User guide

50 MDX-ETHERNET User Guide

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13.6.2 Data access

Data access using Modbus TCP/IP takes the form of a request for data by the master,

followed by a response from the slave indicating either success (response), or failure

(exception response) as shown in Figure 13-2. If no response is received this indicates

that the message has not been received or the node is unable to reply.

Figure 13-2 Modbus TCP/IP- request, response, exception

MBAP and PDU message structure

The following tables document the structure of the MBAP and PDU, specifically the

functions of each data byte within the overall message structure.

Table 13.5 MBAP

Table 13.6 PDU

Byte Description

0 Transaction identifier MSB

1 Transaction identifier LSB

2 Protocol identifier MSB

3 Protocol identifier LSB

4 Length MSB

5 Length LSB

6 Unit identifier

PDU Consist of

Request Function code, 1 byte

Function data, > 1 byte

Response Function code, 1 byte

Response data, > 1 byte

Exception

Response

Error code, 1 byte

Exception code, 1 byte

Function

Code

Function Data

Function

Code

Response Data

Function

Code

Exception Data

PDU (Request)

PDU (Exception Response)

PDU (Response)