LXI test instruments

- a summary or tutorial about LXI, LAN eXtension for Instrumentation, what it is and how it works, and how it can be used for controlling test instrumentation

Ethernet 802.3 is a technology that has gained almost universal acceptance as a LAN technology, enabling widespread connectivity to be achieved. It is therefore a natural step to adopt Ethernet as a communications medium for linking test instruments.

Although Ethernet has been used in this role for some time, the problem is that the there is no standard and the way instruments communicate over Ethernet varies from one instrument to the next. It is this problem that LXI seeks to address. In this way LXI will simplify the development of automated test programmes and equipment that operate over LANs using Ethernet.

Background to LXI

For many years there has been a growing trend towards automated test systems using bus technologies to enable a central controller to coordinate several test instruments, initiate measurement and store and process the results. With electronic equipment becoming more sophisticated, so too the need for more comprehensive testing has developed. As already mentioned the GPIB was developed in the late 1960s and early 1970s. This provided the first generic standard that could be widely used. Realizing the size of many test systems, VXI technology was developed. It used as its core the VME bus. Although fast, and gaining wide acceptance it was nevertheless expensive. As a result PXI and a number of other technologies came to the fore.

However with Ethernet appearing to offer an ideal medium for fast, efficient and cheap communications there was a need to develop an industry standard to address the problem. If no common standard were developed, then different instruments would have different interfaces and complete implementations of ATE systems using Ethernet communications would not operate as well as would be possible using a standardised interface.

To overcome this problem a consortium was set up in 2004. Known as LAN-based eXtensions for Instrumentation, LXI, its aim was to ensure that there would be interoperability between local area network (LAN)-based instruments. As such LXI builds on the Ethernet standard. It was thought that a host of independent implementations for instrumentation over Ethernet would result in poor interoperability and prevent this becoming a viable successor to GPIB.

Ethernet has been in existence for many years. However it is still at the forefront of technology as a result of the fact that it has been continually developed. By adopting it as a standard, LXI technology is likely to see a long life. This is good news for any company investing in the technology either as a developer of equipment, or as a user buying equipment and systems.

Basic concepts

The LXI standard uses an Ethernet backbone. Instruments are designed to be rack mounted and contain their own power supplies. This gives a number of advantages. The first is that it removes all the problems of heat dissipation, crosstalk and the like that occurred in systems such as VXI. Secondly, the fact that Ethernet can allow communication over a wide area enables the creation of test systems with LXI that cover a wide area. Conceivably using switches, routers, or hubs, an LXI test system could be created to cover a whole production area using a single controller and in this way provide a highly integrated system.

The LXI standard

The LXI standard addresses both the mechanical and electrical aspects of the equipment. In this way it means that equipment not only operates together, but can also be mounted in the same way, allowing systems to be built more easily.

The LXI standard defines that test equipment is rack mounted, either one or two units high, and either full or half width. LXI also defines that the equipment contains its own power supply, processor and LAN connections. Although these may appear obvious, setting down a standard enables full compatibility.

Additionally as the LXI instruments are controlled via a LAN connection, they do not require controls of displays. Instead the user can interact with them via web browsers and COM drivers. In this way redundant controls are eliminated and the cost of the LXI instruments is reduced. Additional mechanical constraints call for the LXI instruments to have signal input / output on the front panel, and AC power and LAN connections at the rear.

The electrical of functional elements of LXI are being defined. A wide variety of issues are being defined so that the instruments can operate together in a similar manner. To achieve this it is necessary to address a variety of factors including triggering, interrupt handling, discovery, synchronization of multiple devices, and software interfaces. Of these timing, synchronisation and triggering are some of the most challenging as the distances involved and delays in the system can introduce small delays that may be unacceptable in some circumstances.

The triggering will use a Trigger Bus that will be similar to that used on GPIB, and will also be compatible with VXI. It will use a high speed differential LVDS interface.

Timing is another important issue, especially where sub-microsecond synchronisation is needed. To enable the LXI to be able to achieve the required levels of performance the IEEE 1588 Precision Timing Protocol has been adopted. This allows enables users to simplify cabling by triggering instruments over the LAN instead of using the Trigger Bus. It ahs also been developed for use over LAN applications to provide accurate synchronisation regardless of the separation of the elements.

Summary

With Ethernet usage growing, and the need to fast data communications to link test and measurement instrumentation, LXI is bound to make a large impression on the market place when it is fully defined and launched. The specification is now in its advanced stages, before being fully adopted. Even before its final ratification, it is likely that product will hit the market in anticipation. When this happens it may sound the death knell for GPIB which has given magnificent service for around 40 years.