
AXM-WEB2 for Acuvim II Series Power Meter
www.accuenergy.com
V: 1.10 Revised: January 2020
22
9. Description of Modbus-TCP Protocol
The Modbus-TCP protocol is used as one of the communication protocols in the AXM-WEB2. 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. Figure below
shows how the Modbus-TCP protocol works.
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 (MBA Header) Field
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
Modbus Client
Modbus Server
Request
Indication
Response
Confirmation