Understanding of RS-485 Communications

Introduction:

RS-485 is a standard protocol for transmitting data. It can be used to establish a reliable, high-speed, real-time, multi-node data communication network connection. RS-485 is also called TIA-485. RS-485 is a standard that defines the electrical characteristics of drivers and receivers used in serial communication systems. RS485 is widely used in industrial control systems and can handle up to 32 devices on a single network. RS-485 is commonly used in industrial automation to monitor and control PLCs, variable frequency drives, DCS, etc. This article will mainly introduce the basic principles, characteristics, wiring and practical application cases of RS-485 communication.

Basic principles of RS-485 communication:

RS-485 is an asynchronous serial communication protocol that enables multi-node communication. RS-485 communication is based on differential signaling, where information is transmitted over two complementary signals sent over two wires (often called A and B). The voltage difference between the two wires is what conveys the information, not the voltage between the individual wire and ground. This makes RS-485 systems highly resistant to common mode noise. And it can improve the transmission distance and transmission speed. The RS-485 protocol stipulates that a master node can communicate with up to 32 slave nodes, and the communication between each node is coordinated through the master node.

Features of RS-485 communication:

RS-485 communication has the characteristics of high speed, reliability, stability, real-time and low cost. Because RS-485 supports multi-node communication, it eliminates the need for complex signal forwarding mechanisms and makes it easier to expand the network. The RS-485 protocol is standardized, so compatibility issues can be avoided. In addition, due to the application of differential transmission technology, RS-485 communication has high anti-interference capabilities against electromagnetic interference. At the same time, RS-485 communication can ensure the stability and reliability of communication when the communication distance reaches 1.2 kilometers. RS-485 signals are transmitted without acknowledgment. Interruptions or interference in differential signals can corrupt data without being repeated or received; a "fire and forget" system.

RS-485 wiring:

The wiring of RS-485 requires the twisted pair mechanism as shown in the figure below. A twisted pair composed of a positive and negative pair of data lines is laid. At the same time, since RS-485 uses differential signals for transmission, we also need to provide an additional common signal ground for the two data lines. In order to avoid interference from other interfering signals, we can add an RS-485 interference-resistant attenuator in the middle of the wiring.

RS-485 communication case:

Let's consider a simple example of an RS-485 network with one master and two slave devices.

Idle state: When there is no device transmitting, the line is in idle state. In this state, the differential voltage between line A and line B is zero.

Master Transmission: When the master wants to send data, it changes the voltage difference between the A and B lines. For example, a "1" might mean that A has a higher voltage than B, and a "0" might mean that B has a higher voltage than A.

What the slave will get: All devices on the network, including the slave, will continuously monitor the voltage difference between the A and B lines. When they detect a change, they interpret it as some data.

Slave Response: If the master sends a command that requires a response from the slave, the slave will wait until the master completes the transmission and then changes the voltage difference between the A and B lines to send its response.

Master Reception: The master device, like the slave device, constantly monitors the voltage difference between the A and B lines, so it will receive the response from the slave device.

Return to idle state: After all data has been transmitted, the line returns to the idle state and the voltage difference between lines A and B is zero.

In this way, data can be sent back and forth over the RS-485 network. It's important to note that all devices on the network need to use the same logic to interpret voltage differences as bits (i.e. does A having a higher voltage than B represent a "1" or a "0"). In a network with multiple devices, each device needs to have a unique address so that it knows when to listen and when to ignore traffic on the line. This is usually handled by a protocol used over RS-485, such as Modbus or Profibus.

For example, in a Modbus network, every message sent by the master begins with the address of the target device. When devices see a message with their address, they know to process the message and possibly send a response. If the address does not match your own address, the message is ignored.

Summarize:

Compared with TCP/IP, USB, I2C and other protocols, although the transmission speed of RS-485 is not particularly fast, it has unparalleled advantages: it can realize multi-node communication, has strong anti-interference ability, and has long communication distance. These characteristics are No other protocol can compare. As a communication protocol widely used in industrial control, automation and other fields, RS-485 still has broad prospects for future use.

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