Is LXI the future of test ?
07 June 2006
With the launch of the LXI Consortium in September 2004 and the definition of the LXI specification just a year later it is now possible to review the benefits that this new interface standard brings. Stefan Koop outlines 10 good reasons why many perceive LXI as the way forward for test system interfacing.
1: Ease-of-use
LXI uses Ethernet as its basis and this provides some significant advantages in ease of use.
At the physical layer (see Figure 1), LXI recommends Auto MDIX, a feature that eliminates the need for crossover cables in direct (peer-to-peer or controller-to-instrument) connections. In addition there is automatic negotiation of the speed and duplex mode of the LAN connection.
At the network (IP) layer, LXI instruments support automatic IP configuration through a DHCP server or through dynamic configuration of link local addresses.
At the application layer, LXI instruments support the VXI-11 protocol (based on RPC) for automatic discovery of new instruments and identification through the *IDN? query.
LXI instruments include a web server for instrument configuration and trouble-shooting. Typically, the web page also allows you to interactively exercise instrument functions and is an invaluable tool during system test and commissioning.
For programmatic control, the LXI standard recommends IVI-COM instrument drivers. These drivers are based on the Microsoft COM architecture and work with all modern programming environments. With their object-oriented nature and hierarchical API, they can take advantage of the advanced features of modern, object-oriented environments.
2: Performance
LXI uses Ethernet's TCP protocol for communication between the system's components.
Figure 2 shows the structure of an Ethernet frame with TCP/IP packets enclosed as payload. Assuming the use of IPv4 and maximum frame size, the bandwidth remaining for application data is just a little less than 95% of the underlying physical layer transmission rate.
With a Fast Ethernet connection (IEEE 803.2u, 100MBits/s), the payload data rate is approximately 11.9 Mbytes/s. Gigabit Ethernet (IEEE 802.3z), recommended by the LXI specification, boosts the performance by a factor of ten to approximately 119 Mbytes/s.
With Gigabit Ethernet, LXI exceeds the original VXI (revision 1) performance of 40 Mbytes/s. Early in 2004, the VXIbus Consortium released revision 3.0 of their specifications, increasing the data rate performance up by a factor of four, to 160 Mbytes/s.
However, since LXI can take advantage of developments in IT and particularly the desire for ever-increasing data rates, the performance of LXI will continue to increase. 10GBit Ethernet will be readily available soon and will allow LXI to surpass the performance of VXI 3.0.
3: Cost
A test system based on LXI components is likely to be less expensive compared to GPIB, VXI and PXI because it doesn't require a costly card-cage, Slot-0 controller or specialised communication interfaces and cables. With card-cage systems, these items typically cost between five and ten thousand US$, depending on the configuration.
The LAN interface required for LXI comes with the system controller at no additional cost. Also, LAN infrastructure like hubs, switches and routers is either already available or can be purchased at very moderate cost.
One big advantage with regards to test system cost is somewhat less obvious. There has always been a natural gap between R&D applications (typically based on bench-top equipment) and automated applications (often based on system components like VXI or PXI). LXI can bridge that gap, since it accommodates both interactive bench-top and high-performance, faceless instruments. Using the same architecture for both types of systems, it will be much easier to move from one type of test system to the next, for example from system verification to manufacturing test. There is a potential for reuse of existing code and other elements, resulting in lower cost and less risk and implementation time.
Manufacturers will be able to create both bench-top and smaller faceless (system) versions of their instruments, based on the very same building blocks and firmware. The economies of scale that will result will eventually lead to lower cost instruments.
4: Scalability
Scalability results from modularity. As long as the test system architecture is truly modular, different types of measurement resources can be mixed and matched and measurement channels, digital I/O lines, switches and signal sources can be added as required.
Since LXI modules do not require a card-cage, there is no limit to the number of modules that can be added to the system.
The LXI standard also includes an optional mechanical specification, which simplifies the integration of LXI modules in system racks. The mechanical standard specifies units to be full or half rack width and one or several rack units high. Measurement signals are routed to the front of the module, whereas power, Ethernet, LXI trigger bus and other control connectors are placed on the rear.
