Communication between devices must be reliable in modern industrial, automotive, and energy applications. Yet, engineers often encounter strange situations where everything seems to be connected well but the networks start acting strange out there in the field. Intermittent loss of communication and unexpected breakdown of nodes become tremendously expensive issues-a major reason for frustration, for such problems hardly pop their heads in the lab.
After more research, it is discovered that the sneaky evil is common mode voltage swings brought on by external circumstances. The swings push the signal outside the limits set in the original RS-485 standard, thereby maring the data exchange. Today’s extended CM range RS-485 transceivers are meant to protect networks against such eventualities.
When Noise Invades the Network
Figure 1: Scope Trace
A plain scope probe can narrate a grand story. At 500 kHz, it is not unusual to observe differential signals severely distorted due to external coupling. The transceiver receives data swathed in extraneous noise, and communication becomes unstable.
The 1983 EIA-485 standard provided for such situations by specifying a large common mode voltage range of -7 V to +12 V. Theoretically, this would be safe for signals. Practically, long cable lengths, wire resistance, and common mode external interference can move common mode levels far beyond that range.
Sources of Common Mode Swings:
A number of real-world conditions can inject interference currents into RS-485 networks:
- Motors and High-Voltage Power Supplies
Figure 2: Common Mode Swing Sources – Motors and High Voltage Power Supplies
Motors used in industry and switching power supplies produce powerful electromagnetic fields. These fields tend to couple readily into surrounding cabling, inducing unwanted RF interference on the data lines.
- Power and Data Bundled Together
Figure 3: Common Mode Swing Sources – Cables with Power and Data Bundled Together
Running AC power cables alongside RS-485 data cables within the same cable causes capacitive coupling, which injects currents directly into the communication network.
- Ground Potential Differences Between Buildings
Figure 4: Common Mode Swing Sources – Ground Potential Differences Between Buildings
Figure 5: Ground Voltage Differences Explained
When networking across buildings, ground potential differences can create large voltage shifts. Leakage currents from safety grounds or neutral wiring often raise the common mode voltage, pushing it outside the RS-485 tolerance.
Why Extended Common Mode Range Transceivers Help
Rather than depend solely on costly shielded cables, a better solution is to implement RS-485 transceivers that have wider common mode ranges. These chips are designed to maintain reliable data transfer even when external noise forces voltages well outside the legacy standard.
Examples include:
MAX33070E – MAX33074E: Half-duplex, 3.3 V to 5 V products with ±40 V common mode tolerance
LTC2862A and LTC2863: Half- or full-duplex products with improved noise immunity
Through the implementation of these resilient solutions, engineers are able to safeguard networks against downtime, minimize field failures, and lengthen the lifespan of industrial systems.
Conclusion:
Installing and operating RS-485 networks under regulated settings is nearly impossible. Motors, power supplies, bundled cables-if anything-shift voltages around in ways that disrupt communication.
With that said, the way to go will be to design with extended common mode range transceivers that offer a safety margin against any such disruptive swings and thus ensuring communication stays steady and reliable even in the harshest industrial weather.
(This article has been adapted and modified from content on Analog Devices.)
