What Is LXI?

Figure 1. Current LXI Standard 1.4 Core With Optional Extensions

GPIB (the IEEE 488 standard) has been around for over 30 years and is a common bus for instrument control. While the LXI Consortium advertises LXI as the successor to GPIB systems, GPIB is a rugged bus that is highly relied on and not likely to disappear in the foreseeable future. 

With GPIB and serial communications, customers can automate the control of their instruments and test systems. Newer buses such as USB and Ethernet are giving customers additional options when choosing a bus to control their test systems.  When the test and measurement industry adopted Ethernet as an instrumentation bus, it leveraged the benefits of existing LAN technology.  To ensure interoperability, the LXI Consortium created the LXI standard as a guide for instrument vendors.

Each system’s requirements determine which buses are appropriate for that particular application. Sometimes engineers must work with test systems at remote locations and trigger their tests over the Internet. LAN-based instruments and systems allow engineers to control test systems from a distance. Even simple systems may need to interact with LAN-based systems and instruments, and the rules in the standard help solve this problem. Over time numerous LXI specification changes have modified the back-end technologies used by LXI devices. These changes can affect performance, usability, and functionality.

All versions of the specification document back to Version 1.1 are available for download from the LXI Consortium website.  Whether your device includes the latest technology depends on when your particular device was designed and tested and which specification was current at that time. Engineers can refer to their device documentation to determine with which specifications their devices comply. Knowing exactly which specification a device complies with can help customers avoid possible setbacks.  For example, a digital multimeter (DMM) model from Agilent, the 34411A, conforms to the LXI 1.0 specification. Although this DMM is LXI compliant, it does not offer the features of later LXI specifications such as a connection LED on the instrument face or LAN Configuration Initialization (LCI), the option to restore the device to factory defaults. Similarly, the ways that networked computers discover LXI compliant devices vary depending on which versions of the standard the devices comply. Table 1 outlines some of the different features and technologies available with different versions of the LXI specifications.

     *Available With Extension

Table 1. Specification Versions Versus Technologies and Features

The majority of certified LXI instruments (about 56 percent as of January 2, 2013) are power supplies. Additionally, the latest specification, Version 1.4, changed the hierarchy of device grouping. Previous versions of the standard organized LXI-compliant devices into three classes: A, B, and C. Class C defined the base functionality that all LXI devices provided. A handful of devices (Class B) provided advanced IEEE 1588 timing options, and a few others (Class A) could use a wired triggered bus (WTB). In the new device hierarchy of the 1.4 standard, all devices are part of a Core Standard, with optional extensions for IEEE 1588 and WTB. Extensions have also been added for Event Messaging, Timestamped Data, Event Logs, and HiSLIP. More information on IEEE 1588 and WTB is mentioned in the LXI Synchronization Options section.

Different applications have different requirements that can be met with different communication buses. A comparison of the most common buses is outlined in Table 2, and a simple hybrid system is shown in figure 2.  Customers must consider timing, latency, and bandwidth when deciding on a bus. Latency measures the delay of data transmission, while bandwidth measures the rate at which data is sent across the bus, typically in megabits per second (Mb/s). When comparing an instrumentation bus to a highway road, the latency correlates to the delay of stoplights in the road while the bandwidth correlates to the width of the road and the speed of travel. Lower latency generally improves the performance of applications that make many small measurements, such as DMM measurements, switching, and instrument configuration. Higher bandwidth is generally important in applications such as waveform generation and acquisition or RF measurements. 

While most desktop and laptop computers currently use 1 Gb/s Ethernet, most LXI instruments today are limited to 100 Mb/s. In the future, instrument vendors will incorporate faster Ethernet interfaces, but these updates are likely to lag behind the highest available Ethernet speeds.

Table 2. Bus Comparison

Figure 2. Simple Hybrid System

There are three types of LXI devices: instruments, bridges, and adapters. LXI instruments are the most common. They are single instruments that either lack submodules or feature submodules connected via a proprietary interface (for example, a DMM with switch modules). These devices are fully LXI compliant and undergo the LXI compliance process. LXI bridges provide a bridge or gateway from LXI to non-LXI buses (for example, LXI-to-VXI bridge). The LXI bridge is fully LXI compliant, but the “downstream” non-LXI devices are not. LXI adapters, on the other hand, offer a fully compliant LXI interface for each of the downstream non-LXI devices.

Figure 3. Instrument Types and Components

LXI devices, which come in many shapes and sizes (Figure 3), are required to use the standard RJ45 Ethernet connector. Because users want flexibility in designing their test systems, many LXI devices also support GPIB and/or USB interfaces on the same device.

Many LXI devices have a traditional vendor-defined front panel like other box instruments. However, some LXI devices are faceless and do not have a fixed front panel. These devices rely on software programming or the LXI web page interface to function.

Different LXI use cases affect the configuration of the network that’s needed. Three typical LXI use cases result in different LAN configurations (Figure 4). Instruments may be on an isolated subnet, connected across a building or campus, or even viewable anywhere in the world on either the public Internet or a virtual private network (VPN). Each of these three scenarios adds complexity.

Figure 4. Network Use Cases

The isolated subnet case uses a switch or hub to connect the instruments with the PC to optionally provide a firewall between the instrument network and a corporate network. Hubs, which are less expensive and simpler to install, generate less packet jitter but can potentially increase network collisions depending on the number of instruments and traffic load. Switches, on the other hand, cost more and decrease network collisions but increase packet delays and jitter.

