Technology Overview

LXI is the LAN-based successor to GPIB. The LXI standard goes beyond GPIB to provide additional capabilities that reduce the time it takes to set up, configure and debug test systems by connecting directly to the standard ports on a PC. It also removes the inherent cable length limitations of GPIB. It does not require special cards or cables, or Slot 0 controllers. Also, LXI software and drivers simplify test system set-up. LXI helps integrators leverage the time and effort already invested in system software and architecture.

Classes of LXI conformance

LXI-conformant instruments or modules are divided into three classes, A, B and C, with the functionality of the classes hierarchically based one upon the other. All instruments of these three classes can be integrated in a test system and combined with each other according to the requirements and application at hand.

The basic class C comprises of a standardized LAN implementation with VXI-11 discovery and a web server for easy configuration through a regular browser. For remote control of a class C conformant instrument an IVI-API (application programming interface) is prescribed. The required IVI (interchangeable virtual instrument) driver uses a standardized set of commands for an easy interchangeability of similar devices. Class B conformance includes all capabilities of class C plus a real time clock with synchronization mechanism to IEEE 1588. This allows time precise triggering using PTP (precision time protocol) and highly accurate time stamping of events. The highest class A of LXI conformance includes all capabilities defined in classes B and C. An additional eight port high-speed trigger interface enables triggering over distances up to 20 m with an even improved accuracy.

Class C

LXI instruments complying with class C are characterized by a common LAN implementation. This includes the capability of automatically detecting LXI instruments in a LAN via the Discovery protocol in accordance with VXI-11. Other definitions and functions such as LAN configuration initialize (LCI) – which resets the LAN configuration – make it easy to integrate these LXI instruments in a LAN-based test system. Owing to a uniform interface, the instruments can be configured via a web browser.

Class C instruments are programmed by means of IVI drivers (APIs), which were standardized by the IVI Foundation. The architecture of these drivers and their interoperability are important prerequisites for system integration.

Class B

Class B compliant instruments are additionally equipped with synchronization mechanisms defined in the IEEE 1588 standard. Thus, high-precision time synchronization of better than 10 ns can be achieved in a 100BaseT LAN. This technique largely avoids the latency times typical of LAN and ensures precise timing, which is indispensable in T&M applications.

Class B is based on common timer events tied to absolute times, which consequently allow very precise synchronization of test system sequences. Data that is to be transferred via LAN can be marked with the exact detection time by means of time stamps. Thus, the recorded data can also be correlated from different sources in distributed systems. Especially the capability to exactly trigger and synchronize widely distributed systems via the precision timing protocol of the IEEE 1588 standard opens up completely new applications.

Class A

In addition to the functions of the other two instrument classes, LXI instruments in accordance with class A are equipped with an eight-channel hardware trigger interface (LVDS interface), whose type of connector, pin assignment and electrical characteristics are defined in the LXI standard. Via this interface, instruments can be connected either in a daisy chain or star configuration. Trigger cable lengths of up to 20 m are thus feasible. The trigger channels can be configured individually as input or output channels; plus, they offer a wired-or function.

Instruments of classes A and B can generate and receive software triggers via LAN messages (UDP and TCP/IP messages) – as currently done in test systems as well. Thus, LXI instruments can communicate with each other without involving the controller (peer to peer). The different trigger and synchronization capabilities in LXI can be configured via the controller by means of an enhanced IVI interface (LXI Sync).