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2. MODBUS Protocol Description
2.4 ASCII Frame
2.4.1 Byte Format
The delivery sequence of byte or word is sent in the following order (left to right): LSB (Least Significant Bit) ---> MSB (Most
Significant Bit).
The format for each byte (10 bits) in ASCII mode is:
• Bits/Byte: 1 start bit
• 7 data bits, LSB sent first
• 0 parity bits
• 2 stop bits
Start Bit
1
1
2.4.2 ASCII Frame
ASCII frame is made up of start symbol, replica address, function code, data, LRC check and over character.
The maximum size of a MODBUS ASCII frame is 513 bytes. The maximum data size is 2*252 bytes.
Start
1 char
:
2.4.3 LRC Checking
The LRC field is one byte, containing an 8-bit binary value. The LRC is calculated by adding together successive 8-bit bytes
of the message, discarding any carries, and then two's complementing the result. It is performed on the ASCII message field
contents, excluding the 'colon' character that begins the message and the CRLF pair at the end of the message. In ASCII
mode, the resulting LRC is ASCII-encoded into two bytes and placed at the end of ASCII mode frame prior to the CRLF.
2.4.4 Data
DATAB: Value type is one byte char. The message is sent two ASCII characters. The data range:
Sign Char type: -128 ~ +127
No Sign Char type: 0 ~ +255
DATAI: Value type is two bytes int. The message is sent four ASCII characters. The data range:
Sign Int type: -32768 ~ +32767
No Sign Int type: 0 ~ +65535
Table 2.4.1 Bit Sequence in ASCII Mode
Data Bit
2
3
Table 2.4.2 ASCII Frame
Address
Function
2 char
2 char
4
5
6
Data
0–2*252 char
6
Stop Bit
Stop Bit
7
1
LRC
End
2 char
2 char
CR, LF
1
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