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.

Network Connectivity

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.