You are on your way to catch a flight to India where you will help an important customer integrate your company’s solar-powered base station into a rural area near Bangalore. As you board the plane, you want to check the status of ongoing environmental tests being performed in your lab on an updated base station designed for hot and humid rural climates. You whip out your iPhone, connect to your Agilent 34972A LXI DAQ unit, and read the results from the most recent temperature scans. Everything checks out, so you raise the temperature of the chamber via the digital control module on the DAQ unit. You power down your smart device for take-off and settle in for a long flight.
Is it a reality or a pipedream?
Smart devices, including smartphones and tablet PCs, are rapidly becoming pervasive. At this point, no one can say with certainty what impact these devices will have on the test and measurement industry. These smart devices are a natural fit with the LXI standard since they can communicate with an Ethernet network either using Wi-Fi or through a cellular provider’s network via the Internet. The wireless freedom of smart devices further increases the remote flexibility of LXI instrumentation.
Smart devices are a new remote instrument control platform, so there is not much software and not many programming tools available to assist you. However, free code to get you started controlling instruments remotely on the iOS platform (iPhone, iPod, iPad) is available.
Communicating at the Network Level
There are three different network hierarchies for using a smart device to connect to an LXI instrument: Wi-Fi local network, Wi-Fi nonlocal network, and wireless cellular network.
Wi-Fi Local Network
The local network is the easiest but least flexible hierarchy. In this configuration, the smart device is connected via Wi-Fi to the same local network that the LXI instrument is connected to, which means there are no firewalls or network security barriers between them (Figure 1).
Figure 1. Connecting Via Local Network
You can test this method easily using the LXI instrument’s built-in Web server. Simply connect to the local Wi-Fi network with your smart device, obtain the IP address of an LXI instrument connected to the same local network, open the Web browser on your smart device, enter the IP address of the instrument, and press go. The instrument’s introductory Web page should pop up. Of course, connecting to the instrument via a local Wi-Fi network does not provide much distance flexibility since a Wi?Fi network range typically is limited to 32 meters indoors and 95 meters outdoors.
Wi-Fi Nonlocal Network
A Wi-Fi nonlocal network involves connecting to a Wi-Fi network with your smart device and reaching across the cloud to the local network where your LXI instrument is connected (Figure 2). You encounter this type of hierarchy when you try to connect to an instrument in the office or lab from your home wireless network. The challenge is getting through network security measures such as firewalls.
Figure 2. Wi-Fi Nonlocal Network
Suppose you are fortunate enough to work from home, and you have your LXI instrument connected to your home router, which is connected to the Internet. By default, your router is configured to let connection requests leave your local network but not come in.
Network software assumes everything on your local network is a client, not a server, and it blocks incoming connection requests to protect the machines on your local network. Your LXI instrument is a server, so when your smart device tries to connect to it from across the cloud, your router will block it.
To overcome this problem, use the port-forwarding or demilitarized-zone (DMZ) capability on your router. Port forwarding tells your router to forward all network traffic on a particular port or ports that you define to a local IP address on your network (in this case your LXI instrument). With DMZ capability, your router makes a specified machine on your local network appear as if it is directly connected to the Internet.
I do not recommend the DMZ method for any instruments running a Windows operating system because it leaves them wide open to cyber attacks.
For most of us, our LXI test and measurement instruments are on a company or organization network heavily fortified from the Internet. In this case, you will need company- or organization-approved virtual private network software on your smart device to gain access to your office network. Also, you would want to consult with your IT department in case your network is configured to restrict certain types of incoming network traffic that encompasses your smart device.
Figure 3. Connecting Across Wireless Provider’s Network
Wireless Cellular Network
The wireless cellular network hierarchy uses your smart device’s wireless provider’s network to connect across the cloud to your instrument (Figure 3). In this scenario, we face the same network security challenges we encountered in the nonlocal network hierarchy. However, using the cellular provider’s network allows greater remote flexibility because you no longer are restricted by the range of typical Wi-Fi networks, but your data bandwidth is limited compared to a Wi-Fi connection.
