Modbus/Tcp Interface; Modbus/Tcp Data Frame - ICP DAS USA PET-AR400 User Manual

5.1. Modbus/TCP Interface

Modbus TCP/IP uses port 502 to listen and receive Modbus messages over Ethernet.

5.2. Modbus/TCP Data Frame

The complete Modbus/TCP ADU (Application Data Unit) is embedded into the data field of a
standard TCP frame and sent via TCP on port 502. The Modbus/TCP ADU is shown below.
Transaction ID
2 bytes
MBAP Header (Modbus Application Header)
MBAP Header
The MBAP is a 7-byte header that contains the following information:
Transaction ID
2 bytes
byte 0 ~ 1
Transaction ID：2 bytes are set by the Master to uniquely identify each request.
Protocol ID：2 bytes are set by the Master, will always be 00 00, which corresponds to the Modbus
protocol.
Length：2 bytes are set by the Master, identifying the number of bytes in the message that follow.
It is counted from Unit Identifier to the end of the message.
Unit ID：1 byte is set to Master. It is repeated by the Slave device to uniquely identify the Slave
device. Availed unit identifier is in the range of 0 to 247. Unit identifier 0 is used for general
broadcast, while unit identifier 1 to 247 is given to individual Modbus devices.
PET-AR400 User Manual
Modbus/TCP ADU (Application Data Unit)
Protocol ID Length
2 bytes 2 bytes
Protocol ID
2 bytes
byte 2 ~ 3
V1.0.0
Unit ID Function Code
1 byte
PDU (Protocol Data Unit)
Length Unit ID
2 bytes 1 byte
byte 4 ~ 5 byte 6
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Data
1 byte n bytes
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