Reference

RS485 Communication Protocol

RS485 communication is one of the typical serial communication standards.

RS485 interface is widely used in industry because of its good noise immunity, long transmission distance, multi-station capability and simple wiring.

RS485 communication protocol only defines the voltage, impedance and other electrical characteristics of transmission, but does not define the software protocol. The data signal adopts differential transmission mode, so it has the ability to suppress common mode interference. The bus transceiver has high sensitivity and can detect voltage as low as 200mV. RS-485 is very convenient for multi-point interconnection and can save many signal wires. It uses a pair of twisted pairs, one of which is defined as A and the other as B. Generally, the positive level between A and B is + 2 ~ + 6V, which is in the logic 1 state, and the negative level is - 2V ~ 6V, which is in the logic 0 state. The maximum transmission distance is about 1200 meters, and the transmission rate is inversely proportional to the transmission distance. Only in a very short distance can the highest transmission rate be obtained. RS485 needs two terminal resistors whose resistance value should be equal to the characteristic impedance of the transmission cable (generally 120Ω). In short distance transmission, there is no need for terminal resistance. Terminal resistors should be connected at both ends of the transmission bus.

ModBus Communication Protocol

Modbus protocol is a master/slave architecture protocol. It has one master node and the other nodes using Modbus protocol are slave nodes, each with a unique address. A MODBUS command contains the Modbus address of the device to be executed. All devices will receive commands, but only the device at the specified location will execute and respond to the commands (except for address 0, which is a broadcast command. All devices that receive the commands will run, but will not respond to the commands). All MODBUS commands include check code to make sure that the arrived command is not damaged. The basic MODBUS command can instruct an RTU to change a value of its register, control or read an I/O port, and ask the device to send back one or more data in its registers.

There are many variants of Modbus protocols:

• Modbus RTU — This is used in serial communication and makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods.

• Modbus ASCII — This is used in serial communication and makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon (":") and trailing newline (CR/LF).

• Modbus TCP/IP or Modbus TCP — This is a Modbus variant used for communications over TCP/IP networks, connecting over port 502.[6] It does not require a checksum calculation, as lower layers already provide checksum protection.