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8.4. "03 (0x03) Read Holding Registers" function
The Modbus request is used to read the values of a number (No. of Points) of adjacent
words starting at the address indicated (Starting Address). The values read are restored
one after another, at the end of the response (First Point Data -> Last Point Data).
Request Format:
Byte Meaning
0 Function Code = 03h
1 Starting Address Hi
2 Starting Address Lo
3 No. of Points Hi (0)
4 No. of Points Lo (1 - 125)
Response format:
Byte Meaning
0 Function Code = 03h
1 Byte Count (B = 2 × No. of Points)
2 First Point Data Hi
3 First Point Data Lo
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B Last Point Data Hi
B+1 Last Point Data Lo
Exception response format:
Byte Meaning
0 Function Code = 83h
1
Exception Code =
01 (Illegal Function)
02 (Illegal Data Address)
03 (Illegal Data Value)
Notes
If the communication number that doesn't exist is read, the option returns 0x8000.