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b. Modbus Application Header (MBA Header) Field
The Modbus application header eld is the start of the data frame and consists of seven bytes.
Field Length Description
Transaction Identier 2 Bytes
Identication of a Modbus Request/Response
transaction
Protocol Identier 2 Bytes Modbus Protocol = 0
Length 2 Bytes Number of following bytes
Unit Identier 1 Bytes Slave address, in the range of 0-247 decimal
c. Function Field
The function code eld of a message frame contains eight bits. Valid codes are in the range of
1-255. When a message is sent from a client to a server device, the function code eld tells the
server what kinds of action to perform.
Code Meaning Action
05 Control Single Relay Output Force Relay to a state of ON or OFF
01 Read Relay Output Status Obtain current status of Relay Output
02 Read Digital Input (DI) Status Obtain current status of Digital Input
03 Read Data Obtain current binary value in one or more registers
16 Write Multiple Registers
Place specic value into a series of consecutive
multiple registers
Chapter 5: Connection Method
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