Summary of Errors in RS-485 Wiring

Because of its simple wiring, stable and reliable, RS485 bus is widely used in video surveillance, access control intercom, building alarm and other fields. However, there are many problems in the 485 bus wiring process due to many incomplete and accurate concepts. Now let's summarize some of the wrong concepts.

1. The 485 signal line can be routed together with the strong power line. In the actual construction, since the traces are all taken through the pipeline, the construction party sometimes directly binds the 485 signal line and the power line for the convenience of the figure, because the strong electric power has a strong electromagnetic signal to interfere with the weak electricity, thereby The 485 signal is unstable, resulting in unstable communication.

2. 485 signal lines can use parallel lines as wiring, or unshielded lines can be used as wiring. Since the 485 signal is transmitted using differential mode, the voltage difference between 485+ and 485- is used as the signal transmission. If there is an external interference source to interfere with it, when using twisted pair for 485 signal transmission, due to its twisted pair, the interference effect is the same for 485+, 485-, and the voltage difference is still unchanged. The interference to the 485 signal is reduced to a minimum. By the same token, if a shielded wire is used as a shielding function, the influence of external interference sources on it can be reduced as much as possible.

3. Choose to use the ordinary super five shielded twisted pair cable or network cable. Due to the rising prices of raw materials, the wire rods on the market are now mixed, and some bad businessmen use some kind of alloy to replace the copper wire to make the network cable, and copper plating on the outside to mix customers. The specific difference method: look at the cross section of the network cable. If it is copper, it is copper wire. If it is white, it is filled with alloy. Alloys are generally brittle, easy to break, and are far less conductive than copper wire, which can easily cause problems in engineering construction. Generally, it is recommended to select the standard 485 line, which is a shielded twisted pair. The transmission line is not a single strand of copper wire like a network cable, but a plurality of strands of copper wire are twisted together to form a line, so that even a small copper wire Broken, it will not affect the entire use.

4. 485 wiring can be arbitrarily arranged into star wiring and tree wiring. The 485 wiring specification is a wiring that must be hand-held. Once it is not directly connected to a star connection and a tree connection by means of a 485 hub and a 485 repeater, it is easy to cause signal reflection to cause bus instability. Many construction parties use star wiring and tree wiring in the 485 wiring process. Sometimes the whole system is very stable, but sometimes there are always problems, and it is difficult to find the cause, generally due to irregular wiring. Caused by it. If you have to make a star connection or a tree connection due to site restrictions, you can use the 485 hub and 485 repeater of Shenzhen Fuyongtong Technology Co., Ltd. to solve the related problems.

5. The 485 bus must be grounded. In many technical documents, it is mentioned that the 485 bus must be grounded, but there is no detailed description of how to ground. Strictly speaking, the 485 bus must be reliably grounded at a single point. The single point is that there can only be one point grounding on the entire 485 bus, and it is not possible to ground more points because it is grounded because the voltage on the ground wire (generally the shielded wire is used as the ground wire) is kept consistent to prevent common mode interference. If multiple points are grounded, it is counterproductive. When grounding reliably, the ground wire of the entire 485 line must have good contact, so as to ensure the voltage is consistent, because in actual construction, for the convenience of wiring, the wire is cut into multiple sections and then connected, but the shielded wire is not well connected. The ground wire is divided into multiple sections, and the voltage cannot be consistent, resulting in common mode interference.