![]() ![]() Unit Identifier: 1 byte is set to Master. It is counted from Unit Identifier to the end of the message. Length: 2 bytes are set by the Master, identifying the number of bytes in the message that follow. Protocol Identifier: 2 bytes are set by the Master, will always be 00 00, which corresponds to the Modbus protocol. These bytes are repeated by the Slave device in the response, since the responses of the Slave device may not always be received in the same order as the requests. ![]() Transaction Identifier: 2 bytes are set by the Master to uniquely identify each request. We drop the address of the SlaveID device and the CRC checksum and get the PDU: 03 006B 0003Īt the beginning of the received PDU message, a new 7-byte header is added, which is called MBAP Header (Modbus Application Header). The number of required registers (reading 3 registers from 40108 to 40110) The following is an example of a Modbus RTU request for obtaining the AI value of the holding registers from registers # 40108 to 40110 with the address of the device 17.įunction Code (read Analog Output Holding Registers)Īddress of the first register (40108-40001 = 107 = 6B hex) ![]() The Modbus TCP command consists of a portion of the Modbus RTU message and a special header.įrom the Modbus RTU message, the SlaveID address at the beginning and the CRC checksum at the end are removed, which forms the PDU, the Protocol Data Unit. The interface is an Ethernet network, the data transfer protocol is TCP / IP.īack to contents Description of the Modbus TCP protocol 20 when using the most common subnet mask 255.255.255.0. Typically, devices are on the same subnet, where IP addresses differ by the last two digit 192.168.1. In an Ethernet network, the device address is its IP address.
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