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9.3 Modbus Communication
9.3.1 Format of Modbus Communication:
Function Code Modbus Name Description
0x03 Read Holding Registers Read the contents of read location
0x06 Preset Single Register Preset the contents of written location
0xFE Read Data Log Read the contents of data log
9.3.2 Modbus Communication Protocols
(1) Modbus RTU mode is adopted with Modbus Master sending out the Request, in which the Function
Code uses 0x03 to request response from Slave to correspond to values in Modbus address. In
Response, Modbus Slave responds to the values of Modbus address in the Master request. The
packet format of IEEE754 is used for the address of floating point numbers that corresponds to the
register values found in table 7.1, using 2's complement packet format. The format are as follows:
Low Word High Word
High Byte Low Byte High Byte Low Byte
The packet formats for the address of integers that corresponds to the register values found in table 7.1
are shown in the example below.
Read out:
Master
Request
Response
Slave
Request
Slave Address 1 ~ 255
Function Code 03h
Start Address (High) 00h ~ FFh
Start Address (Low) 00h ~ FFh
Number of Point (High) 00h
Number of Point (Low) 00h ~ FFh
Error Check (Low) CRC
Error Check (High) CRC
Response
Slave Address 1 ~ 255
Function Code 03h
Byte Count 00h ~ FFh
Data (High) 00h
Data (Low) 00h ~ FFh
Error Check (Low) CRC
Error Check (High) CRC
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