RS485 bus is a standard serial communication protocol that enables communication between multiple nodes and maintains high speed and reliability over long transmission distances.

In practice, RS485 bus is commonly used in industrial automation, power monitoring, environmental monitoring, building automation and other fields. Next will be from the RS485 bus is what line and RS485 bus wiring two aspects of a detailed introduction.

1. What is the RS485 bus?

RS485 bus is a differential signal transmission line for two-way communication. Unlike the RS232 communication protocol, RS485 transmits data through differential signaling, which has strong anti-interference performance. At the same time, RS485 bus also adopts a half-duplex communication mode to support multiple device access, making it one of the most common communication methods in the field of industrial automation.

RS485 bus in the application can use shielded or unshielded twisted pair (commonly used four pairs) for wiring, and need to be careful not to connect directly to the terminating resistor at both ends of the bus, to avoid baud rate error caused by signal reflection. In addition, in the design of the RS485 bus also need to consider the load capacity and rate and other series of issues.

2.RS485 bus wiring method

RS485 bus wiring is usually "star", "ring" or "bus type" three wiring methods.

(1) Star-shaped wiring

Star-shaped wiring, that is, a trunk line in series with each slave device, all slave devices do not communicate with each other, just send data to the trunk line. This type of wiring avoids malfunctions that may affect the entire system.

(2) Ring wiring

Ring cabling connects all the devices into a closed loop and realizes data transmission through circular transmission. This type of wiring is characterized by low delay and high interference immunity, but any failure will affect the whole system.

(3) Bus Wiring

Bus-type cabling that is, using a large trunk line in series with all the equipment, the equipment through the allocation of the address to realize the transmission of data. This wiring method is simple in structure, high reliability, and is currently the most widely used one.

Different wiring methods have their own advantages and disadvantages in practical application, and need to be selected according to specific needs.

According to the theory of 485 bus structure, under the premise of ideal environment, 485 bus transmission distance can reach 1200 meters. The condition is that the communication wire quality standard, baud rate of 9600, only load a 485 device, in order to make the communication distance to 1200 meters, so usually the actual stability of the 485 bus communication distance often does not reach 1200 meters. If the load of 485 devices, wire impedance is not in line with the standard, wire diameter is too thin, poor quality converters, equipment lightning protection complex and baud rate increase and other factors will reduce the communication distance.