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Modbus Agile 41 02/2011
7.3.10.2.1 Character Format
One character consists of 10 bits:
− 1 start bit
− 7 data bits, least significant bit sent first (value = 0x00 .. 0x7F)
− 1 parity bit
− 1 stop bit
Note:
If no parity is used then an extra stop bit is added.
Character format with parity checking:
Start B1 B2 B3 B4 B5 B6 B7 Parity Stop
Character format without parity checking:
Start B1 B2 B3 B4 B5 B6 B7 Stop Stop
7.3.10.2.2 Modbus Message ASCII Framing
A MODBUS message is placed by the transmitting device into a frame that has a known beginning and
ending point. This allows devices that receive a new frame to begin at the start of the message, and
to know when the message is completed. Partial messages must be detected and errors must be set
as a result.
The address field of a message frame contains two characters.
In ASCII mode, a message is delimited by specific characters as Start-of-Frame and End-of-Frame. A
message must start with a trigger "colon"- character (":" = ASCII 0x3A) and ends with a "carriage
return/line feed" pair (ASCII 0x0D and 0x0A).
The allowable characters transmitted for all other fields are hexadecimal 0–9, A–F (ASCII coded). The
devices monitor the bus continuously for the colon character. When this character is received, each
device decodes the next character until it detects the End-of-Frame (EoF)-character.
Intervals of up to one second may elapse between characters within the message. Unless the user has
configured a longer timeout, an interval greater than 1 second means an error has occurred.
A typical message frame is shown below:
SoF Address Function Data LRC EoF
1 char
:
2 chars 2 chars 0 up to 2* 252 chars 2 chars 2 chars
CR, LF
7.3.10.2.3 LRC Checking
In ASCII mode, messages include an error–checking field that is based on a Longitudinal Redundancy
Checking (LRC) calculation that is performed on the message contents, exclusive of the beginning
‘colon’ and terminating CRLF pair characters. It is applied regardless of any parity checking method
used for the individual characters of the message.
The LRC field is one byte, containing an 8-bit binary value. The LRC value is calculated by the device
that emits, which appends the LRC to the message. The device that receives calculates an LRC during
receipt of the message, and compares the calculated value to the actual value it received in the LRC
field. If the two values are not equal, an error results.
The LRC field is one byte, containing an 8-bit binary value. The LRC value is calculated by the device
that emits, which appends the LRC to the message. The device that receives calculates an LRC during
receipt of the message, and compares the calculated value to the actual value it received in the LRC
field. If the two values are not equal, an error results.