LXI Test Instruments Tutorial

- summary or tutorial about the basics of 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.

Some of the main features of LXI instrumentation include:

  • Ethernet connectivity:   With Ethernet being one of the major formats used for networking and data transfer, LXI test equipment has adopted this format to build on a standard that is already widely used and adopted.
  • TCP/IP:   Using Ethernet, the adoption of TCP/IP follows on as a necessary requirement. IPv4 and IPv6 are both supported.
  • Trigger:   In order to enable precise triggering and timing, LXI test equipment is able to have a hard wired trigger. Where sub-nanosecond timing is required, software controlled triggers over the LXI interface are not sufficiently fast.

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.

By Ian Poole

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