LMV Series
Technical Instructions
Document No. LV5-1000
SCC Inc.
Page 1
Section 8
8-1: Modbus
Master-slave principle
Communication between Modbus users takes place according to the master-slave principle. The AZL5
always works as a slave.
Data Transmission
Transmission mode (RTU)
– The transmission mode used is RTU (Remote Terminal Unit)
– Data is transmitted in a binary format (hexadecimal) with 8 bits
– The LSB (least significant bit) is transmitted first
– ASCII operating mode is not supported
Structure of data blocks
All data blocks use the same structure with the following four fields:
Slave Address
Function Code
Data Field
Checksum CRC16
1 byte
1 byte
x byte
2 bytes
Slave Address
Device address of a certain slave
Function Code
Function selection (reading / writing words)
Data Field
Contains the following information:
- Word address
- Number of words
- Word value
Checksum
Identification of transmission errors
Checksum (CRC16)
The checksum (CRC16) is used to detect transmission errors. If, during evaluation, an error is detected,
the relevant device will not respond.
Calculation
scheme
CRC = 0xFFFF
CRC = CRC XOR ByteOfMessage
For (1 through 8)
CRC = SHR (CRC)
if (flag shifted at right = 1)
then
CRC = CRC XOR
0xA001
else
while (not all ByteOfMessage handled)
The low-byte of the checksum is transmitted first.
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