In the 485 bus application, if the 'A' and 'B' ends of each interface are simply connected by a pair of twisted pairs, and the interconnection of the signal ground is neglected, this connection method can be used in many occasions. It works normally, but it has buried a lot of hidden dangers. There are two reasons for this:
1. Common mode interference problem: Although the 485 bus uses differential transmission signals, it does not seem to need to determine the signal relative to a certain reference point. The system only needs to detect the potential difference between the two lines, but some people often ignore it. Any 485 interface IC always has a certain common mode voltage tolerance range, such as the general -7 ~ +12V, only to meet this condition, 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 the communication, and 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, then the common-mode voltage VCM at the receiver input. Will reach VCM=VOS+VGPD. The RS-485 standard stipulates that VOS ≤ 3V, but VGPD may have a large amplitude such as ten or even hundreds of volts, and may be accompanied by strong interference (fast fluctuation), causing the receiver common mode input to exceed the normal range and on the transmission line. Interference current is generated, which affects normal communication and damages the communication interface circuit.
2. EMI problem: the common mode part of the transmitter output signal needs a return path. If there is no low-resistance return channel (signal ground), it will return to the source end in the form of radiation (note the 485 AC model), the whole The bus radiates electromagnetic waves outward like a huge antenna.
For the above reasons, although the 485 uses the differential balanced transmission method, for the entire 485 network, there must be a low-resistance signal to connect the working ground of each interface, so that the common mode interference voltage VGPD is short-circuited. This signal ground can be an additional line (unshielded twisted pair) or a shielded shielded 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. Since the internal resistance of the interference source is large, a large ground loop current will not be formed after short circuit, which will not have a great influence on communication. If the internal resistance of the common mode interference source is low, a large loop current will be formed on the ground line, which will affect normal communication. In this case, the following solution should be adopted:
(1) Use floating technology to isolate the ground loop. This is a more common and very effective method. When the internal resistance of the common mode interference is small, the above method can not be effective. In this case, it is considered to float the node that introduces interference (such as a field device in a harsh working environment). That is, the circuit ground of the system is isolated from the casing or the ground. This cuts off the ground loop and does not form a large loop current.
(2) Use an 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 isolated interface can be used to isolate the ground loop, but there should still be a ground wire to isolate the common side of the ground. Connected to the work site of other interfaces.
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