87
Acuvim-L
4.2 Modbus Protocol
Framing
Table 4-1 data frame format
Address field
The address field of a message frame contains eight bits. Valid slave device
addresses are in the range of 0~247 decimal. A master addresses a slave by
placing the slave address in the address field of the message. When the slave
sends its response, it places its own address in this address field of the response
to let the master know which slave is responding.
Function field
The function code field of a message frame contains eight bits. Valid codes are
in the range of 1~255 decimal. When a message is sent from a master to a slave
device the function code field tells the slave what kind of action to perform.
Address
Function
Data
Check
8-bit
8-bit
N×8-bit
16-bit
Code
Meaning
Action
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 from one
or more registers
16
Preset multiple-registers
Place specific value into a series of
consecutive multiple-registers
Table 4-2 Function code
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