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According to the technical engineer who has been engaged in this technical work for a long time: 1RS485 communication short distance (within 800m) can generally use unshielded ordinary twisted pair 2 When the interference is heavy, the shielded twisted pair can be used (the shield is suspended) / or only one end of the ground - to be separated from the "strong electric ground", especially the "inverter ground") 3 signal ground is generally suspended (not wired), if the grounding is not good, but can not communicate properly.
Most netizens are also above this opinion. However, some netizens believe that the interconnected devices should be common; otherwise, if the receiving end has a higher common mode input, it may damage the receiving device.
In most cases, when connecting the RS-485 communication link, simply connect the "A" and "B" ends of each interface with a pair of twisted pairs, and leave the signal ground - this connection The method works fine in many situations, but you should pay attention to the following two problems:
(1) Common mode interference problem: The RS-485 interface receives the input signal in a differential manner, and does not need to detect the signal relative to a certain reference point. The system only needs to detect the potential difference between the two input lines. However, people often overlook that the receiver has a certain common-mode voltage range. The common-mode voltage range of the RS-485 receiver is -7~+12V. Only when the above conditions are met, the entire network can work normally. When the common mode voltage in the communication line exceeds the range, the communication is stable and reliable, and even the interface is damaged.
(2) Electromagnetic radiation problem: the common mode part of the driver output signal needs a return channel. If there is no low-resistance return channel (signal ground), it will return to the source end in the form of radiation, and the whole bus will be like a huge The antenna radiates electromagnetic waves outward.