Acuvim-L Series : L-WEB User manual
www.accuenergy.com
V: 1.0 Revised: Sept. 2019
22
9. Description of Modbus-TCP Protocol
The Modbus-TCP protocol is used as one of the communication protocols in the L-WEB. The
protocol establishes a master and slave connection in Ethernet. The master device (client)
first sets up a TCP-IP link with slave device (server). The master device then sends a request
to the slave device and the slave device in return sends a response to the master device. The
figure below shows how the Modbus-TCP protocol works.
Modbus Client
Modbus Server
Request
Indication
Response
Confirmation
9.1 Protocol
9.1.1 Data Frame Format
MBAP Header
Function
Data
7x8-Bits
8-Bits
Nx8-Bits
9.1.2 Modbus Application Header (MBAP Header) Feild
Modbus application header field is the start of the data frame, and consists of seven bytes.
Field
Length
Description
Transaction Identifier
2 Bytes
Identification of a Modbus Request/Response
transaction
Protocol Identifier
2 Bytes
Modbus protocol = 0
Length
2 Bytes
Number of following bytes
Unit Identifier
1 Byte
Slave address, in the range of 0~247 decimal.