What are the grounding problems of RS-422 and RS-485?

Last Update Time: 2021-12-01 10:59:45

Electronic system grounding is important, but it is often overlooked. Improper grounding treatment often leads to the inability of the electronic system to work steadily or even endanger the system safety. The grounding of RS-422 and RS-485 transmission networks is also very important, because the unreasonable grounding system will affect the stability of the entire network, especially in the case of harsh working environment and long transmission distance, the requirements for grounding More strict. Otherwise, the interface damage rate is high. In many cases, when connecting RS-422 and RS-485 communication links, simply use a pair of twisted pairs to connect the "A" and "B" ends of each interface. While neglecting the connection of the signal ground, this connection method can work normally in many occasions, but it has buried a lot of hidden dangers. There are two reasons:

1. Common mode interference problem: As mentioned above, both RS-422 and RS-485 interfaces use differential transmission of signals, and there is no need to detect the signal relative to a reference point. The system only needs to detect the The potential difference is sufficient. But people often neglect that the transceiver has a certain common mode voltage range, such as the RS-422 common mode voltage range is -7 ~ +7V, and the RS-485 transceiver common mode voltage range is -7 ~ +12V, only meet the above Conditions, the entire network can work normally. When the common-mode voltage in the network line exceeds this range, it will affect the stability and reliability of communication, and even damage the interface. Taking FIG. 1 as an example, when the transmission driver A sends data to the receiver B, the output common mode voltage of the transmission driver A is VOS. Since the two systems have independent grounding systems, there is a ground potential difference VGPD. Then, the common-mode voltage VCM at the input of the receiver will reach VCM=VOS+VGPD. Both RS-422 and RS-485 standards stipulate that VOS3V, but VGPD may be very large (tens of volts or even tens of volts), and may be accompanied by strong interference signals, causing the receiver's common-mode input VCM to exceed the normal range , And produce interference current on the transmission line, which will affect normal communication at least, and damage the communication interface circuit at the worst.


2.(EMI) problem: The common-mode part of the output signal of the driver needs a return path. If there is no low-impedance return channel (signal ground), it will return to the source in the form of radiation, and the entire bus will look like a The huge antenna radiates electromagnetic waves outward.

Due to the above reasons, although RS-422 and RS-485 use differential balanced transmission, for the entire RS-422 or RS-485 network, there must be a low-impedance signal ground. A low-impedance signal ground connects the working grounds of the two interfaces so that the common-mode interference voltage VGPD is short-circuited.

This signal ground can be an extra wire (unshielded twisted pair) or the shielding layer of the shielded twisted pair. This is the most common method of grounding.

It is worth noting that this method is only effective for high-impedance common-mode interference. Due to the large internal resistance of the interference source, it will not form a large ground loop current after short-circuiting and will not have a great impact on communication. When the internal resistance of the common-mode interference source is low, a large loop current is formed on the ground line, which affects normal communication. The author believes that the following three measures can be taken:

(1) If the internal resistance of the interference source is not very small, a current limiting resistor can be added to the ground wire to limit the interference current. The increase in grounding resistance may increase the common mode voltage, but as long as it is controlled within an appropriate range, it will not affect normal communication.

(2) Adopt floating ground technology to isolate ground loop. This is a more commonly used and very effective method. When the internal resistance of the common mode interference is very small, the above method can no longer work. At this time, it is possible to consider floating the node that introduces interference (such as field equipment in a harsh working environment). That is, the circuit ground of the system is isolated from the chassis or the ground), so that the ground loop is cut off, and no large loop current will be formed.

(3) Use isolated interface. In some cases, for safety or other considerations, the circuit ground must be connected to the chassis or the ground and cannot be suspended. In this case, an isolation interface can be used to isolate the ground loop, but there should still be a ground wire to isolate the common side of the isolation side. Connect to the working place of other interfaces.


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