RS-485 Bus Overview

When the communication distance is required to be from tens to thousands of meters, the RS-485 serial bus standard is widely used.

RS-485 uses balanced transmission and differential reception, thus it has the ability to suppress common-mode interference. Additionally, the bus transceiver has high sensitivity and can detect voltages as low as 200mV, allowing the transmission signal to be recovered over kilometers.

RS-485 operates in half-duplex mode, meaning that only one device can be in the sending state at any time; therefore, the sending circuit must be controlled by an enable signal. RS-485 is very convenient for multipoint interconnection and can save a lot of signal lines. Using RS-485, a distributed system can be constructed.

The number of “nodes” in RS-485 is mainly determined by the “receiver input impedance”; According to regulations, the input impedance of a standard RS-485 interface is ≥12KΩ, and the corresponding standard number of driver nodes is 32. To accommodate more nodes in communication scenarios, some chips are designed with input impedances of 1/2 load (≥24KΩ), 1/4 load (≥48KΩ), or even 1/8 load (≥96KΩ), allowing the corresponding number of nodes to increase to 64, 128, and 256. Each RS-485 network can operate under different baud rates: 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200, allowing the same address number. Therefore, relay modules can not only increase communication distance but also increase the number of nodes.

Due to its simple wiring and reliable stability, the RS-485 bus is widely used in video surveillance, intercom systems, building alarms, and various other fields. However, during the wiring process of the RS-485 bus, many inaccurate concepts lead to numerous issues. Here are some clarifications on some misconceptions:

485 signal lines cannot be run together with power lines. In actual construction, since wiring is usually done through conduits, sometimes the construction team binds the RS-485 signal lines directly with the power lines for convenience. Due to the strong electromagnetic signals from the power lines interfering with the weak signals, this leads to unstable RS-485 signals and unstable communication.

RS-485 signal lines can use shielded cables for wiring, and unshielded cables can also be used. Since RS-485 signals are transmitted using differential mode, the voltage difference between 485+ and 485- is used for signal transmission. If there is an external interference source, using twisted pair cables for RS-485 signal transmission minimizes the effect of interference on both 485+ and 485-, keeping the voltage difference unchanged. Similarly, if a shielded cable is used, the impact of external interference can also be minimized.

Choosing to use ordinary Category 5 shielded twisted pair cable (i.e., Ethernet cable) is sufficient. Due to rising raw material prices, the market is flooded with various cables, and some unscrupulous merchants use alloys to replace copper wire for Ethernet cables, plating them with copper to deceive customers. The specific distinction method is to look at the cross-section of the cable; if it is copper-colored, it is copper wire; if it is white, it is an alloy posing as copper. Alloys are typically brittle, easy to break, and have far inferior conductivity compared to copper wire, which can easily cause problems during construction. It is generally recommended to choose standard RS-485 cables, which are shielded twisted pairs, where the transmission wires are not single-strand copper but multiple strands twisted together, ensuring that even if one small copper wire breaks, it will not affect the overall use.

485 wiring can be arbitrarily configured into star and tree topologies using RS-485 hubs and repeaters. The RS-485 wiring specification requires hand-in-hand wiring; if it is directly configured into star and tree connections without using RS-485 hubs and repeaters, it can easily cause signal reflections, leading to bus instability. Many construction teams use star and tree wiring in RS-485 wiring processes; sometimes the entire system is very stable, but at other times, problems arise that are difficult to trace, usually due to non-standard wiring.

RS-485 bus must be grounded. Many technical documents mention that the RS-485 bus must be grounded, but do not detail how to ground it. Strictly speaking, the RS-485 bus must be reliably grounded at a single point. A single point means that there can only be one grounding point on the entire RS-485 bus; multiple grounding points are not allowed because grounding is meant to keep the voltage on the ground wire (usually the shielded wire acting as the ground) consistent, preventing common-mode interference. If there are multiple grounding points, it can be counterproductive. Reliable grounding means that the ground wire of the entire RS-485 line must have good contact to ensure consistent voltage. In actual construction, for convenience, wires are often cut into multiple segments and connected, but if the shielded wire is not well connected, it leads to the ground wire being divided into multiple segments, making the voltage inconsistent and causing common-mode interference.

RS-485 Communication Network Structure Diagram (Including Terminal Resistance Settings)

RS-485 Bus Overview

Structure 1

RS-485 Bus Overview

Structure 2

RS-485 Bus Transmission Distance

When using 0.56mm (24AWG) twisted pair as communication cable, the maximum theoretical transmission distances based on different baud rates are as follows: