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8. Modbus protocol
8. 2. Modes
b RTU mode
The Modbus RTU frame contains no message header byte, nor end of message bytes.
It is defined as follows:
The data is transmitted in binary code.
CRC16: Cyclic redundancy check parameter.
The end of the frame is detected on a silence greater than or equal to 3.5 characters.
The format used for the frames in the rest of this document is RTU mode.
b ASCII mode
The structure of the Modbus ASCII frame is as follows:
Header: By default, ":" = 16#3A. Two other characters are possible: "CR" = 16#0D and "LF" = 16#0A.
LRC: The 2's-complement of the modulo 256 sum (in hexadecimal format) of the frame (excluding the header and end characters) before
ASCII coding.
Example: Write a value of 10 to logic address word 9001 (16#2329) on slave 2
Identical request and response:
In hexadecimal format
In ASCII format
Slave address Function code Data CRC16
Header
":"
Slave
address
Function
code
Data LRC End
Hi Lo "CR" "LF"
3A 30 32 30 36 32 33 32 39 30 30 30 41 41 32 0D 0A
:02062329000AA2CRLF