RS485 MODBUS RTU Communication Protocol

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RS485 MODBUS RTU Communication Protocol [Copy link]

1. RS485 interface standard

RS485 is developed from RS232 and RS422, which makes up for the shortcomings of poor anti-interference ability, short communication distance and low rate, and adds multi-point and two-way communication capabilities, that is, it allows multiple transmitters to be connected on the same main line. At the same time, it increases the transmitter's driving capability and conflict protection characteristics, and expands the bus common mode range.

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

(1) The interface signal level of RS485 is lower than that of RS231-C, which is less likely to damage the chip of the interface circuit. In addition, the level is compatible with the TTL level, making it convenient to connect to the TTL circuit.

(2) The maximum data transmission rate of RS485 is 10Mbps. The length of its balanced twisted pair is inversely proportional to the transmission rate. At a rate of 100kbps, the maximum specified cable length can be used, and the maximum transmission rate can only be achieved at a very short distance.

(3) The RS485 interface uses a combination of a balanced driver and a differential receiver, which has strong resistance to common-mode interference, that is, good resistance to noise interference, so the communication distance is long. The maximum transmission distance is about 1200m, and the actual distance can reach 3000m.

(4) The RS485 interface allows up to 128 transceivers to be connected on the bus, which means it has multi-station capability. It also requires two terminal resistors, whose resistance is required to be equal to the characteristic impedance of the transmission cable. The terminal resistors do not need to be connected when transmitting within a short distance of 300m.

2. MODBUS RTU protocol

This protocol has become a universal industrial standard. Through the protocol, serial communication can be achieved between controllers and controllers, and between controllers and other devices through the network (Ethernet). RTU stands for Remote Terminal Unit, and each 8 bits in the message contains two hexadecimal characters. At the same baud rate, this method transmits more data than the ASCII method.