5: Longevity
Test systems must live as long as the products they were designed to test. In some industries, including Automotive and Defense, product life cycles can span many decades. During this time the test system has to be updated and expanded as new product variants arise.
Ethernet is an extremely stable standard, more than 30 years old. Due to its stability and other virtues, it is being adopted in many industries outside of IT, such as consumer electronics, industrial automation and measurement.
Ethernet is also continually evolving. Across its development life it has added many higher-layer protocols, as well as enhancements at the physical layer (like Gigabit Ethernet) and network layer (like IPv6). Such enhancements, however, are typically made in an "upwards compatible" fashion, protecting the investment in older flavors of the standard.
6: Distributed Systems
With Ethernet, LXI creates new applications, which previously would require more specialised equipment. Using a corporate Ethernet infrastructure or the public Internet, large distances can be bridged transparently to the end user.
With LXI, these solutions can be designed using the same instruments that would be used for local applications and rack-based systems. There is no need to create custom gateways - remote access comes without extra effort.
Security is a concern for many remote applications; however, these issues have been addressed in the IT world. Modern routers have security features like access filtering based on MAC or IP addresses, WLAN encryption etc. VPN (Virtual Private Network) allows IP packets to be sent securely via the public internet, encrypted through IPsec or other encryption protocols.
7: Flexibility
Unlike card-cage systems, LXI modules can be freely distributed in a test rack, lab or building. This allows the instruments to be located where it's best from a measurement or application standpoint.
Typically, the aim is to minimise the cable length between the sensor or device under test (DUT) and the measurement instruments. At the same time, it is important to keep as long a distance as possible between the measurement signals and the power wiring or any sources of EMI. Ethernet's inherent flexibility helps in this respect.
New use models enabled by LXI include that of intelligent sensors, which combine both the sensor and the measurement hardware in a small package, possibly powered over Ethernet. Similarly, for manufacturing test applications, critical signals might be measured or generated in the test fixture, very close to the DUT.
Another important aspect with regards to flexibility is communication. Here, again, LXI enables new use models by allowing instruments to communicate with each other directly without arbitration through the system controller. Ethernet's TCP protocol is used for peer-to-peer communication, whereas UDP is available for multicast (one-to-many) communication.
8: Synchronisation through IEEE1588
IEEE1588 is one of the key enabling technologies behind LXI. With this standard, included in LXI class A and B instruments, Ethernet becomes a much better fit for measurement applications.
In the kind of distributed applications enabled by LXI, there will be intelligent instruments that perform measurement tasks on their own, independent of the system controller. To make this approach practical, instruments will typically have a local clock which enables them to timestamp measurements and events. The purpose of IEEE1588 is to synchronise the various clocks in the system.
The advantage is that IEEE1588 works across Ethernet. No additional cables are required. Depending on the size of the network (or more precisely, its variation in latency times), clocks can be synchronised down to 10ns or better. 100ns of synchronisation are easily achievable in a typical test system with its own local subnet.
9: Rack space
The LXI standard includes an optional mechanical specification. Primarily intended for automated applications, such instruments (called LXI units) will typically be faceless (no display, no buttons), half rack width and one or several rack units high. Without a physical user interface, depending on the type of instrument, manufacturers will be able to shrink their instruments in size considerably.
Another advantage of LXI is that it requires no card-cage. In many applications, card-cages are not fully used, either because the application doesn't require it or because the user intentionally leaves a number of slots empty for future extension. With LXI, system extension is much easier and only the space required will be consumed.
10: Synthetic Instruments
LXI is the ideal enabling technology for synthetic instruments (SI). With SI, instruments are broken down into major their building blocks. This approach is especially well suited to RF instruments, but it applies to other types of instruments as well. For example, an RF vector signal analyzer can be broken down into a down-converter, a digitizer and analysis software.
If the instrument is broken down into more elementary building blocks, the individual components can be re-used for a different application. For example, the analysis software and digitizer may be used with a new, higher frequency down-converter.
As you break traditional instrument down into components, communication between these components becomes a critical factor. With Gigabit Ethernet, LXI offers the data rates and the flexibility of TCP/IP (peer-to-peer and concurrent communication) at the same time and so is an ideal solution for SI interconnection.
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