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ASDA-A3

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Modbus Communication

LRC and CRC transmission error checking

In ASCII mode, the error checking method is LRC (Longitudinal Redundancy Check). In RTU

mode, the error checking method is CRC (Cyclical Redundancy Check). See the following

details.

LRC (ASCII mode):

Start

Slave Address

Function

Start Data Address

Data Quantity

(in words)

LRC

End 1

End 0

‘:’

‘7’

‘F’

‘0’

‘3’

‘0’

‘5’

‘C’

‘4’

‘0’

‘0’

‘0’

‘1’

‘B’

‘4’

(0DH) (CR)

(0AH) (LF)

To calculate the LRC value: add all the bytes, round down the carry, and take the two’s

complement. For example: 7FH + 03H + 05H + C4H + 00H + 01H = 14CH, round down the

carry 1 and take 4CH. The two’s complement of 4CH is B4H.

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