LXI Overview : Introduction
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:
Cooling
Triggering
Interrupt Handling
Mechanical Interfaces
Multiple Device Synchronization
Software Interfaces
Network Routing, Switching
EMI/RFI
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.
Summary
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.
