Regarding the grounding issue of the 485 bus

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Regarding the grounding issue of the 485 bus, in applications involving this bus, if one simply uses a pair of twisted pairs to connect the “A” and “B” terminals of each interface, ignoring the interconnection of the signal ground, such a connection method may work fine in many cases. However, it poses significant risks, for two reasons: 1. Common-mode interference problem: Although the 485 bus transmits signals in a differential manner and it seems that there is no need to determine the signal relative to a certain reference point, as the system only needs to detect the voltage difference between the two wires, people often overlook the fact that any 485 interface IC has a certain range for acceptable common-mode voltages, typically between -7 and +12V. Only by meeting this condition can the entire network function properly. When the **modulation voltage on the network line exceeds this range, it affects the stability and reliability of communication, and may even damage the interface. For example, when transmitter A sends data to receiver B, the output common-mode voltage of transmitter A is VOS. Since the two systems have separate grounding systems, there is a ground potential difference VGPD; as a result, the common-mode voltage at the receiver’s input becomes VCM = VOS + VGPD. The RS-485 standard specifies that VOS ≤ 3V, but VGPD can reach very high values, such as tens of volts or even hundreds of volts, and may be accompanied by strong interference (rapid fluctuations). This causes the receiver’s common-mode input to exceed normal limits, resulting in interference currents on the transmission lines. In mild cases, this affects normal communication, while in severe cases it can damage the communication interface circuitry. 2. EMI issue: The common-mode component in the transmitter’s output signal requires a return path; without a low-impedance return path (the signal ground), it will return to the source in the form of radiation (note the AC model of 485). In this case, the entire bus acts like a giant antenna that emits electromagnetic waves. For the above reasons, although 485 uses differential balanced transmission, a low-impedance signal line must be provided across the entire 485 network to connect the ground terminals of various interfaces, thereby short-circuiting the common-mode interference voltage VGPD. This signal line can be an additional wire (unshielded twisted pair), or the shield layer of a shielded twisted pair. This is the most common grounding method. It is worth noting that this approach is only effective against high-impedance common-mode interference; due to the high internal resistance of the interference source, no large ground loop current is generated upon shorting, so it has little impact on communication. If the internal resistance of the common-mode interference source is low, a large loop current will be generated in the ground wire, affecting normal communication. In such cases, the following solutions should be adopted: (1) Use floating ground technology to break the ground loop. This is a commonly used and highly effective method. When the impedance of common-mode interference is very low, the aforementioned method becomes ineffective; in such cases, it is possible to float the node that introduces the interference (for example, field devices located in harsh operating environments), meaning isolating the system’s circuit ground from the chassis or the earth. This prevents the formation of grounding loops and thus avoids large loop currents. (2) Use an isolation interface. In some cases, for safety or other reasons, the circuit ground must be connected to the chassis or earth and cannot remain floating; in such situations, an isolation interface can be used to break the grounding circuit, but there should still be a ground wire that connects the common terminal on the isolated side to the working ground of the other interfaces.