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Connecting with Other Products
10.3 Modbus TCP Frame
10.3.1 Modbus TCP Frame Structure
MBAP Header (7 bytes)
PDU (5 bytes or greater)
In general, EtherNet communication uses EtherNet II frames.
MODBUS Application Protocol header (MBAP header)
The following table explains the components of a MBAP header.
Section
Length
Description
Transaction
identifier
2 byte
Unique transmission number, which increases by 1 each
time the client sends data frame to the server.
Protocol
identifier
2 byte
Fixed at 0.
Length
2 byte
Data frame length of the Modbus communication, which
represents the length (in byte unit) from the MBAP header
to the unit identifier.
Unit identifier
1 byte
When communications using Modbus TCP and Modbus
RTU are connected via a gateway, the unit identifier
indicates the slave number. The address is fixed to 0xFF
when Modbus TCP communication is used alone.
Protocol Data Unit (PDU)
PDU is the actual data in the Modbus TCP communication, which is composed of a
function code and data.
Refer to "10.3.2 Function codes" below for detailed information.
