LXI instrument networks fit every need

The LAN eXtensions for Instrumentation (LXI) links test instruments through a local-area network (LAN) rather than through an older and slower bus such as IEEE 488, the general-purpose instrumentation bus (GPIB). Because LXI instruments use Ethernet protocols to communicate, they give engineers many ways to set up a test-system network. Thus, LXI based systems can employ private networks, connect to the World Wide Web or mix and match network configurations. The network architecture chosen depends upon engineer’s needs and test objectives. Several configurations that show a range of capabilities and trade-offs are presented in this article.



DEDICATED NETWORK

A basic dedicated-network configuration (Figure 1) for an LXI test system comprises a PC connected to a router that connects to LXI-based instruments. LXI instruments can operate up to 100m from a switch or router, but engineers can add switches and routers to extend that distance almost without limit.



This configuration treats all devices attached to the router as equals. Any device can communicate with any other device in the system without disruption from external network traffic. And, a dedicated network can run at the full communication speed of the computer, routers and instruments.

The inherent isolation of a dedicated network guarantees high throughput and protects the network from external viruses and users. This type of network requires only an Ethernet interface on the PC and a router with enough ports to handle the maximum number of LXI instruments that engineers need. The router should operate at the speed of the fastest instrument or computer.



CONNECT TO THE WEB

Because LXI instruments use the standard TCP/IP protocol, they can connect to the Web (Figure 2). A local server assigns each instrument an IP address consistent with its subnet. Any user who knows an instrument’s address can communicate with it from almost anywhere. But this type of network configuration presents security and performance challenges.



Most test instruments do not provide firewalls or antivirus software. Thus, an instrument connected to the Web could succumb to the same viruses that attack PCs. Instruments such as oscilloscopes, analyzers and signal sources that use a Windows operating system require antivirus and firewall protection. (Common PC viruses would not affect many instruments that use non-PC operating systems.) Instruments connected to the Web also may suffer from throughput limits. OC commands and instrument data must travel through a number of intervening hubs, routers and switches that can decrease throughput along with Internet traffic.



Never connect mission-critical systems to the Web because of possible discovery and configuration problems, data and control-command collisions, data-security issues and vulnerabilities to outside control. On the other hand, a Web connection works well for product and system demonstrations because worldwide users can access them.



CONNECTION TO A COMPANY NETWORK

Instruments connected to a company Intranet (Figure 3) let many users access a test system, and firewalls on company servers protect the instruments fairly well from random or unknown users who might try to control instruments through a Web connection. Test engineers must ensure that only appropriate internal company users have access to the instruments.



Company-network traffic, such as e-mail and file transfers, can delay messages going to and from instruments. So, placing a test network on a company Intranet can degrade performance. Time-critical and mission-critical test systems should have a dedicated network not tied directly to an Intranet.



ROUTER-ISOLATED SUBNET

A router-isolated subnet (Figure 4) separates the test system from company Intranet. The wide-are network (WAN) port on the router connects to the Intranet, and the other router ports connect to a computer and the test instruments. The router assigns private IP addresses to the computer and the instruments, and it blocks company users from accessing the test subnet.



The router also performs the network-address translation between the subnet devices and the company network so that all devices in the subnet can access the company network through the router’s IP address. This approach works well when engineers need to update instrument firmware or pass test data from the PC to a company database. In a router-based system, the router uses the domain host control protocol (DHCP) to dynamically assign IP addresses to networked devices. Thus, every time engineers turn an LXI instrument on, it will receive an IP address, but that address may not match the UP address it had previously. Engineers have two ways to ensure that an instrument remains at the same IP address:

1. Disable DHCP on the instrument and assign it a fixed IP address.

2. Use a dynamic naming system (DNS) to give each instrument a unique name.



The router translates between the name and the address. Some older LAN-based instruments may not support DNS, and some older company networks may not support network-address translation or virtual private networks.



PC-ISOLATED SUBNET

In a PC-isolated subnet, a PC provides two network interface cards (NICs); one NIC connects to the company network, and the other connects to instruments via a switch (Figure 5). The PC assigns IP addresses to the instrumentation and the company network assigns an IP address to the computer’s second NIC. The use of separate NICs isolates the subnet so company-network users can “see” the PC but not the instruments.



Most engineers prefer a PC-isolated subnet for LXI test systems. Only the PC can access the company network, while the instruments on the subnet run at top speed without concerns for network security. External users can access the instruments by running a commercial “remote desktop” application on the test-system PC. In this way, outside users can control the instruments and view data on their own computer as if they sat in front of the test-system PC. The instruments continue to operate at full speed without any knowledge of interactions between the remote user’s computer and the test-system PC.



When an instrument is turned on, the test-system PC assigns it an IP address which could differ from its previous IP address. The approaches described above overcome this problem.



A virtual private network (VPN) could let engineering access this type of test system when they work from a remote location. They connect to the company network via the Web and then access the test system. Again, the instruments in the subnet continue to operate at full speed.



SYNCHRONIZE LXI CLOCKS

LXI Class-A and –B devices can provide precise timing using a boundary clock: a special LAN switch with a built-in IEEE 1588 clock and software protocol (Figure 6). The boundary clock – used in place of the test-system network switch – provides accurate synchronization among Class-B LXI devices. The boundary clock timestamps messages as they enter and exit the switch, eliminating the time required for messages to queue up and go through the router based on calculations prescribed in the IEEE 1588 algorithm. For accurate synchronization, engineers should connect Class-A and –B LXI devices to the boundary clock and connect Class-C and non-time-critical instruments to a router.



The choice of network topology depends upon the objectives of the instrument system. A small company’s R&D department might find it useful to share test instruments by connecting them to the company network, although business traffic may slow instrument operation. A large company may choose to use the PC-isolated approach to group instruments in private subnets. A manufacturing department could also use PC-isolated subnets so that remote users may access the test system which can send test results to a company database.



Grant Drenkow is a strategic planner for future test system products from Agilent Technologies. He holds a BS in electrical engineering and an MBA in marketing.



For further reading

• You can find the LXI Consortium, a list of member companies, and the LXI standard at: www.lxistandard.org.

• The LXI Connexion Website provides many helpful articles and reference information: www.lxiconnexion.com.

• Test and Measurement World Magazine’s Website contains many articles about LXI concepts and instruments: www.tmworld.com. Search for “LXI.”

• The Interchangeable Virtual Instruments (IVI) Foundation promotes the development and adoption of standards for programming test instruments, including those that use the LXI bus. Engineers can download the standards, technical information and code examples. Links from the Foundation’s Website help users locate IVI drivers for specific instruments: www.ivifoundation.org.

• The “Embedded Systems” column in this issue of ECN covers IEEE 1588, a timing standard that synchronized remote clocks on an Ethernet network. LXI instruments can use this synchronization technique to maintain accurate timing across a test-and-measurement network. The National Institute of Standards and Technology maintains a Website devoted to this standard, as well as other precision-timing topics: ieee1588.nist.gov.

• Agilent Technologies offers a non-promotional white paper, “10 Good Reasons to Switch to LXI: Key advantages that enable better systems.” The paper also includes references to many application notes and resources: cp.literature.agilent.com/litweb/pdf/59894372EN.pdf.



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