Emerging LXI Spec Weds Ethernet with Instrumentation Apps
Feeding system developers’ hunger for more bandwidth, the new LXI specification puts Ethernet to work in the instrumentation realm.
JON SEMANCIK, VXI TECHNOLOGY
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The functional test and data acquisition community has been instrumental in establishing many industry standards, ranging from communications interfaces to instrumentation backplanes. The need for ever-increasing bandwidths and higher data transfer rates has helped drive the latest industry initiative, LAN eXtensions for Instrumentation (LXI). LXI is based upon industry=standard Ethernet technology and will provide the flexibility and performance common on backplane-based implementations, such as VXIbus, to the next generation of small and medium size systems (Figure 1). The LXI Consortium was formed in the fall of 2004 by VXI Technology and Agilent Technologies to address this need, and membership has since grown to over 20 leading test and measurement companies from around the globe.
Ethernet has emerged as the clear choice for this next-generation standard based upon technical merit and wide general industry acceptance of the interface by computer manufacturers and users. Technical advantages such as TCP/IP error checking, fault detection, long inter-device connectivity, ease of cable routing and inexpensive networking hardware clearly make Ethernet an attractive alternative to current parallel bus and other serial-based interfaces. Many of these attributes that have made Ethernet so popular to the computer industry are also attractive to the instrumentation community; however, instrumentation manufacturers and users alike demand key functionality not typically associated with this vanilla flavored Ethernet.
Beyond Plain Vanilla Ethernet
Determinism, synchronization, triggering, device discovery and predictable software driver interoperability are all essential functional requirements that extend beyond vanilla Ethernet performance. Different applications areas will also drive different functional
requirements, and these requirements can vary tremendously with applications ranging from bench top to functional test to distributed data acquisition to remote smart sensors. While all of these cases require guaranteed network interoperability, careful evaluation resulted in the identification of three functional groups that are commonly encountered in mechanical and electrical test. The first group includes applications requiring very tight phase relationships between measured data points; this drives the need for deterministic hardware triggering. The second group requires close synchronization, but this can be accomplished by precision triggering across the serial bus. The final group requires minimal triggering and timing control, but must ensure proper network communications.
As a result, the LXI specification has been written to ensure these key technical requirements are addressed, while providing manufacturers the flexibility to design hardware solutions without undue restrictions. Therefore, three distinct specification classes have been defined with the characteristics outlined in Table 1.
One of the primary goals of the LXI Consortium is to provide users an experience that is free of undue frustrations and integration complications, whether they are using a single instrument or combining multiple devices in a larger functional test or data acquisition system. Class C compliance ensures that these expectations are met with network functionality that includes device discovery and web browser interfacing, which are indeed crucial to a positive user experience.
Users of backplane-based systems, such as VXI, are familiar with resource management utilities that identify all of the available hardware in the system, and provide details such as base address, memory utilizations and the like.
Without these utilities the identification of resources would be tedious and prone to errors. Identifying devices and resources in distributed networks poses a similar problem; therefore, Class C requires the use of VXI-11 for the identification of instruments connected to the network.
VXI-11 uses Remote Procedure Calls (RPC) as its underlying technology, which provides utilities, libraries and protocols designed to assist programmers in developing networked applications. During device auto-discovery, a broadcast RPC is performed and sent to the VXI-11 server. After collecting a list of VXI-11 server addresses on the LAN, a VXI-11 connection to each of those addresses is
established and a “*IDN” message is sent. The data received from the response to these queries permits the building of a resource table that can be used by other applications and utilities.
Serial Connection Synchronization
Basic network connectivity and communications may address stand-alone applications, but the synchronization and control of multiple instruments is essential for most functional test and data acquisition applications; therefore Class B-compliant devices address these needs. Historically, Network Time Protocol (NTP) has been used as the de facto standard to synchronize the clocks of multiple computers connected to a network. This approach typically offers synchronization in the millisecond range with many factors affecting the actual precision of NTP timing including network traffic, switches, routers and distances between devices. The non-deterministic nature of this approach was therefore not acceptable.
The most accurate Ethernet-based timing is available through IEEE 1588, referred to as Precision Time Protocol (PTP). PTP is 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.
There are several ways in which PTP can be implemented, ranging from user-level software control, to kernel-level driver modifications, to hardware implementations utilizing dedicated FPGA devices. User- and kernel-level modifications, while out performing NTP, will still be limited by other operating system processes. The highest level of precision is obtained when hardware implementations assist in the time stamping of incoming and outgoing network packets or frames; delay fluctuations can be in the nanosecond range with this approach. Therefore, the incorporation of PTP in Class B devices 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.
Hardware Synchronization and Triggering
While PTP-enabled devices are capable of providing significant improvements over NTP, there are still applications that demand hardware-level triggering to ensure deterministic responses and handshaking. Class C-compliant devices will provide an auxiliary trigger subsystem that can provide extremely precise timing and control signals to multiple instruments with minimum phase skew referred to as Trigger Bus.
The Trigger Bus interface incorporates Low Voltage Differential Signal (LVDS) line drivers utilizing the latest available technology and miniature connector configurations. Flexible twisted pair cabling is used to ensure signal integrity and maximize transmission distances between devices. Standard implementations will include bi-directional triggering capability, and will support both star and daisy-chain implementations.
Class C-compliant devices will provide the means for deterministic triggering and synchronization of LXI instruments using a consistent hardware interface and program interface (Figure 2). The Trigger Bus interface also provides a means to interface with other open standard platform architectures, such as VXIbus, with ease, thus maximizing the users test and measurement investment dollars.
Test and Measurement LAN Implementations
Selecting the right network topology is also an important consideration that can have a significant impact on the overall performance and timing of LXI instrumentation. Ethernet-based communication involves the transmittal and receipt of data packets across a single data path, and a typical corporate network can have hundreds or even thousands of devices competing for their share of the available bandwidth. Limitations in system bandwidth, coupled with overall network communications loading, can increase latencies and increase the likelihood of data packet collisions and associated transmission delays.
Another aspect that must be considered is corporate information technology (IT) policies and procedures. Many corporations closely monitor network implementations and may restrict the use of certain configurations and topologies. This issue is further complicated by security concerns and the use of mandated security tools and firewalls. Therefore, the cleanest approach for most implementations addresses both of these issues, and involves the establishment of a dedicated test local area network (TLAN).
Dedicated networks are inexpensive to install and provide the necessary isolation between corporate-wide network traffic and the test system. The basic system, shown in Figure 3, uses a second Ethernet interface to connect the TLAN 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 and 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 manufacturers.
The LXI Consortium has ensured that features such as software interoperability, maintainability and reusability are maintained by mandating that compliant devices be provided with IVI driver sets. The manufacturer is free to determine which variant best fits the product, IVI-COM or IVI-C, but one of these must be provided. Utilizing a standard API greatly reduces program development and software maintenance costs and ensures that application programs can be written in any standard language, such as C/C++, LabVIEW, Visual Basic or VEE, all with the same familiar interface.
Real World Adoption
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. This standard will provide the basis for long life cycle instrumentation implementations that are not limited by bandwidth, software or computer-dependent architectures. Designs based upon the LXI standard are ideal for a wide range of functional test and data acquisition applications by providing instrumentation that features open architecture, modularity and computer independence.
The LXI Consortium continues to evolve the preliminary specification with a planned official release date in September of 2005. A preliminary release of the specification will be available for initial design purposes by the end of the first quarter of 2005, and this will be followed by several interoperability test sessions (plugfests) in the following months. The Consortium encourages anticipation by instrumentation manufacturers and users. Additional information can be found at the Consortium web site www.lxistandard.org.