Diagnosing Signal Disturbances

Signal disturbances are signal integrity deviations induced onto low voltage digital network signals by harsh industrial operating environments. Typical causes include transients, static discharge, and harmonics generated by plant machinery.

Disturbances can be continuous, periodic or random by nature:

• Continuous – causes include Variable Frequency Drives, High Voltage HID lighting systems, or ground loop currents caused by improper wiring, loose or corroded connectors.
• Periodic or random – causes include static discharge from a conveyer belt or breaker contacts opening and closing. Signal drop outs are often due to loose connections caused by vibration.

If industrial network signals are conveyed differentially on two conductors, most noise is ultimately canceled out at the transceiver. However, certain disturbances can still impact network performance:

• In the case of static discharge, devices may be subject to periodic bursts of high energy. These bursts will cause devices to reset, and, over time, cause permanent failure.
• Accidental ground loop currents can "confuse" the state of device transceivers, disrupting communications.

Always baseline segments at installation. Any change in waveform characteristics is usually an indicator of a problem's root cause.

Analyzing signal integrity:

• Use an oscilloscope to capture signal disturbances. Inspect the waveform's amplitude or frequency parameters, looking for disturbances that correlate to other devices on the plant floor. (See figure 1)
• Capture waveforms along the path of the network segment in question. Watch for changes in the waveform: increasing amplitude indicates you are moving closer to the source.
• Using process of elimination, toggle surrounding devices on then off, watching the waveform to see if disturbances appear and then disappear.
• To capture random disturbances, use an oscilloscopes with a pass fail mode. Create a waveform reference template. Any waveform points that fall outside the template will then be automatically internally stored for later analysis.

Figure 1:Examples of signal disturbances of an oscilloscope