1. Protocol description: Modbus communication protocol has three modes.
(1) Modbus RTU (Remote Terminal Unit)
(2) Modbus ASCII (American Code for Information Interchange)
(3) Modbus TCP (Internet connection), in line with the trend of development, the latest mode; Modbus RTU and Modbus ASCII used physical hardware interface are serial communication ports (RS232, RS422, RS485). The physical hardware interface used for Modbus TCP is the Ethernet port, i.e,the crystal-head connected network port, the RJ45 connection port. In a modbus communication system, only one mode can be selected, and no mixture of the two modes is allowed.

2. Interface description: RS-232, RS-422 and RS-485 are serial data interface standards, without involving connectors, cables or protocols.

RS-232 has a maximum transmission distance of about 15 meters and a maximum rate of 20kb/s, and RS-232 is designed for point-to-point (i.e, only a pair of receiving and transmitting devices) communication. Therefore, RS232 is just suitable for local communication.

RS-422 is proposed to make up for the shortcomings of RS-232. The transmission rate is increased to 10Mb/s, the transmission distance is extended to 1200m (when the rate is lower than 100kb/s), and it allows up to 10 receivers to be connected on a balanced bus.

RS-485 adds multipoint, bi-directional communication capability, i.e, it allows multiple transmitters to be connected to the same bus, while increasing the drive capability and conflict protection features of the transmitters and extending the bus common-mode range.

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