Modbus protocols include ASCII, RTU , TCP, etc., and do not specify the physical layer. This protocol defines the message structure that controllers can understand and use regardless of the network over which they communicate. The standard Modicon controller uses RS232C to implement serial Modbus. The ASCII and RTU protocols of Modbus stipulate the structure of messages, data, commands and response methods. Data communication adopts the Maser/Slave method. The Master sends a data request message, and the Slave can send data to the Master after receiving the correct message. To respond to the request; the Master can also directly send messages to modify the data on the Slave to achieve bidirectional reading and writing.
Modbus has the following characteristics:
1. Standard and open. Users can use the Modbus protocol for free and with confidence. There is no need to pay license fees and it will not infringe on intellectual property rights. Currently, there are more than 400 manufacturers supporting Modbus and more than 600 products supporting Modbus.
2. Modbus can support a variety of electrical interfaces, such as RS-232, RS-485, etc., and can also be transmitted on various media, such as twisted pair, optical fiber, wireless, etc.
3. The frame format of Modbus is simple, compact and easy to understand. It is easy for users to use and easy for manufacturers to develop.
In fact, Modbus protocols include ASCII, RTU, and TCP.
The standard Modicon controller uses RS232 to implement serial Modbus. The ASCII and RTU protocols of Modbus stipulate the structure of messages and data, the command and response methods, and the data communication adopts the Maser/Slave method.
Modbus protocol requires data verification. In addition to parity check, the serial protocol uses LRC verification in ASCII mode and 16-bit CRC verification in RTU mode. Modbus TCP mode does not specify additional verification because the TCP protocol is a reliable connection-oriented protocol.
The RTU and TCP protocols are very similar. Just remove the two-byte check code of the RTU protocol, then add 5 0s and a 6 at the beginning of the RTU protocol and send it out through the TCP/IP network protocol.
Modbus RTU and Modbus TCP are both MODBUS protocols in essence, and both rely on MODBUS register addresses to exchange data; but the hardware interfaces used are different. Modbus RTU generally uses the serial port RS232 or RS485, while Modbus TCP generally uses the Ethernet port. . There are many protocol converters on the market today that can easily convert these different protocols to each other.
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