Schneider ElectricModbus TCP
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What a modbus message consists of?
What a modbus message consists of?
Schneider ElectricModbus TCP
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Modbus message consists of 4 phases: Device address, function code, data and error check.
The Modbus message has the same format for the request and response messages, but it depends on the variation of the protocol used:
- Modbus ASCII - serial protocol that is using an ASCII characters subset
- Modbus RTU - serial protocol that uses 8-bit binary messages
- Modbus TCP (which we are interested in) - TCP/IP protocol
- Modbus RTU over TCP - which is a TCP/IP protocol with additional CRC check
So the Modbus TCP uses a TCP/IP link to send or receive Modbus messages and this is the following structure:
In this message type, the specific fields can be explained as the following:
- The Transaction Id is a field that identifies the transaction
- The Protocol identifies 0 - indicating a Modbus protocol
- Length field identifies the number of bytes to follow
- Unit Address field is the address of the PLC that is encoded as a single byte
- And the Message field represents the Modbus PDU, with a maximum length of 253 bytes
The whole Modbus TCP message has a maximum length of 260 bytes.
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