Ethernet Module
Sinus H
23
5.
Modbus/TCP
5.1 Modbus/TCP Frame Structure
MBAP Header( 7 bytes)
PDU (5 bytes ~)
Generally, Ethernet uses Ethernet II Frame.
[MODBUS Application Protocol Header (MBAP Header)]
Header
Length
Description
Transaction
Identifier
2 Bytes
It is increased by 1 each time as a unique
transmitting number when Data Frame is sent
from Client to Server.
Protocol
Identifier
2 Bytes
Fixed as 0
Length
2 Bytes
It is Modbus Data Frame length which means
the length by Byte from MBAP Header to Unit
Identifier.
Unit
Identifier
1 Bytes
When Modbus/TCP and Modbus/RTU are
connected by Gate, Slave number is written.
When Modbus/TCP is only used, it is fixed as
0xFF.
Protocol Data Unit (PDU): AS an actual Data of Modbus/TCP, It
is consists of Function Code and Data.
5.2 Function Code Description
Modbus/TCP can be divided into Client and Server. Client gives
the command and Server responds to the command. Generally,
as Client, there are PLC, HMI and PC so on, and Server means
inverter.
