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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
