RS485 MODBUS RTU Communication protocol

1, RS485 interface standard

RS485 by the development of RS232 and RS422, to compensate for the poor anti-jamming ability, short communication distance, low rate of the shortcomings, increased multi-point, two-way communication capability, that allows multiple transmitters connected to the same main line, while increasing the transmitter's drive capability and conflict protection characteristics, extended the bus common-mode range.

RS485 can be used in two-line and four-wire two ways, second-order system can achieve true multi-point two-way communication. Its main features are as follows:

(1) RS485 interface signal level is lower than rs231-c, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which facilitates the connection of the TTL circuit.

(2) The maximum data transfer rate of RS485 is 10Mbps. The length of the balanced twisted pair is inversely proportional to the transmission rate, at a rate of 100kbps, it is possible to use the longest cable length specified and the maximum transfer rate can be obtained at a very short distance.

(3) RS485 interface using a combination of balanced driver and differential receiver, anti-common mode interference ability, that is, anti-noise interference is good, so the communication distance, the maximum transmission distance of about 1200m, the actual up to 3000m.

(4) The RS485 interface allows the connection of up to 128 transceivers on the bus, with a multi-station capability. At the same time, two terminal resistors are required, and their resistance is equal to the characteristic impedance of the transmission cable. The terminal resistor is not connected when transmitting within a short distance of 300m.

2. MODBUS RTU Protocol

This protocol has become the general industry standard, through the Protocol, the Controller and controller, controller through the Network (Ethernet) and other devices can achieve serial communication. RTU is a remote terminal unit with two hexadecimal characters per 8 bits in the message. At the same baud rate, this method is larger than the amount of data transmitted in ASCII mode.

3. Reference

(1) Li Jiang full. Visual Basic.NET serial communication and measurement and control application Typical example [M]. Electronic industry press,.

(2) Tri Jing, Xu Xiaoquang, Kan, ET. C # Programming realizes Modbus Communication protocol [J]. Journal of Xinxiang University, 2014, 41 (8): 30-33.

(3) Modbus function code list: https://wenku.baidu.com/view/693da73782c4bb4cf7ec4afe04a1b0717fd5b3c9.html

Huairen, but true reality.

RS485 MODBUS RTU Communication protocol