Schneider Electric Modbus TCP
Schneider Electric
Modbus TCP
expert (2762)djuricin00q
asked by djuricin00q

What a modbus message consists of?

What a modbus message consists of?



expert (2139)mechatronic-alex
Top answer by mechatronic-alex

Modbus message consists of 4 phases: Device address, function code, data and error check.

pioneer (323)jayson-phillis
answered by jayson-phillis

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:

Answer what-a-modbus-message-consists-of

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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