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MODBUS Communication ASDA-A3
9-6
Revision December, 2017
9
Communication data structure
Definitions for the data frames in the two modes are as follows:
ASCII mode:
Start Start character ‟:” (3AH)
Slave Address Communication address: 1 byte consists of 2 ASCII codes
Function Function code: 1 byte consists of 2 ASCII codes
Data (n-1)
Data content: n word = 2n- byte consists of n x 4 ASCII codes, n ≤ 10 …….
Data (0)
LRC Error check: 1 byte consists of 2 ASCII codes
End 1 End code 1: (0DH) (CR)
End 0 End code 0: (0AH) (LF)
The start character in ASCII communication mode is the colon ‟:” (ASCII code: 3AH). The ADR is
two characters in ASCII code. The end code is CR (Carriage Return) and LF (Line Feed). The
codes for data such as communication address, function code, data content and error checking
(LRC: Longitudinal Redundancy Check)), are between the start character and the end code.
RTU mode:
Start A silent interval of more than 10 ms
Slave Address Communication address: 1 byte
Function Function code: 1 byte
Data (n-1)
Data content: n-word = 2n- byte, n ≤ 10 …….
Data (0)
CRC Error check: 1 byte
End 1 A silent interval of more than 10 ms
The start and end of the communication in RTU mode are silent intervals. The codes for data
such as communication address, function code, data content and error checking code (CRC
(Cyclical Redundancy Check)), are between the start interval and the end interval.