By Bryson Barney
What is going on when the network is connected correctly, but strange things are happening? Communication only intermittently works, and node failures resulting in field returns are more common than you would expect. The confusing part is that none of these issues show up in a lab test mimicking a typical network.
Probing with a scope on one of the receiver inputs reveals that the differential signals are swinging way beyond the RS-485 standard limits at 500 kHz. As seen in the scope plot below, the C3 trace shows the data input into the transceiver, while the C1 and C2 signals show a 500 kHz signal from the environment being coupled onto the cable that the transceiver is driving. The receiving transceiver will see this data with 500 kHz superimposed onto it.
Figure 1: Scope Trace
Explanation
In 1983 the EIA-485 standard was published, which wisely defines a wide common mode range of -7 V to +12 V. The common mode voltage is defined as the common DC voltage on the A and B lines of a differential pair relative to a reference voltage, typically ground at the receiver. In ideal circumstances the common mode voltage would stay close to zero volts well within the standard range, but the longer the cable distance between nodes, the more susceptible the network will be to external conditions that will cause the common mode voltage to move beyond acceptable limits. This happens because there is now substantial cable resistance between nodes. Per ohms law, any current that passes over that cable resistance will shift the voltage at the other end by I*R. There are a few different things that can cause dynamic current to flow, such as capacitive coupling, inductive coupling, and radio frequency (RF) coupling. These are common in real RS-485 networking applications such as industrial automation systems, renewable energy installations, and automotive and transportation networks. Higher quality cables with shielding can be used to reduce the amount of coupling to outside noise sources that the electrical signals experience, but these cables are costly. An alternative approach is to use transceivers with extended common mode range. These transceivers are specifically designed to deal with large common mode shifts. ADI offers several options within its selection of robust RS-485 transceivers. For example,
±40V Common Mode Range
- MAX33070E/MAX33071E/MAX33072E/MAX33073E/MAX33074E – Half-Duplex, 3.3 V to 5 V, Common Mode Range RS-485 Transceivers
- LTC2863 and LTC2862A – Half- or Full-Duplex, 3.3 V to 5 V, RS-485 Transceivers
Should I Consider Transceivers with an Extended Common Mode Range?
The answer depends on common mode swing. If you anticipate having any of the typical sources (as described in the following sections) in your application, then extended common mode range transceivers will bring your protection to a higher performance level across your network and avoid costly communication problems.
Figure 2: Common Mode Swing Sources – Motors and High Voltage Power Supplies
Motors and High Voltage Power Supplies Nearby
Motors and high voltage power supplies on the factory floor can radiate large amounts of electromagnetic interference (EMI) due to their operating principles where large amounts of current are switched. These switching currents induce strong magnetic fields that can couple to cable runs. This is an example of RF coupling.
Figure 3: Common Mode Swing Sources – Cables with Power and Data Bundled Together
Power and Data Bundled in the Same Cable
RF coupling can occur when network nodes are connected to power supplies that use AC power over cables and have a local ground reference. Cabling with AC supply wiring bundled in the same cable with the data signals can cause unintended current to flow in the data lines through capacitive coupling.
Figure 4: Common Mode Swing Sources – Ground Potential Differences Between Buildings
Building-to-Building Networking
Ground potential differences from one area to another are especially common when networking cables go from one building to another.
Figure 5: Description of Ground Voltage Differences between Buildings
Figure 5 shows how a generalized system transmits data between two widely separated buildings and shows the earth currents created between ground points in a single-phase power distribution system. Similar currents are created in 3-phase, Y-connected systems. Note: The 5 V and 8 V values shown here are examples of how much potential could exist in such a system. The numbers are arbitrary for the purpose of the explanation and would be different in a real-world application.
Bonding a power-line subscriber's neutral line to a ground rod sunk in the earth at the power entry point establishes the power line neutral as a safety ground. From that point, a bare or green-insulated wire carries the safety-ground reference to all electrical outlets and equipment installed throughout the premises. Industrial chassis frames are bonded to the safety ground at the chassis' power input point, where it becomes frame ground.
Leakage currents in the safety-ground wire due to AC primary or secondary neutral currents in the power distribution system, can produce a potential difference between the neutral and frame ground, ranging from a few volts to several tens of volts.
These potential ground loops and leakage current paths cause a multitude of common mode problems via coupling through all sources (inductive, capacitive, and RF).
Conclusion
If disruptive signal swings plague your network, don’t despair. Extended common mode range RS-485 transceivers will allow transmitted data to be received even in the presence of external coupling that pushes the common mode beyond RS-485’s original limitations. Interested in getting started, go to analog.com and search for Interface and Isolation Products.
See the blogs in the TranscendingConventionalFieldBus series.