The across-campus case requires directly connecting to a corporate intranet and usually involves the IT department to help address security concerns such as OS updates, user access, and virus scanning. For most implementations, the instruments remain “hidden” inside a corporate firewall and are not available to remote users, but campus networks also enable multiple clients to access the same instruments at the same time. Users must implement a resource management scheme to ensure that only one client at a time can change the state of an instrument. An LXI common resource management standard is currently under development.

Remote access from anywhere in the world requires the use of a router with VPN and security features. The IT department usually installs and manages the routers. The benefits of access anywhere in the world are remote collaboration, debugging, and maintenance. The drawbacks include security, high packet jitter, router cost and maintenance, and resource management across many users.

Bus synchronization options are another important bus evaluation criterion. Most ATE systems require some level of synchronization between the instruments and the device under test (DUT). The amount of synchronization depends on the DUT and test requirements. The extension options for IEEE 1588 over Ethernet and LXI Wired Trigger Bus are discussed below.

Figure 5. Time Synchronization

If an application requires use of IEEE 1588 synchronization it is important that customers determine whether their instruments are compliant with a spec prior to Version 1.3.  Instruments compliant with LXI Versions 1.1 and 1.2 adhere to the IEEE 1588-2002 standard. Instruments that comply with later versions 1.3 and 1.4 comply with the more recent IEEE 1588-2008 standard.  These standards are not compatible. When building hybrid systems, please follow the suggestions in Recommendations for LXI Systems Containing Devices Supporting Different Versions of IEEE 1588.

To use the advanced timing of the IEEE 1588 protocol, National Instruments provides the NI PCI-1588 and the NI PXI-6683. The PCI-1588 and PXI-6683 can support either standard using NI-Sync driver software. NI-Sync versions 3.0 and earlier support IEEE 1588-2002. NI-Sync versions 3.1 and later support IEEE 1588-2008. For new test systems needing network synchronization, NI suggests using the IEEE 1588-2008 standard.  Also, NI CompactRIO devices support IEEE 1588-2008 for real-time applications.

The IEEE 1588 precision time protocol (Figure 5) synchronizes time-of-day clocks on multiple devices to the same absolute time. Synchronized time-of-day clocks provide a common time base for the following:

1. Event time stamping on multiple nodes for later correlation
2. Phase-locked loops (PLLs) to local clocks and signals (for example, sampling clock)
3. Execution of synchronized events using future time events

For applications that have less strict timing requirements, instrument vendors may implement a software-only version of IEEE 1588 that usually generates tens of microseconds of clock skew. The more common option for test and measurement is the hardware-assisted implementation that provides sub microsecond clock skew.

In addition to the implementation of the network interface, the accuracy of the synchronization depends on the topology of the network as well as the quality of the master timing source. The more complex the topology, the less accurate the synchronization. Some of this can be mitigated using specialized IEEE 1588 network hardware such as boundary clocks or transparent clocks.  Without this additional hardware, routers can introduce timing jitter caused by network delays that compound en route to triggered devices. Because of these issues, users often create a separate network for LXI systems to help improve network and timing performance. The private subnet also prevents the IEEE 1588 packets from loading other networks.

For more information on IEEE 1588, see the recommended resources at the end of this document.

The WTB is defined in a separate specification to allow for changes in the WTB specification independent of the core LXI specification. WTB may be used with traditional box instruments (for example, some Rohde & Schwarz spectrum analyzers), faceless instruments (for example, the Agilent N82xx family), and LXI bridges (for example, the VTI Instruments LXI-to-VXI bridge).

Figure 6. Typical WTB System

WTB cables use eight low-voltage differential signal (LVDS) cable pairs with terminators at both ends to reduce reflections. The WTB supports both Driven and Wired-OR modes. In Driven mode, one device drives the pair while other devices monitor the state of the pair. With Wired-OR mode, multiple devices can drive the pair with the last device causing the state to change (for example, indicating a trigger condition).

While only a few devices come with wired trigger bus connections, the WTB provides a lower-latency and lower-jitter synchronization option than the LXI IEEE 1588 extension. The WTB supports a 5 ns pulse for 10 m or a 10 ns pulse for 20 m between devices. The challenges with the WTB are that cables and terminators are fairly expensive and that  trigger fidelity, latency, and skew are affected by cable length and topology.

Figure 7. LXI Instrument Types

WTB increases the debugging effort because the timing and fidelity of the trigger signals need to be tested during development. An LXI WTB breakout box (bottom left-hand corner of Figure 7) is recommended to analyze the timing and fidelity of WTB signals with an oscilloscope. 

Beyond the emergence of the LXI standard in the industry, it is important to remember that no single bus or technology is the ideal solution for every application.  You should understand the specific instrument control needs of your application before choosing an instrument control bus.  For your applications involving LXI, NI simplifies LXI and LAN complexities by offering a complete suite of instrument control software. Additionally, as the LXI Consortium continues to revise the standard with new options, NI software and hardware for LAN and LXI instrument control minimize the changes to your test systems.

• Introduction to Distributed Clock Synchronization and IEEE 1588 PTP
• See PXI-6683 PXI Synchronization Module details