- summary of the overall LXI standard and the other standards that are used to deliver the functionality.
With Ethernet and Ethernet technology widely used for data communications, the LXI standard was developed to utilise this technology while providing a common standard that would enable test instruments and controllers from different manufacturers to operate in the same system.
The LXI standard or specification has been developed and is maintained by the LXI Consortium. This industry body ensures that all interested parties are represented and therefore that LXI technology meets the needs of industry, while maintaining compatibility between equipment from different vendors.
The LXI standard addresses all aspects of LXI equipment. 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 uses 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 has also been developed for use over LAN applications to provide accurate synchronisation regardless of the separation of the elements.
External LXI standards
While the LXI standard manages to specify many aspects of its operation, it also utilises a number of other specifications and standards.
These other LXI related standards include:
- Ethernet IEEE 802.3.
- TCP/IP (both versions IPv4 and IPv6 are supported.
- Discovery protocols (VXI-11, etc)
- W3C web browser standards
- IEEE 1588 precision timing protocol
- M-LVDS TIA/EIA-899
Using all these standards means that all aspects of LXI operation is standardised and means that the equipment can be built to conform with these standards and hence interoperability should be maintained.
By Ian Poole
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