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3.2 Modbus TCP protocol
The Modbus protocol defines a so-called Modbus PDU (Protocol Data
Unit) which is independent of the underlying communication layers.
This Modbus PDU consists of the fields "Function Code" and "Data".
Depending on the mapping to the different network protocols, the
Modbus PDU is extended by additional fields in the so-called Modbus
ADU (Application Data Unit). The Modbus PDU and the Modbus ADU
constitute the Modbus message, also referred to as "Frame".
Function code DataMBAP Header
MODBUS PDU
MODBUS TCP/IP ADU
Figure 3: Structure of a Modbus message
The "Function Code" of a message specified the Modbus service to be
triggered. The "Data" field can contain additional information, depend-
ing on the "Function Code".
Due to the encapsulation of "Function Code" and "Data" in the Mod-
bus PDU, the Modbus services and the object model can be the same
in the case of all Modbus versions.
In the case of a "Function Code" for which the "Data" field in the Mod-
bus PDU has a fixed length, the "Function Code" is sufficient.
In the case of a "Function Code" for which the "Data" field in the Mod-
bus request or the Modbus response has a variable amount of data,
the "Data" field contains a byte counter.
The maximum size of a Modbus ADU is 260 bytes. The size of an
embedded Modbus PDU is 253 bytes.
NOTE: The fields are encoded in Big Endian format (highest-value
byte first).
LXM32M
3 Basics
Modbus-TCP module 17
0198441113843, V1.01, 01.2012