The RS485 bus is widely used in various fields such as video surveillance, access control intercoms, and building alarms due to its simple wiring and reliable stability. However, many inaccurate concepts during the wiring process can lead to various issues. Here are some summarized misconceptions.
1. The 485 signal line can be run alongside high-voltage power lines. In actual construction, since wiring is usually done through conduits, contractors sometimes bind the 485 signal lines directly with the power lines for convenience. However, the strong electromagnetic signals from high-voltage lines can interfere with the low-voltage signals, causing instability in the 485 signal and communication.
2. The 485 signal line can use parallel wires or unshielded wires for wiring. The 485 signal is transmitted using differential mode, which means that the voltage difference between 485+ and 485- is used for signal transmission. If there is an external interference source, using twisted pairs for 485 signal transmission minimizes the interference effect because the differential voltage remains unchanged. Similarly, if shielded wires are used, the impact of external interference can also be reduced as much as possible.
3. It’s sufficient to use ordinary shielded twisted pair cables, like network cables. Due to rising raw material prices, the market is flooded with mixed-quality cables, and some unscrupulous merchants use alloys to replace copper wire in network cables, coating them with copper to deceive customers. A specific way to distinguish is to look at the cable cross-section; if it is copper-colored, it is copper wire, while a white cross-section indicates an alloy. Alloys are generally brittle and have much poorer conductivity than copper, which can easily cause problems during construction. It is generally recommended to choose standard 485 cables, which are shielded twisted pairs, where the transmission wires are not single copper strands like network cables but multiple strands twisted together, ensuring that even if one small copper strand breaks, it won’t affect overall functionality.
4. 485 wiring can be arbitrarily set up in star and tree configurations. The 485 wiring specification requires a daisy-chain layout. If star or tree configurations are directly set up without using 485 hubs and repeaters, it can easily cause signal reflection and bus instability. Many contractors use star and tree wiring during 485 installations; sometimes the entire system is very stable, but other times issues arise that are hard to diagnose, typically due to non-compliant wiring. If star or tree connections must be made due to site constraints, solutions can be found using 485 hubs and repeaters from Shenzhen Fuyongtong Technology Co., Ltd.
5. The 485 bus must be grounded. Many technical documents mention that the 485 bus must be grounded, but they do not detail how to ground it. Strictly speaking, the 485 bus must have a single reliable ground point. This means there should only be one grounding point along the entire 485 bus, as multiple grounding points can lead to common-mode interference. Reliable grounding requires that the ground wire (usually the shielded wire acting as a ground) has good contact to ensure consistent voltage. In actual construction, for convenience, wires are often cut into segments and connected, but if the shielded wire is not properly connected, it can lead to multiple segments of the ground wire, causing inconsistent voltage and common-mode interference.