AccuEnergy Accuvim II, User Manual

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Ethernet Modual
3.3 Description of Modbus/TCP protocol
The Modbus/TCP protocol is used for communication in Ethernet module.
The protocol sets up master/slave link in Ethernet. First, master device
(client) sets up TCP link with slave device (server). Second, master
device sends request frame to slave device, and slave device receives
request frame and returns response frame to master device. Figure 3-16
displays working mode of Modbus/TCP protocol.
Figure 3-16
1. Protocol
1) Data Frame Format
MBAP Header Function Data
7x8-Bits 8-Bits Nx8-Bits
Table 3-1
2) Modbus Application Header (MBAP Header) Field
Modbus application Header field is start of data frame, and it is consist 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.
Table 3-2
Modbus Client Modbus Server
Request Indication
Confirmation Response