DOCA0172EN-12

Modbus TCP/IP Functions

General Description

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:

  • Two device applications.

  • Device application and other device.

  • HMI/SCADA applications and devices.

  • A PC and a device program providing on line services.

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

Table of Modbus Functions

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)

QR Code is a registered trademark of DENSO WAVE INCORPORATED in Japan and other countries.

Was this helpful?