Modbus TCP/IP Functions

The Modbus messaging service provides a client/server communication between devices connected to an Ethernet TCP/IP network.

The client/server model is based on four types of messages:

Modbus Request, the message sent on the network by the client to initiate a transaction.
Modbus Indication, the request message received on the server side.
Modbus Response, the response message sent by the server.
Modbus Confirmation, the response message received on the client side.

The Modbus messaging services (client/server model) are used for real time information exchange between:

A dedicated header is used on TCP/IP to identify the Modbus Application Data Unit. It is called the MBAP header (Modbus Application Protocol header).

The MBAP header contains the following fields:

Fields	Length	Description	Client	Server
Transaction Identifier	2 bytes	Identification of a Modbus Request/Response transaction	Initialized by the client	Recopied by the server from the received request
Protocol Identifier	2 bytes	0 = Modbus protocol	Initialized by the client	Recopied by the server from the received request
Length	2 bytes	Number of following bytes	Initialized by the client (Request)	Initialized by the server (Response)
Unit Identifier	1 byte	Identification of a remote server connected to a serial line or on other buses	Initialized by the client	Recopied by the server from the received request

The following table describes in detail the functions supported by the Modbus TCP/IP devices supported by the EcoStruxure Panel Server:

Function code	Function name
03	Read n words
06	Write 1 word
16	Write n words
43-14	Read device identification (refer to Function 43-14: Read Device Identification)
100-4	Read non-adjacent words where n ≤ 100 (refer to Function 100-4: Read Non-Adjacent Registers)