Correctly understand the wrong ideas in RS-485 wiring process

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RS485 bus is widely used in video surveillance, access control intercom, building alarm and other fields due to its simple wiring, stability and reliability. However, many problems arise in the wiring process of 485 bus due to many inaccurate concepts. Now we will clarify some wrong concepts.

1. The 485 signal line can be routed together with the strong power line. In actual construction, since the lines are routed through pipelines, the construction party sometimes directly ties the 485 signal line and the power line together for convenience. Since the strong current has a strong electromagnetic signal that interferes with the weak current, the 485 signal is unstable, resulting in unstable communication.

2. 485 signal lines can be wired using parallel lines or unshielded lines. Since 485 signals are transmitted using differential mode, that is, the voltage difference between 485+ and 485- is used as the signal transmission. If there is an external interference source, when using twisted pair cables for 485 signal transmission, due to the twisted pair, the interference effect on 485+ and 485- is the same, and the voltage difference remains unchanged, and the interference to the 485 signal is minimized. In the same way, if there is a shielded cable to play a shielding role, the interference effect of external interference sources on it can also be minimized as much as possible.

3. Choose to use ordinary Category 5e shielded twisted pair cables, i.e. network cables. Due to the rising prices of raw materials, the wires on the market are mixed. Some unscrupulous merchants use some alloys to replace copper wires to make network cables, and copper-plated them on the outside to deceive customers. Specific distinguishing method: look at the cross-section of the network cable. If it is copper-colored, it is copper wire. If it is white, it is an alloy. Alloys are generally brittle and easy to break, and their conductivity is far inferior to that of copper wire, which can easily cause problems in engineering construction. It is generally recommended to choose standard 485 wires, which are shielded twisted pair cables. The transmission line is not a single strand of copper wire like a network cable, but multiple strands of copper wire twisted together to form a wire, so even if a small copper wire breaks, it will not affect the entire use.

4. 485 wiring can be arranged in star or tree configurations. 485 wiring specifications require hand-in-hand wiring. Once it is directly arranged in star or tree configurations without the help of 485 hubs and 485 repeaters, it is easy to cause signal reflection and bus instability. Many construction parties use star or tree configurations during 485 wiring. Sometimes the entire system is very stable, but sometimes problems always occur and it is difficult to find the cause. This is usually caused by non-standard wiring. If a star or tree connection is required due to site restrictions, the 485 hub and 485 repeater of Shenzhen Fuyongtong Technology Co., Ltd. can be used to solve 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 suggestion on how to ground it. Strictly speaking, the 485 bus must be reliably grounded at a single point. A single point means that there can only be one point grounded on the entire 485 bus, and multiple points cannot be grounded. The reason for grounding it is to keep the voltage on the ground wire (usually the shielded wire is used as the ground wire) consistent to prevent common-mode interference. If multiple points are grounded, it will be counterproductive. When reliably grounded, the ground wire of the entire 485 line must have good contact to ensure consistent voltage. Because in actual construction, in order to facilitate wiring, the wire is cut into multiple sections and then connected, but the shielded wire is not well connected, so that the ground wire is divided into multiple sections, the voltage cannot be kept consistent, resulting in common-mode interference.