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LXI, the "Next Generation" Replacement for GPIB

 By: Fred Bode, LXI Consortium Administrator and Executive Director*

Where is LAN / LXI Best used for Instrument Control?

 By:  Patrick Webb, Instrument Control Product Manager, National Instruments

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This issue's focus is LXI Products. 
You can view and add to our existing list of Test Products/Services, Test Literature, Test Definitions, Test Vendors, containing "LXI"

 By: Fred Bode, LXI Consortium Administrator and Executive Director*

GPIB has been around for over 30 years, and there is no question it is still the most popular interface for Instrument to Computer connection.  However, there is also no question that it is getting a little long in the tooth, and a number of different interfaces have been considered as replacements.  USB and IEEE-1394 (Firewire) were each thought to be candidates for replacing GPIB, but they failed to become popular and have not had much impact in replacing GPIB.

Lately, instrument companies have been increasingly putting an Ethernet interface on their instruments in addition to the GPIB connector.  They also have been experimenting with adding additional functionality to the Ethernet port.  The potential benefits are great.  By building on the well established Ethernet components and software protocols, the interface is very well understood, the components are inexpensive, CAT 5 cabling is pennies per meter, data transfer speed dramatically improves, and interconnect distance limitations of GPIB disappear.

Let’s see...  It is cheaper, better, longer and faster...  What are we missing?

Well, for one thing, as different companies started to add Ethernet connectors and started to add new interface capability, the customer solutions started to diverge, and different implementations from different companies were incompatible.  That is when the LXI Consortium stepped up.  Started in 2004 by Agilent and VXI Technology, and quickly joined by Keithiley, Measurement Computing, Rohde & Schwarz and other leading Instrument companies, the LXI Consortium has grown to over 40 members in less than two years.  It’s purpose is to define the LXI standard in a way that can simplify the integration of Ethernet connected (LXI compatible) products so that developing an automated test system is even easier than with GPIB.  

By the way, there is no question in anyone’s mind that GPIB instruments will be around for a long time, as will card based instruments such as VXI and PXI systems. Therefore, part of LXI’s success depends on making sure that current generation instruments with GPIB, VXI and PXI interfaces and form factors can be easily integrated with LXI instruments.  LXI calls these Hybrid systems and is busy defining adapters or other required components to assure flawless integration is part of the LXI specification.

LXI simplifies the systems integrator’s job by offering an effortless transition from GPIB.  The initial step is as simple as replacing a GPIB cable with a CAT-5 LAN cable and changing a global address variable.  It’s that easy.  There is no software to change except the address string.  Specs remain the same, and systems designers can use existing non-LXI GPIB, PXI, and VXI equipment in hybrid systems using bridge adapters.  

There are three types of LXI instruments, which LXI describe as Classes, with increasing capability.

• LXI Class C: If basic LAN connectivity is all that is required for local or remote applications, then Class C would suffice and be most cost effective.  However, LXI goes much farther by offering new tools and capabilities.  LXI simplifies system set up with automatic discovery, automatic speed negotiation, automatic IP address configuration through a DHCP server, and DNS naming.  In addition, the lower cost interface advantages of base Class C instruments compare favorably with the relatively expensive and often cumbersome GPIB adapters and cabling.

• LXI Class B: If the application requires or could benefit from scheduled triggering down to the 10's or 100's of nanoseconds, then Class B devices should be a consideration.  A new IEEE Standard, IEEE-1588 Precision Time Protocol, offers the system designer the ability to integrate time into his programming arsenal. A unified trigger model has been defined that treats LAN triggers, hardware triggers, and time triggers the same.  This greatly simplifies programming.  By using this new timing standard, systems designers can utilize precision timing in a variety of ways.  One is by adding precision timestamps to the data.  Another is by using the capability of peer-to-peer triggering, a new capability for most test and measurement applications and not available with GPIB.  If a power supply detects an over-voltage situation and needs to shut the system down, it can now communicate with all instruments in the systems without going through the systems controller, saving time and clock cycles in a potential dangerous situation.

• LXI Class A:   If the application requires VERY tight event timing (low nanoseconds), then Class A devices would be the best choice.  Fashioned after the VXI high speed trigger bus, the LXI wired trigger bus features a high speed 8 channel LVDS design that can be used in a star or daisy chain configuration to bring ultra high speed, low latency, low jitter trigger signals to applications requiring a direct link between two devices.  With the Unified Trigger Model, LAN triggers, time triggers, and wired triggers all operate using an identical syntax, allowing programmers to quickly change trigger types by simply changing global variable names.  

For simple applications such as a single instrument that requires PC control and data connectivity (as opposed to completely manual operation), LXI also shines.  The significant LXI benefit for simple, single instrument applications (perhaps even a cluster or 2-3 instruments) is the zero installation, web interface capability.  While LXI minimum implementations do not require sophisticated browsable GUI interfaces to be baked into the instruments, Agilent, Keithley and many other LXI implementations have already shown such advanced capabilities in currently released LXI products.  So when choosing an LXI instrument, a selection might be made on the basis of which LXI instrument has sufficient embedded control and data capabilities to eliminate the need for companion application software packages saving the customer some money and associated complexity.

