Control System Checks Need More Than Multimeter

Increased Complexity Requires More Monitoring Tools to Verify Data Integrity in a Network

By Ian Verhappen

Often taken for granted, the physical layer and associated protocols that connect your field devices to your control system I/O and then the various nodes of your control system are fundamental to reliable operation. Being able to predict the health of a system requires monitoring changes over time. In the case of networks, that translates ideally into real-time monitoring tools or at minimum preventive maintenance measures of regular data gathering from which you can prepare trend charts.

Because control systems are becoming more complex, a breakout box and multimeter are no longer the only tools required to verify data integrity in a network. Networks and digital protocols typically rely on a change in voltage or current at a "rapid" rate for communications, so measuring a single voltage will not tell you about message integrity.

You won’t need an optical time-domain reflectometer (OTDR) often, but because fiber tends to be used for backhauls, you likely can share an OTDR with the IT team, or alternately ask IT to include your fiber as part of their testing.

Oscilloscopes are able to capture waveforms, but then you must figure out which waveform is coming from which device to identify which node is causing the problem. Once you make the waveform association, interpreting what it means often requires expertise for each unique protocol.

With Ethernet becoming more commonly deployed in modern systems, the typical engineer and systems maintenance person now is required to be proficient in at least capturing that data for wired — and now wireless — networks as well. Open-source tools such as Wireshark, which allows you to investigate the contents of each packet, and Network Diagnostic Tool (NDT), which is designed to quickly and easily identify a specific set of conditions that are known to impact network performance, can get you started at little cost. NDT performs the following tasks to monitor network performance: simple bi-directional test to gather end-to-end (E2E) data, gather multiple data variables from the server, compare measured performance to analytical values, and then translate network values into plain text messages.

Fortunately, with the increased use of wireless technology, associated network tools also have been developed, with the most well-known and oldest of them being Netstumbler. Netstumbler begat NetSurveyor, which is similar but also includes a recording/playback feature that’s useful for the comparative aspect required of maintenance systems. NetSurveyor also has a useful "add-in" comparison tool called NetStress.

A tool similar to Netstumbler is inSSIDer, which is designed to detect wireless networks and report on their type, maximum transfer rate, and channel usage. It includes a graphical representation of each wireless network’s amplitude and channel usage.

Another tool that shows how well your wireless network is performing is Wireless Wizard, which includes a spectrum analyzer that recommends the best wireless channel to use. This feature is useful when designing networks or determining why a network connection is "overloaded."

A final wireless tool, CommView for WiFi (available as a free download and commercial product) supports more than 70 protocols. It is almost the wireless equivalent to Wireshark for 802.11a/b/g/n networks because this program allows you to capture packets and search them for specific strings, packet types to display protocol layers and packet headers.

Network monitoring tools are handy for gathering data, but are not very useful until the meaning of the data is interpreted. Fortunately, the tools themselves are getting smarter, providing interpretation; and several manufacturers have developed commercial tools that are able to help interpret the data more fully.

The above will allow you to see your network, but understanding what is happening often requires protocol-specific tools, especially for field-level bus networks. There are tools available for each of the fieldbus protocols for continuous and offline monitoring. When you purchase protocol-specific tools, be aware of the infrastructure that the tools themselves might require such as power supply, computer, data collected and its presentation, and, of course, communication of captured data to the appropriate maintenance or monitoring tool.