RS485 bus is widely used in various fields such as video surveillance, access control intercom, and building alarms due to its simple wiring and stable reliability. However, many problems arise during the wiring process due to several inaccurate concepts. The following is a summary of some common misconceptions.
1. The 485 signal cable can be run alongside the power supply cable. In actual construction, since the cables are usually run through conduits, sometimes the construction team, for convenience, directly bundles the 485 signal cable and the power supply cable together. Because high-voltage electricity has a strong electromagnetic signal that interferes with low-voltage electricity, this can cause the 485 signal to become unstable, leading to unstable communication.
2.485 signal lines can be wired using either parallel or unshielded wires. Since the 485 signal uses differential mode transmission, meaning it's transmitted based on the voltage difference between 485+ and 485-, if an external interference source interferes, using twisted-pair cable for 485 signal transmission ensures that the interference has the same effect on both 485+ and 485- due to the twisted pair; the voltage difference remains constant, minimizing interference to the 485 signal. Similarly, if shielded cable is used, the interference from external sources can be reduced as much as possible.
3. Ordinary Cat5e shielded twisted-pair cable (network cable) is sufficient. Due to rising raw material prices, the market is flooded with substandard cables. Some unscrupulous merchants use alloys instead of copper wires to make network cables, plating them with copper to deceive customers. To distinguish them: check the cross-section. If it's copper-colored, it's copper wire; if it's white, it's an alloy. Alloys are generally brittle and easily break, and their conductivity is far inferior to copper wire, easily causing problems during installation. It's generally recommended to choose standard 485 cable, which is shielded twisted-pair cable. Unlike network cables with single-strand copper wires, the transmission line consists of multiple copper wires twisted together to form a single wire. Therefore, even if one small copper wire breaks, it won't affect the overall functionality.
4.485 cabling can be laid out in either a star or tree topology. However, the standard for 485 cabling requires a daisy-chain configuration. If a star or tree topology is directly laid out without the use of 485 hubs and repeaters, signal reflection can easily occur, leading to bus instability. Many contractors use star and tree topologies during 485 cabling, sometimes resulting in a very stable system, but other times causing persistent problems that are difficult to diagnose. These issues are generally caused by improper cabling. If site constraints necessitate a star or tree topology, 485 hubs and repeaters from Shenzhen Fuyongtong Technology Co., Ltd. can be used to resolve these problems.
5. The 485 bus must be grounded. Many technical documents mention that the 485 bus must be grounded, but they don't specify in detail how. Strictly speaking, the 485 bus must have a single-point reliable ground. A single point means that only one point on the entire 485 bus can be grounded; multiple grounding points are not allowed. This is because grounding is necessary to maintain a consistent voltage on the ground wire (usually the shielded wire) to prevent common-mode interference. Multiple grounding points would have the opposite effect. For reliable grounding, the ground wire of the entire 485 line must have good contact to ensure consistent voltage. In actual construction, for ease of wiring, the wire is often cut into multiple segments and then connected, but the shielded wire is not properly connected, resulting in multiple ground wire segments and inconsistent voltage, leading to common-mode interference.