Large bandwidth is not really needed in this remote instrument model. Since smart devices are used on the go, you cannot guarantee a constant connection between you and your instrumentation, so you can only use it as a measurement monitoring tool and to make simple noncritical control decisions. Data traffic that consists of only measurement readings and simple control SCPI commands does not require much bandwidth.
Communicating at the Code Level
In addition to the various ways you can connect a smart device and communicate with an LXI instrument from a network standpoint, an important piece of the puzzle is the software that tells the hardware and network how to facilitate communications.
Unfortunately, there currently are no apps for connecting and communicating with LXI instrumentation with smart devices. The LXI standard requires each instrument to have a built-in server that can host a Web interface accessible via a Web browser. In its most basic form, the Web interface must provide users access to the LAN settings of the instruments. Most test and measurement vendors have gone the extra step and added instrument control capability to the Web page.
Smart devices have Web browsers so you can use them to connect and control LXI instruments that provide control capability, right? The connection part works fine when you use a smart device’s Web browser. You enter the IP address into the browser, hit go, and see the instrument’s introduction page show up in the browser.
But, the majority of instrument control Web interfaces requires some type of plug-in, typically Java, to access the control user interface. These plug-ins currently are not available on smart device operating systems, which means using the instrument’s LXI Web pages to control the instrument won’t work with a smart device in most cases.
That leaves us only one choice if we want to use our smart device to control our LXI test and measurement instrument: create the software ourselves. When you create software on a Windows-based PC, many tools are available, such as VISA and IVI-COM drivers that make instrument programming easier by doing all the low-level connection management
and data handling for you.
As of the writing of this article, the only way to connect and communicate with an LXI instrument using a smart device is through network sockets. Anyone who has worked with sockets knows network programming is not easy.
When you use drivers, you don’t have to worry about some of the programming challenges that low-level sockets normally present. For example, you do not have to manage connections and format incoming and outgoing data.
There are many aspects to managing a connection to an instrument, but one of the trickiest is dealing with methods that block, or stop, execution until the method completes. A good example is the well-known C BSD socket API. In BSD, the connect() and recv() methods in their default state will block until they get a reply. If the device your program is communicating with loses its network connection and you call the connect() or recv() function, your program will stop execution with no way to recover other than restarting it. Obviously, this is undesirable.
There are multiple ways to prevent these functions from blocking, such as setting up a timeout or executing them on a separate thread. Of course, this adds complexity to your software development.
Data handling is another aspect of using low-level sockets that adds time and complexity to programming. For example, you need to be able to handle data streams of various lengths from the instrument. The data comes in strings, so you’ll need to convert to integers and floats at your desired precision level.
Some of the smart device development environments provide tools that add levels of abstraction to low-level sockets. These tools can make managing a connection or handling data easier. But keep in mind when using these tools that they are mainly created to connect and interact with Internet websites, not test and measurement instrumentation.
Conclusion
Smart devices offer a portable and interactive way to access information and data wirelessly from almost anywhere. They access information and data through Ethernet using a provider’s cellular network or a Wi-Fi connection. Since the LXI connection standard is Ethernet based, LXI is a natural fit for monitoring and controlling test and measurement instrumentation with smart devices.
Using smart devices to remotely monitor and control test and measurement instrumentation provides unmatchable location flexibility. It allows you to access instrumentation from almost anywhere so you are no longer tied to a desk or lab during long tests.
At the time this article was written, there was no software or drivers for using smart devices to control instrumentation. This means the only way to use a smart device to monitor and control test and measurement instruments is to get down and dirty with the code yourself. One thing you can count on: As smart devices continue to become a bigger part of our digital and wireless lives, they will find their way into the test and measurement industry.
About the Author
Neil Forcier is an application engineer for the Systems Products Division at Agilent Technologies. Previously, he served in the U.S. Navy as an electronic test equipment calibration technician on-board the USS Harry S. Truman CVN 75. Mr. Forcier earned a bachelor’s degree in engineering from Pennsylvania State University. Agilent Technologies, 5301 Stevens Creek Blvd., Santa Clara, CA 95051, 970-679-5272, [email protected]