Perhaps the most surprising aspect of this whole LXI development is the speed at which it is catching on.  It was about two years ago that the LXI Consortium was formed (November 2004) and about a year since Rev 1.0 of the specification was released at AUTOTESTCON (September) 2005. By June of 2006 there were over 100 LXI compatible products which have passed LXI conformant testing, been released and are shipping. To view a list of LXI conformant products go to the LXI Consortium web site.  While you are there, look around the LXI web site and learn more about this exciting emerging new standard.

(*Fred Bode wrote this article exclusively for The BestTest Newsletter in June - just before he retired.)

  By:  Patrick Webb, Instrument Control Product Manager, National Instruments

Today there are numerous methods to connect to your test instrument. This includes GPIB, PXI, USB, and LAN to name a few. Each of these has advantages in different applications. Test systems should be architected to take advantage of the best instrument control bus for the application. National Instruments suggests designing your system to support any bus in any programming language to provide this flexibility. This article will focus on where LAN-based instruments are most useful and common pitfalls of LAN-based instrument control.

Ethernet, or LAN, has been used for instrument control for more than 15 years. Its most obvious strength is in the distance allowed between instrument connections which is practically limitless through the use of LAN switches and routers. This capability is crucial when systems need to distribute measurements over a long distance, or to put measurement instruments close to a source, but far away from a controlling or monitoring PC. With a properly configured secure network, LAN can also be used for remote diagnostics; for example to view the configuration of an instrument in a remote test location.

The LXI Consortium, for LXI (LAN-based eXtensions for Instrumentation), was created in 2005 to supplement the traditional specifications for LAN with requirements for test and measurement instruments. Figure 1 compares the features of LXI with existing LAN instruments.

Figure 1 -  A comparison of LAN and LXI instrument features

IEEE-1588 allows synchronization over a LAN network. Using specialized LAN hardware, IEEE-1588 devices are capable of achieving synchronization in time of within +/-100 ns. This capability makes IEEE-1588 attractive for applications with relatively low acquisition rates (below 1 MS/s) that require synchronization over large distances. The LXI Hardware Trigger bus is a shared set of LVDS (Low-Voltage Differential Signaling) that provide greater synchronization accuracy using a separate specialized cable in addition to the Ethernet cable. This specialized cable limits the distance between LXI instruments, counteracting the benefit of the LAN infrastructure for distributed applications.

However, when used in a non-distributed fashion, such as in desktop or rack mount use, LAN based equipment has a number of weaknesses. LAN based instruments have long latency specifications, require high processing overhead, are complex to configure, and there are a limited number of LAN instruments due to the slow adoption of LAN as an instrument control bus.

Latency is an important specification to consider due to its impact on throughput. Throughput is determined by both the latency and the bandwidth of the bus. Latency measures the delay of transmission of data, while bandwidth measures the rate at which data is sent across the bus. Lower latency improves the performance of applications that require a large number of small commands or data sets to be transferred such as control applications while higher bandwidth is important in applications such as waveform generation and acquisition. Figure 2 compares the latency and bandwidth of various instrumentation buses. Because LXI is LAN-based, it exhibits the same latency and bandwidth specifications as LAN.

Figure 2 - LAN provides good bandwidth but poor latency performance

The processing overhead on LAN can increase the cost of a LAN instrument in two ways. First, in high speed systems, a desktop or server class processor will likely be required to process the TCP/IP stack. Second, when real-time data rates can not be achieved over LAN, the instrument designer must instead embed processing for data reduction into the instrument. This raises the cost of the instrument and system and reduces the user’s flexibility.

Another weakness of LAN is the setup required to establish a LAN network. While for complex applications this may not be a concern, it can be a burden when compared to USB in desktop applications. LAN requires an IP address and other network configuration and may be subject to IT policies of the network on which it is installed. In fact, many of the benefits of remote diagnostics of a LAN instrument may be negated by a company’s particular IT policy with regards to firewalls and other network security.

Even though Ethernet has been around longer than GPIB, and used in instrument control for at least 15 years, it remains a niche bus in instrument control. There are still only a few hundred LAN instruments (mostly Fast Ethernet) compared to over 10,000 GPIB instruments. Today, LAN is used primarily for systems where a long distance between instruments is a requirement. For desktop applications you may consider using USB because of the ease of configuration.  When connecting to legacy equipment, GPIB might be the logical choice.  If you are designing a test system with limited space such as a manufacturing floor and demanding high throughput, PXI is the sensible choice.

Real-world systems use multiple bus technologies within modular system architecture to take advantage of the best attributes of each. For example, National Instruments recommends you build your test system around PXI as the core. This allows the flexibility to connect to any instrument control bus, LAN/LXI, USB, GPIB, while leveraging the modularity and high bandwidth / low latency specifications of PXI based instruments. When selecting instruments, make sure the instrument comes with a software instrument driver so that you can easily architect a hybrid system in your software of choice.

For more information about LAN/LXI and other Instrument Control options, please take a look at the National Instrument tutorial, Instrument Control Fundamentals.

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