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Modbus Software Components for Developers

 
Raw Frames Tab Raw Frames

iPad Raw Frames

The Raw Frames tab shows all of the raw data bytes that make up the request and response of the last Modbus/TCP message transmitted.  Viewing the raw frames is very beneficial when troubleshooting communications problems with your Modbus/TCP device or for just understanding how Modbus/TCP messages are constructed.

Request Data

The Request Data field includes the Modbus Application Protocol (MBAP) header and the Protocol Data Unit (PDU).  The total length for all bytes of the Request is shown as a decimal Length to the left of the data.  The MBAP Header contains the following fields.

Request MBAP

Field Length (bytes) Description Example
Transaction Identifier 2 Identification of a Modbus Request/Response transaction. 076B
Protocol Identifier 2 0 = Modbus Protocol 0000
Length 2 Number of following bytes. 0006
Unit Identifier 1 Identification of a remote slave connected on a serial line or other buses. 01

The remaining bytes are for the Protocol Data Unit (PDU) which is function code dependant.  All PDUs contain the following bytes.

Request PDU

Field Length (bytes) Description Example
Function Code 1 The Modbus function code. 03 (Read Holding Registers)
Data variable Function code specific data bytes. 00 00 00 20

 

Response Data

The Response Data field includes the Modbus Application Protocol (MBAP) header and the Protocol Data Unit (PDU). The total length for all bytes of the Response is shown as a decimal Length to the left of the data.  The MBAP Header contains the following fields.

Response MBAP

Field Length (bytes) Description Example
Transaction Identifier 2 Identification of a Modbus Request/Response transaction. 076B
Protocol Identifier 2 0 = Modbus Protocol 0000
Length 2 Number of following bytes. 0043
Unit Identifier 1 Identification of a remote slave connected on a serial line or other buses. 01

The remaining bytes are for the Protocol Data Unit (PDU) which is function code dependant.  All PDUs contain the following bytes.

Response PDU

Field Length (bytes) Description Example
Function Code 1 The Modbus function code. 03 (Read Holding Registers)
Data variable Function code specific data bytes. 40 00 00 ... 00

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