technical advantages, such as TCP/IP error checking and fault detection, as well as long inter-device connectivity clearly exceed the limitations of parallel bus and other serial based interfaces.
The TCP/IP stack provides error detection and correction that will typically not interfere with throughput rates, especially when a dedicated test system network is used. Furthermore, Ethernet connections can span 100 meters point-to-point, encompass a radius of 200 meters with the use of a hub, switch, or router, or extend to thousands of kilometers if fiber interfaces are used.
There are, however, instrumentation-specific requirements that must be addressed before Ethernet can be accepted as a next generation platform for modular instruments. Some of these include:
Multiple Device Synchronization
Network Routing, Switching
Some of these performance areas, such as mechanical interfacing and cooling, while important, do not represent significant technical challenges. The most challenging aspects of LXI™ implementation involve instrument synchronization, test network architecture, and software inoperability. The following sections will discuss different implementation approaches to address these issues from within the LXI™ framework.
LXI™ Device Synchronization
The first area of concern is device synchronization. The synchronization and control of multiple instruments is a prerequisite for most functional test and data acquisition applications which are dependent on phase relationships or stimulus/response handshaking.
Several approaches can be utilized for synchronization utilizing Ethernet interfaces, and the most common approaches include an auxiliary trigger subsystem (Trigger Bus), Network Time Protocol (NTP), or IEEE-1588
An auxiliary trigger subsystem can provide extremely precise timing and control
signals to multiple instruments with minimum phase skew. Additionally, this approach
provides the means to synchronize with ease LXI™ instruments and other standard
platform implementations, such as VXIbus
Another approach that is unique to Ethernet-based interfaces, and inherent in LXI™, is the Network Time Protocol (NTP), which is the current de-facto standard for network time synchronization. This approach is designed to synchronize the clocks of networked devices; however, it only offers synchronization in the millisecond range at best. There are many factors that can affect the actual precision of NTP, including network traffic, bus, switches, and routers. This approach will provide a reasonable level of synchronization, and may be acceptable for low speed measurements such as thermocouple devices.
The most accurate Ethernet-based timing is available through IEEE-1588, a standard that defines a precision clock synchronization protocol for networked measurement and control systems. The protocol is designed to enable the synchronization of systems that include clocks of different precision, resolution and stability. Sub-microsecond accuracy can be achieved with minimal network and local clock computing resources, and with little administrative attention from the user. This standard will provide LXI™ a level of synchronization that will easily address many typical multiple instrument installations without the need for additional cabling or clock distribution systems.
Test & Measurement Local Area Network Implementations
Selecting the right network topology is also a concern, and can have a significant impact on the overall performance and timing of Ethernet-based instrumentation. Ethernet networks function by sending packets of data between different nodes on the network. If both nodes transmit data at the same time, a collision will occur, thus affecting system throughput.
Consequently, more collisions will occur as the number of nodes increase, and data throughput will be reduced. Therefore, the performance of an LXI™ based instrumentation network will be optimized if it is isolated from general purpose corporate network paths.
Dedicated networks are inexpensive to install and provide the necessary isolation between corporate wide network traffic and the test system (See Figure 3). Additionally, this network can be easily interfaced with the rest of the corporation, or the World Wide Web, with little effort.
Isolated instrumentation networks also eliminate many of the logistical issues that may arise when trying to conform to corporate network requirements. Security concerns can also be addressed by simply not allowing physical access to outside network connections.
Software & Programming
The final implementation issue involves software drivers and programming interfaces. Well-planned network topologies and synchronization schemes are still dependent on software drivers and programming interfaces if multi-vendor interoperability and operating system independence are to be assured. The very nature of Ethernet and LXI™ based instrumentation implies that many different configurations will be implemented, utilizing hardware from different instrumentation, computer, switching, router, and computer interface hardware manufacturers.
Software interoperability, maintainability, and reusability are ensured by implementing drivers that conform to open standards for instrument drivers, such as VISA and LXI Overview
VXIplug&play and IVI. VISA supports network communications through a required implementation of VXI-11, developed as a standard for communicating with Ethernet-based instruments (VXI-11 is a subset of SICL-LAN developed by Agilent Technologies). VXI-11 uses Remote Procedure Calls (RPC) as its underlying technology; RPC provides utilities, libraries and protocols designed to assist programmers in developing networked applications.
The selection of VISA as the LXI™ programming interface also provides the user with a standard application programming interface (API). Application programs can then be written in any standard language, such as C/C++, LabVIEW, Visual Basic, or VEE, all with the same familiar interface.
The successful adoption of any standard requires more than simply articulating technical merits, it includes dedicated real world implementation teams identifying potential issues and ensuring complete functionality. VXI Technology, Inc., in partnership with Agilent Technologies, is committed to providing the test and measurement community with LXI™, a world class LAN based, open standard for instrumentation.
This standard will provide the basis for long life cycle instrumentation implementations that are not limited by bandwidth, software, or computer dependent architectures. LXI™ is the ideal solution to your next engineering challenge.